Mosquitoes may not be the first thing most people associate with Greenland, but as arctic temperatures rise we may start hearing more about these pesky critters. Dr. Lauren Culler uses these mosquitoes and the snow-melt ponds they breed in as a fascinating study system to investigate changes in mosquito life cycles within a rapidly changing environment
Honey bees face a myriad of interacting stress factors including pesticide exposure and poor nutrition in intensive agricultural landscapes. Andrew Garavito spent his masters looking at how these factors interact in real-world landscapes to affect honey bee health. Comparing factors such as pollen diet, pesticide contamination of pollen, and drought stressing of pollen, he gained some interesting insights into what bees face on any given day in the field.
Natural threats among the flowers lurk. Dr. T'ai Roulston delves into the somewhat macabre world of bumble bee parasitism by conopid or thick-headed flies.
"Dr. Arnaud Martin details his research adapting the CRISPR-Cas9 gene editing system to crustaceans and butterflies, providing further evidence that supports previous findings surrounding the use of genetic tool kits found in all animals."
The Greek philosopher Heraclitus was channeling his inner taxonomist when he declared, “everything is in flux.” Depending on who you ask, taxonomy as a field precariously teeters between being either the foundation of all other biological sciences or the most esoteric debate topic two scientists can choose. Scientists rely on taxonomists to draw the lines between the species that we study so we can all be on the same page while we study them (imagine trying to plan a trip to the zoo without names for the animals). As Dr. Jason Mottern explained during the last Entomology Department colloquium of 2016, these lines are often drawn in pencil.
Conservation biological control – a fancy term for the support of beneficial insects in a cropping system to enhance natural pest control - has long been of interest in organic agriculture. Lauren Hunt of the Department of Entomology’s Hooks Lab has been investigating the potential for conservation biological control in organic field corn using partridge pea (Chamaecrista fasciculata) to attract and support insects that feed on and parasitize pests. Many predators and parasitoids, collectively called beneficial insects or natural enemies, need nectar resources offered by flowers to successfully develop and reproduce. Partridge pea offers these nectar resources to insects in the form of flowers and via special glands, termed “extrafloral nectaries” located at the base of the petioles. These nectaries are particularly attractive to wasps known to parasitize a variety of insect pests, including stink bugs, corn earworms, and corn borers – common pests of corn and key pests in Lauren’s study.
The kudzu bug (Megacopta cribraria) (Family Plataspidae) (Fig. 1) has become a widespread invasive pest in the southern United States since it was first detected in Georgia in 2009. This small, round bug is closely related to stinkbugs (Pentatomidae) and has plagued soybean farmers under non-treatment and high density conditions with yield losses as high as 59%! It feeds on the soybean plants’ leaves and stems with its piercing-sucking mouthparts, which can affect the number of seeds per pod and overall seed weight. The overall damage is astounding, especially when considering all kudzu bugs in North America originated from a single female bug from Japan. Luckily, many studies have been done to track and monitor this pest, such as those done by Jessica Grant.
It can be a lonely world for a bed bug researcher! Dr. Mark Feldlaufer began his presentation by extending an open invitation to visit his bed bug research lab at the United States Department of Agriculture-Agricultural Research Center in Beltsville, Maryland. He assured us that the offer rarely gets takers, and it is really not surprising, all things considered. As parasites that feed on people and cause itchy welts, bed bugs give people the heebie-jeebies. Just a picture or two of the bed bugs (Fig. 1) was enough to have several attendees visibly cringe, as they imagine the marks and persistent itches that usually follow after bed bug bites. However, Dr. Feldlaufer’s fascinating research, which aims to improve bed bug detection and control, may contribute to a future where everyone can rest assured that there truly are no creatures lurking under the bed.
often do not come in contact with them. It is impractical, dangerous, and against labelled usage, to coat every surface and crack of a home or business in insecticide, so bed bug chemical treatments often fail. The silver bullet control method appears to be heat treatments, but, like silver, heat treatments are expensive. Homes are heated to between 120-140⁰F for several hours to kill all bed bug life stages, which tend to cost anywhere between $2,500-$7,000. Though bed bugs themselves do not discriminate against the rich or poor, our methods of remediation tend to do just that. The most effective way to get rid of bed bugs is unaffordable to many.
In the course of his bed bug research, Dr. Feldlaufer has worked on other projects to help to detect and control bed bugs early in their infestations with a number of students, including our very own Dr. Kevin Ulrich. The team found that bed bugs consistently avoided a common chemical insect repellent, DEET, the main ingredient in most mosquito repellents. They did not, however, respond with greater avoidance to higher dosage. Although this may seem like a quick and easy fix, wearing DEET to bed may just cause bed bugs to merely move on to other surfaces (e.g. the couch). On the other hand, they also found that aldehyde compounds produced by bed bugs elicit a strong attractive response to other adult and immature bed bugs. These aldehydes may be one way bed bugs attract each other to form aggregations (Ulrich et al. 2016), suggesting that the aldehydes may be used to develop an inexpensive and more discreet option to lure and trap bed bugs. Dr. Feldlaufer currently aims to develop and test reduced risk pesticides that show promise in killing bed bugs.
Bed bugs can be a terribly menacing presence in anyone’s home, but thankfully Dr. Feldlaufer has dedicated much of his career to keeping us informed about these insects and their crafty activities. By pairing lures with reduced risk pesticides, Dr. Feldlaufer aims to develop options to safely, affordably, and discreetly ensure we can sleep tight assured that the bed bugs will not bite.
Lit L., Schweitzer J.B., and Oberbauer A.M. 2011. Handler beliefs affect scent detection dog outcomes. Animal Cognition 14: 387.
Pfiester M., Koehler P.G., and Pereira R.M. 2008. Ability of bed bug-detecting canines to locate live bed bugs and viable bed bug eggs. Journal of Economic Entomology 101(4):1389-96.
Ulrich, K.R., Kramer, M. and Feldlaufer, M.F. 2016. Ability of bed bug (Hemiptera: Cimicidae) defensive secretions (E)-2-hexenal and (E)-2-octenal to attract adults of the common bed bug Cimex lectularius. Physiological Entomology 41: 103-110.
Hanna Kahl is a master’s student at University of Maryland in Dr. Cerruti Hooks’ lab researching the effects of red clover living mulch on arthropod pests and pollinators.
Samuel Ramsey is a PhD student at University of Maryland in Dr. Dennis vanEngelsdorp’s lab researching Varroa destructor.
Skunk, cilantro, burnt rubber—just a few of the many scents you might associate with the pungent odor produced by stink bugs. Like many animals, these bugs produce a variety of chemical odors (called semiochemicals) that modify the behavior of recipient organisms in different ways. That “stink” that these bugs get their name from is just one example. Stink bugs (family Pentatomidae) represent an extremely diverse family of insects that includes both agricultural pests and beneficial predators. Some of the agricultural pest species can cause millions of dollars in damages to crops. At the USDA Agricultural Research Service in Beltsville, Maryland, Dr. Don Weber studies stink bug semiochemicals in hopes of being able to take advantage of their communication to monitor their populations. In particular, Dr. Weber studies pheromones – one kind of semiochemical used for communication within a species. For example, male stink bugs emit pheromones to attract females, or in other cases, both females and males. In other insects, pheromones can also act to signal danger, food resources, or aggregation sites.
Because semiochemical signals often influence insect behavior, synthetically replicating these scents equips growers with a powerful tool to manage agricultural insect pests. This strategy works most effectively if the chemical composition of the natural odor is identified and isolated, and if the behavior it elicits is fully understood. The invasive brown marmorated stink bug (BMSB) (Halyomorpha halys) and the harlequin bug (Murgantia histrionica) (Figure 1) both respond to traps baited with pheromone lures which can be used in order to monitor levels of these pests to determine whether or not to take action against them with insecticides (Figure 2).
With stink bugs, there are some challenges that researchers such as Dr. Weber have faced in using pheromones as lures. Pheromones released by male stink bugs are not exclusively sex pheromones;females, males and nymphs alike are attracted by the male pheromones. This indicates to researchers that those pheromones might have some of the other roles mentioned above, such as signaling aggregation sites.
Yet another piece to this complicated smell puzzle stems from cross-species attraction, meaning that the deployment of a pheromone by one stink bug species can attract another stink bug species. Scientists can monitor BMSB by using MDT, the aggregation pheromone of a completely different stink bug species (the Asian brown-winged green bug, Plautia stali). However, because the pheromone is most attractive to BMSB in the fall, which is after most crop harvests have finished or already starting, so this is not very helpful for most growers1.
To produce better attractants for BMSB, researchers successfully identified the specific aggregation pheromone produced by BMSB adult males. The pheromone (commonly called murgantiol) is comprised of two different isomers, or molecular conformations, with a specific ratio. This ratio is important to know because there can be variation between species, and even between individual bugs, for what ratio is most attractive. Dr. Weber and colleagues determined which isomer ratio of murgantiol was most attractive separately to BMSB, and then combined this pheromone with MDT to see if a blend of the two was attractive to the bugs. The two pheromones together were much more attractive than either alone – with an added advantage of this blend being attractive to the bugs all season long, unlike MDT by itself. These pheromones also do not need to be extremely pure, which is good news for keeping the cost of lure production low.
Harlequin bug males produce murgantiol as well – so the same types of lures can be used to monitor both pest species, though the two species have different ideal isomer ratios5. Harlequin bugs, however, specialize in feeding on cruciferous plants – unlike BMSB, which are unspecific in their preferred hosts (polyphagous). Harlequin bugs were found by Dr. Weber to also be highly attracted to pheromones mixed with mustard oils, which are chemically derived from their host plants’ defensive compounds6,7. Adding these oils to murgantiol therefore could enhance trap performance for harlequin bugs, much like MDT for BMSB.
While Dr. Weber and his colleagues uncovered a lot of information about stink bug pheromones, there is still room to further our understanding. Now that researchers have evidence for which pheromone blends work most effectively for the two species, there is a lot of fine-tuning to be done about how exactly to use them in traps, and the most economical way to produce them. Dr. Weber and his colleagues also have plans in the works to investigate using pheromones for other pest-combating purposes, such as attracting insect predators of pest species like stink bugs, and genetically engineering plants to produce insect pheromones and act as trap crops. Overall pheromones offer an exciting approach to manage not only stink bugs, but many different pest insects, to better protect our agriculture.
To learn more about Dr. Weber’s research or contact him, visit his USDA homepage. For more information on BMSB or Harlequin bugs, you can visit the University of Maryland Extension pages on BMSB and Harlequin bugs.
Authors: Elizabeth Brandt, Aditi Dubey, Morgan Thompson
However, asymptomatic bees are common and can have either low or high virus loads (de Miranda et al. 2012). The story is complicated by the fact that DWV is very closely related to a series of other RNA viruses, such as Varroa destructor virus-1 (VDV-1). And the story gets even more complicated… because of recombination.
Using next generation gene sequencing, Dr. Ryabov (currently a visiting scientist at the United States Department of Agriculture (USDA) Bee Research Lab in Beltsville, MD) and his colleagues at the University of Warwick, UK, decided to characterize the virus diversity in honey bees. They found that the genome of DWV-like viruses could be divided into three functional parts, or “modules”, any of which were sometimes crossed-over between DWV and VDV. They identified three distinct types of genomes: the 100% DWV genome, and two types of recombinants formed by the association of “modules” from DWV and VDV, which they named VDV-1DVD and VDV-1VVD (Moore et al. 2011).
So what was thought to be a single population of viruses is actually a group of variants (VDV-1DVD, VDV-1VVD and DWV). Dr. Ryabov then compared the levels of each variant in honey bee pupae and associated Varroa mites. They found that individual honey bees were usually infected by a mixture of the three variants. Of the three viruses, the recombinant VDV-1DVD’s levels in honey bee pupae was highly associated with its level in associated mites. This suggests this recombinant is more efficiently transmitted between the mite and the honey bee.
When Varroa acts as a vector for the DWV, it increases the levels of viruses in contaminated colonies, causes deformities in the affected workers (Fig. 2), and overall results in increased risk of mortality of the whole colony. But what remains to be determined is whether this is caused by 1) Varroa amplifying and introducing more virulent strains of the virus and/or by 2) Varroa suppressing the honey bee immune response.
To test those two hypotheses, Dr. Ryabov exposed Varroa-naïve honey bees (collected from a Varroa-free region in Scotland) to DWV either orally (in brood food) or through Varroa mite feeding. They monitored the change in DWV diversity and loads within the host as well as changes in honey bee expressed genes to identify potential antivirus immune responses (Ryabov et al. 2014). They detected changes in expression for a number of genes associated with the immune response of honey bees while in presence of the mites. This suggests that the second hypothesis should be further explored.
This study also showed that in Varroa-free colonies (controls), honey bees had highly diverse DWV, though at low levels. When the bees were infected orally, DWV levels remained low, but the composition of the DWV strains changed compared to the controls. When bees were infected through Varroa, two outcomes would happen. Honey bees either showed low levels of diverse DWV strains, or they developed high levels of a single specific variant of DWV or very closely related variants, even though the infecting mite contained a high diversity of strains. By inoculating honey bees through injections (which simulates Varroa feeding), the researchers observed high levels of replication for the recombinant strains containing VDV-1 derived structural gene block. This suggests that these particular strains have an advantage due to the route of transmission. All of this largely supports the first hypothesis that Varroa amplifies more virulent strains of the virus.
In conclusion, this example of the shift in virulence of the DWV – from a benign and asymptomatic virus to a serious disease – illustrates the importance of the process of recombination in the generation of various strains of viruses, and how the addition of a vector, and a new route of transmission, can increase the impact of a virus by altering the relative composition of its strains.
Meghan McConnell is a Master’s student in Dennis vanEngelsdorp’s Lab. She is currently studying honey bees, with a focus on non-chemical control of Varroa mites.
Nathalie Steinhauer is a PhD candidate working in Dennis vanEngelsdorp’s Lab on honey bee health and management practices. Her projects aims to identify and quantify the effects of risk factors associated with increased colony mortality.
Bowen-Walker, P. L., S. J. Martin, & A. Gunn. 1999. The Transmission of Deformed Wing Virus between Honeybees (Apis mellifera L.) by the Ectoparasitic Mite Varroa jacobsoni Oud. Journal of invertebrate pathology 73(1), 101-106.
Highfield, A. C., A., El Nagar, L. C. Mackinde, M. L. N. Laure, M. J. Hall, S. J. Martin, & D. C. Schroeder. 2009. Deformed wing virus implicated in overwintering honeybee colony losses. Applied and environmental microbiology 75(22), 7212-7220.
de Miranda, J. R., L. Gauthier, M. Ribiere, and Y. P. Chen. 2012. Honey bee viruses and their effect on bee and colony health. In D. Sammataro & J. Yoder (Eds.) Honey bee colony health: challenges and sustainable solutions. CRC Press. Boca Raton. 71-102.
Moore, J; Jironkin, A; Chandler, D; Burroughs, N; Evans, DJ; Ryabov, EV (2011) Recombinants between Deformed wing virus and Varroa destructor virus-1 may prevail in Varroa destructor-infested honeybee colonies. Journal of General Virology, 92(1): 156–161. DOI:10.1099/vir.0.025965-0
Ryabov, EV; Wood, GR; Fannon, JM; Moore, JD; Bull, JC; Chandler, D; Mead, A; Burroughs, N; Evans, DJ (2014) A Virulent Strain of Deformed Wing Virus (DWV) of Honeybees (Apis mellifera) Prevails after Varroa destructor-Mediated, or In Vitro, Transmission. PLoS Pathogens, 10(6): 1–21. DOI:10.1371/journal.ppat.1004230
Aliens are invading the forests of the United States! Not the green, bug-eyed aliens from outer space; no we are talking about the, well… green, bug-eyed aliens from Earth. With the globalization of trade, insect introductions leading to invasive pest problems have steadily increased over the last few centuries, causing massive economic and environmental devastation in the systems where these pests permeate. These invaders are especially difficult to manage when they are pests of our native North American forest trees due to the large spatial scale associated with them, making pesticide applications impractical.
having a warmer climate than Connecticut, created an asynchronous relationship between the host (EHS) and the parasitoid (E. citrina) in Connecticut. This means that the scale and parasitoid are developing at different times of year, preventing the wasp from being able to effectively attack the scale in its introduced range. With the colder climate of Connecticut, it was hypothesized that the EHS scales developed more slowly. Wasps, as a result, would have fewer suitable 2nd instar hosts to parasitize. Dr. Abell tested this by observing scale abundance and parasitism by E. citrina at three distinct latitudes in the U.S. (Connecticut [“coldest”], Pennsylvania, North Carolina [“warmest”]), hypothesizing that he would find more parasitoid-host synchrony as he moved further south where warmer temperatures would allow for multiple generations of scales.
Ultimately, Dr. Abell did not observe any increase in synchrony between EHS and E. citrina at any of his three field sites. Instead he found continuous reproduction of EHS, and all life stages were present throughout the year. This led Dr. Abell to Japan to better understand how EHS behaves in its native range. While surveying hemlock scales and their associated parasitoids, Dr. Abell found 11 new species attacking EHS in Japan, some of which may have potential as classical biological control agents.
a wasp that is less than 1mm in length that attacks EAB eggs. Research done by Duan et al. in 2013 indicated that T. planipennisi was effectively established in Michigan and is a strong disperser. However, they observed that there was no parasitism of EAB in larger trees. In a study done by Dr. Abell, it was determined that the bark thickness was preventing this small wasp from attacking the EAB larvae. The ovipositor (egg-laying mechanism) of T. planipennisi is too short to reach the EAB larvae underneath the thick bark.
The bark was also placed in emergence chambers to collect any parasitoid wasps that emerged from the bark remnants that were missed in earlier screening. After two years of testing these methods, Dr. Abell concluded that the bark-sifting method was a more effective way to measure the rate of O. agrili egg parasitism in the field because significantly more parasitoids were recovered with this method. Invasive insects continue to attack our forests today, therefore it is very important to continue to understand and utilize biological control methods to preserve our forests. Dr. Abell continues his work on EAB biological control in the Shrewsbury lab here at the University of Maryland where he is evaluating other introduced and native parasitoids and additionally an integrated approach that combines pesticides with classical biological control methods.
About the Authors:
Olivia Bernauer is a second year Master’s student in Dennis vanEngelsdorp’s bee lab working to better understand the floral preferences of Maryland’s wild, native pollinators.
Jackie Hoban is a second year Master’s student working on emerald ash borer biological control in Paula Shrewsbury’s lab.
What makes a species a species? What are the characteristics that make it unique enough to be distinguished from similar creatures? What are the genetic underpinnings that allow species to evolve to create a distinct entity?
These are the type of questions Dr. Carlos Machado is trying to answer. Answering them are key to understanding our planet's biodiversity. A traditional definition of speciation centers around the concept of reproductive isolation. While members of two different species may mate, if they either produce no offspring or sterile offspring, these two individuals represent different species. This is known as the biological species concept. However, what factors lead to reproductive isolation? One way is through physical separation of populations (e.g., a mountain range) or allopatric speciation. On the other hand, what about sympatric speciation, or speciation in overlapping geographical distributions?
Dr. Machado’s work brings us a step closer to understanding the roots of speciation. Following from this work, his future research ideas center on correcting inversions using the latest genetic techniques, such as CRISPR (Clustered regularly interspaced short palindromic repeats), to study in further detail these regions that are responsible for species’ differences.
Jonathan Wang is a PhD student in Raymond St. Leger’s lab studying host-pathogen interactions.
Jen Jones is a PhD student in Bill Lamp’s lab, studying how socioeconomic factors influence the distribution of mosquito populations.
As the world’s population continues to grow, a key issue facing society is how to balance feeding the world while protecting the environment. Dr. Kate Tully has worked on this problem in both East Africa and the United States, where the solution will require different approaches, in part due to the different use of crops. In developed nations, many grain crops are grown for animal feed and fuel compared to developing nations where most are grown for human consumption. As such, different goals and approaches need to be taken for a sustainable future.
While crop yield has increased in many parts of world since the 1960s, this trend has not occurred in sub-Saharan Africa. Dr. Tully is working in Kenya and Tanzania to determine what changes can improve yield without excessive fertilizer use. Africa as a whole plans to increase nitrogen-based fertilizer use, but there is an upper limit to the amount plants can use before the rest is lost to the environment.
Large amounts of fertilizer nitrate were leached through the sandy soils to groundwater. Dr. Tully concluded that, in addition to using better overall management practices such as irrigation and increasing organic matter content, site characteristics should determine the optimal nitrogen fertilizer amount; in these cases, farmers in Kenya can increase yield by adding 75-100 kg N/ha while those in Tanzania should only use 50 kg N/ha. These levels can increase yield while protecting the environment from excess nitrogen leaching into waterways and gassing off into the air.
Back in the U.S., Dr. Tully turned her attention to soil nutrient dynamics on the Eastern Shore, Maryland. The dominant agricultural crops feed—corn and soybeans support their primary industry—poultry. Over the years, phosphorus from fertilizers and chicken litter has accumulated in the soil to the point of saturation. Phosphorus typically binds to soil particles with minimal nutrient leaching to the surrounding environment. Yet, as human-accelerated sea-level rise causes saltwater intrusion of coastal farmland, soil dynamics are changing. Saltwater can significantly alter soil chemistry, which may result in the release of bound soil nutrients as runoff. While nitrogen and phosphorus are essential nutrients for living organisms, too much can throw entire ecosystems off balance. Excess phosphorus in particular has led to algal blooms in fresh water estuaries. Over time algal blooms deplete life supporting oxygen resulting in ‘dead zones.’ But to what magnitude, rate, and extent are saltwater intrusions increasing the movement of phosphorus?
Read more of Dr. Tully’s work!
Tully K, Hickman J, McKenna M, Palm CA, Neill C. 2016. Effects of increased fertilizer application on inorganic soil N in East African maize systems: vertical distributions and temporal dynamics. Ecological Applications 26: 1907-1919. doi:10.1890/15-1518.1
Becca Eckert is a Ph.D. student in Bill Lamp’s lab. Her research examines how changes in light and nutrient availability affect macroinvertebrate growth and diversity in heterotrophic headwater streams as mediated by changes in leaf-associated algal quantity and quality.
Lisa Kuder is a Ph.D. student in Dennis vanEngelsdorp’s Bee Lab. Her research focuses on road ecology, specifically improving highway rights-of-way for pollinators.
Dr. César Nufio has examined a vast number of grasshoppers to understand the impacts of climate change on insects in the Rocky Mountains of Colorado. To be precise, the number of specimens he has captured and processed over the last 10 years has exceeded 180,000 from grassland communities found along a high plains to subalpine gradient. Nufio’s National Science Foundation funded research combines extensive field surveys with comparisons of museum collections, weather data, and laboratory and field experiments. The entire project started with his discovery of a collection of 25,000 pinned and label grasshopper specimens and three data notebooks at the University of Colorado’s Natural History Museum. These pinned specimens and notebooks, which are part of the Gordon Alexander Collection, were part of several field studies conducted over 50 years ago.
In 2006, Nufio began resurveying the same sites that Alexander had nearly half a century ago. Nufio wanted to understand how climate had changed along the gradient and how this change might impact the timing of grasshopper life history events (when they hatched and how fast they developed), their elevational ranges and demography (population size, longevity, reproductive rates), and body sizes. All of these questions could be addressed comparing his recent findings to the observations made by Alexander.
In addition to examining phenology, Nufio more recently was interested in the demographic changes among species over the elevational gradient. He observed that species with short wings showed a reduction in body length with no change in reproductive output with increasing elevation. Conversely, grasshoppers with long wings showed no change in body size but a reduction in reproductive output. Nufio also examined changes in weight, longevity, and reproduction over time in response to temperature using a caged field experiment. During a warm year, he found that females tended to be heavier, live longer, and laid more eggs (more on demography: Article). While these subalpine grasshoppers appear to be benefiting from warming, his surveys suggest that those at the bottom of the mountain may be negatively affected by warm years.
Please join us for as many as you can! Lunch will be provided for all attendees.
What drives species apart: how does evolution turn one population into two populations that can no longer mate? This process is called speciation, and we have a lot to learn when it comes to this subject. Dr. Catherine Linnen, from the University of Kentucky, is using pine sawflies (genus Neodiprion) as a model to better understand the origins of biodiversity, from its proposed start as genetic mutations to eventual speciation.
Pine sawflies have particularly diverse characteristics (or phenotypes) in terms of their body shape, larval coloration, social behavior, and host preference, though most use some type of pine trees. Since some of these species are pests, much their biology has been well studied. Speciation is easier to study in organisms showing divergent behaviors, preferences, and morphologies because these are all characteristics that evolution is proposed to act upon in the process of speciation. These qualities, in addition to the ease of their capture and lab-rearing, make them perfect for studies on speciation.
How are differences in phenotype, such as host selection, driving populations apart into distinct species? What specific mutations are responsible for variations in phenotypes? Dr. Linnen proposed a phenotypic approach, which can be used to identify underlying genetic variation, using Neodiprion populations to help answer these questions
1. Can host preference separate populations into distinct species?
In theory, this could happen in three steps: first, the colonization of a novel host; second, divergent selection on different traits; third, a reduction in gene flow leading to speciation. With a sizeable amount of biological data, Dr. Linnen was able to show that host use was the best predictor to model the speciation seen in Neodiprion species. Dr. Linnen showed an interesting example of this from her work on the saw-shaped egg-laying apparatus of adult sawflies, which is where these organisms get their common name. Different sawfly species have saws and eggs that fit well into the needles of specific host plants. In this case, she was investigating saw and egg differences of sawflies on pine trees with thick or thin needles. Since individuals with a thick saw would have lower fitness on host plants with thin needles, and since thin-sawed sawflies would ill-equipped to exploit hosts with thick needles, this trait could drive speciation.
When looking at mitochondrial genes for proof of introgression (exchange of genes between distinct populations as a result of mating) between species, she showed that host difference seems to act as a barrier for introgression, as well.
Image shows variation in saw size and shape, egg size, and the typical spacing of eggs laid on different pine tree needles by Neodiprion lecontei and Neodiprion pinetum. These differences allow for these two species to specialize on different species of plants leading to potential spatial isolation even within the same geographic range.
2. Is there evidence of the same type of processes at the population level?
If yes, two individuals from the same species, but with different hosts should show higher genetic variation than a pair from the same host. The observed data supported this trend. After accounting for genetic variation due to geographical clusters, Dr. Linnen showed that divergent selection of hosts could drive differentiation between populations of a single Neodiprion species, potentially leading to speciation.
3. But how? Is host use reducing inter-breeding or is there a cost associated with inter-breeding in terms of fitness?
Neodiprion females show preference when egg-laying: each prefer to lay eggs on their typical host. This behavior alone explains some spatial isolation resulting in reduction in gene flow. In addition, when females and males are placed together to test for mating preference, they prefer to mate with their conspecific (an individual of the same species), even in the absence of any host. This behavior indicates some level of sexual isolation, which would also reduce gene flow.
However, in the lab, some individuals still chose a non-conspecific mate from the other host, resulting in hybrids. Hybrids were also collected in the field. Those observations, plus the results showing some level of mitochondrial introgression suggests that sexual isolation between Neodiprion species is incomplete.
Host preference is associated with both survival and fecundity. Each species survives and reproduces better on its typical host. This amounts to a fitness cost for hybrid phenotypes resulting in underperformance compared to specialists on either host. Egg mortality was strongly linked to the matching of oviposition traits to the hosts. In particular, traits associated with thick-needles resulted in high mortality rates when eggs were laid on pines with thinner needles.
What can sawflies tell us about speciation? Dr. Linnen’s work suggests that pine needle width is generating divergent selection pressure on host traits within and between populations of sawflies resulting in fitness costs for hybrid individuals and eventually driving speciation. By just considering how they lay eggs, sawflies can tell us a great deal. There is still more to learn as we explore the diversity of sawfly traits.
Linnen, C.R. and B.D. Farrell. 2010. A test of the sympatric host race formation hypothesis in Neodiprion
(Hymenoptera:Diprionidae). Proceedings of the Royal Society of London B. 277: 3131-3138.
Bagley, R.K., V.C. Sousa, M.L. Niemiller, and C.R. Linnen. In Revision. History, geography, and host use shape genome-wide patterns of genetic differentiation in the redheaded pine sawfly (Neodiprion lecontei).
Bendall, E.E., Vertacnik, K., and C.R. Linnen. In Prep. Selection on oviposition traits generates reproductive isolation between two pine sawfly species.
About the Authors:
Nathalie Steinhauer is a PhD candidate working in Dennis vanEngelsdorp’s lab on honey bee health and management practices. Her projects aims to identify and quantify the effects of risk factors associated with increased colony mortality.
Samuel Ramsey is a doctoral student in Dennis vanEngelsdorp’s lab. His work focuses on honey bee parasites specifically targeting ectoparasite Varroa destructor and endoparasite Nosema ceranae. His project seeks to establish a deeper understanding of key aspects of both parasites’ biology with the aim of optimizing current IPM practices in respect to these organisms.
Since its accidental introduction into the Allentown, Pennsylvania area in the late 1990’s, the brown marmorated stink bug (BMSB), Halyomorpha halys, has established itself as a severe pest across much of the eastern United States. Much to farmers’ dismay, most conventional management techniques, including insecticides, do not effectively control the insect. This fact spurred the development of a working group in 2012 to research potential management strategies for the brown marmorated stink bug. The work of Chris Taylor investigating the importance of BMSB symbionts was part of this research. Taylor’s research focused on the relationship between the brown marmorated stink bug and a species of bacteria (Candidatus Pantoea carbekii) that resides in the insect’s gut, as well as on the surfaces of BMSB egg masses. He conducted a series of laboratory experiments looking at symbiont acquisition, nymphal survival, and fecundity under a variety of conditions.
Are brown marmorated stinkbugs reliant on the bacteria smeared on egg mass surfaces?
Previous research has shown varying levels of dependence on symbionts by different stink bug species, who obtain these symbionts be feeding on bacteria smeared on the surface of the egg mass. This resulted in the need to first confirm the level of symbiont reliance by the brown marmorated stink bug. To do this, aposymbiotic insects (those without symbionts) were generated by surface sterilizing egg masses using bleach and subsequently rearing these insects through all life stages. Survival,
development time, and egg mass production was tracked and compared to stink bugs allowed to obtain their symbiont. Results
indicated that BMSB is heavily reliant on its symbiont, showing significantly reduced survival, increased number of days to
developmental peak, and reduced fecundity in aposymbiotic insects.
Does environmental stress impact symbiont transmission or survival after transmission?
Previous research has shown that some insects lose their symbionts under higher temperature conditions. In the case of BMSB, the effects of heat on the symbionts present both on the egg mass surface, and within the gut of the insect, had never previously been studied. To determine the effects of temperature on egg mass surface symbionts, egg masses were exposed to three fluctuating temperature conditions meant to mimic different mid-Atlantic summer conditions. The temperatures were classified as ‘normal’, ‘above average’ and ‘extreme’. The egg masses were exposed to one of these three temperatures, and returned to
optimal rearing conditions as soon the eggs hatched. Hatch rate was calculated for each egg mass, and the ‘extreme’ condition was found to significantly reduce hatch rates and was therefore discarded. Hatch rate, survival, and percent inoculation were calculated for the ‘normal’ and ‘above average’ conditions, and no effect of temperature on the symbiont was detected.
Next, Chris wanted to look at symbiont survival within the gut under heat stress. He settled for comparisons as 25 versus 30 degrees Celsius (mimicking previous studies) due to an inability to use the fluctuating conditions chambers for extended periods of time. Looking at the effects of these two temperatures on symbionts within the gut, a significant decrease in survival over time was seen in insects exposed to the higher temperature that were allowed to obtain their symbionts at birth. Additionally, decreases in fecundity were seen in the bugs at the higher temperature, as well as a drastic reduction in the number of bugs that still had their symbionts as adults.
Results from these experiments indicated that the symbionts present on the surface of the egg mass are not likely affected by heat stress, but that heat stress over time has the potential to kill the symbiont within the gut of the bug.
Can antimicrobial products remove the symbiont from the egg mass, and can this be used as a possible management strategy?
The final experiments investigating the relationship between the brown marmorated stink bug and its’ gut symbionts addressed the potential for exploiting this relationship as a possible avenue of BMSB control. Using commercially available products with antimicrobial properties, Chris treated the symbionts on the egg mass surface like any other field crop disease, and tested to determine if any of these products had the potential to sterilize the egg mass surfaces. A total of six products were tested, and nymphs from each treatment were screened for the presence of gut symbionts. Of these six products, three were found to significantly reduce the number of BMSB able to obtain their symbiont from the egg mass surface.
The findings of Chris Taylor’s research contribute much to the understanding of the complicated relationship between this insect and its symbionts. Future research is needed to determine sterilization efficacy of these products in a field setting, and to tease apart the different abiotic conditions that may limit BMSB’s host range.
Taylor CM, Coffey PL, DeLay BD, Dively GP (2014) The Importance of Gut Symbionts in the Development of the Brown Marmorated Stink Bug, Halyomorpha halys (Stål). PLoS ONE 9(3): e90312. doi:10.1371/journal.pone.0090312
About the Author:
Veronica Johnson is a Master’s student whose research focuses on determining the role of different post-harvest litter management practices in the degradation of Cry proteins in genetically modified Bt corn. She is also looking to determine the effects of winter temperature and precipitation on both tissue decomposition and protein degradation under these various litter management practices.
In the mountains of California there are 9 species of millipede in the genus Motyxia, that do something no other millipede on earth can do. Every night they emerge from underground burrows and begin to glow with a constant soft blue-green light, and until recently no one knew why. While emitting light makes Motyxia unique among the millipedes it is hardly novel in the animal kingdom, and animals do it for a variety of purposes. Anglerfish in the deep sea glow to attract prey, but Motyxia are vegans. Fireflies flash with light to attract mates, but Motyxia, like all members of the order Polydesmida, are completely blind, so they aren’t searching the night for a glowing partner. While working at the University of Arizona Dr. Paul Marek set out to solve this mystery.
Bioluminescent millipedes are exclusively found along the foothills of the Sierra Nevada, Tehachapi, and Santa Monica mountain ranges in Southern California. They live in majestic live oak and giant sequoia thickets, where they feast on decaying plant material and are sadly eaten in turn by rodents, centipedes, and phengodid beetles. These gentle and slow moving creatures appear to lack any defense, and glowing seems like the opposite of camouflage. It seems like they’re asking to be eaten! It turns out that glowing isn’t the only shocking chemical reaction taking place in these tiny legged bodies, they’re packed full of the deadly poison cyanide.
Marek hypothesized that glow of these millipedes warns potential nocturnal predators that they will be an unpleasant meal, just as the bright colors of a wasp or a poison dart frog warns predators in the daytime to stay away! Scientists call the presence of warning signals like these aposematism.
To test his theory of the function of bioluminescence, Marek created millipedes out of clay, painted some of them with glowing paint, and placed them in Motyxia habitats. After a night the clay millipedes were collected and checked for signs of predation. Rodents avoided attacking the glowing clay millipedes, indicating that rodents know to avoid the green glow and it’s accompanying mouthful of cyanide.
Furthermore, at higher elevations the rodent populations are greater and more diverse, which means there is a much higher risk of being devoured by a predator. Over time the millipedes living at higher elevations have coped with this disadvantage by evolving brighter bioluminescence than those living at low elevations. This brighter and stronger warning keeps hungry rodents at bay!
How they produce their light remains a mystery...
Video courtesy of Marek Lab
Marek, P.E., D.R. Papaj, J. Yeager, S. Molina, W. Moore. (2011). Bioluminescent aposematism in millipedes, Current Biology, 21, R680-R681.
Pieribone, V., & Gruber, D. F. (2005). Aglow in the dark: The revolutionary science of biofluorescence. Cambridge, MA: Belknap Press of Harvard University Press.
About the Authors:
Gussie Maccracken is a PhD. student in the Mitter Lab at the University of Maryland and the Labandeira Lab at the Smithsonian Institution National Museum of Natural History. She studies plant-insect interactions in the Late Cretaceous fossil record of North America.
Peter Coffey is a Master's student whose research focuses on using cover crops to optimize sustainable farming economics. His current projects using lima beans and eggplant as model systems focus on plant nutrition, weed suppression, influences on pest and beneficial insects, and crop yield value. Follow him on twitter at @petercoffey.
Follow Paul Marek on twitter at @apheloria.
Increased Temperature Aids Pests in Overtaking Cities
It has been observed for over one hundred years that plant insect pests such as scales, white flies, and thrips are more prevalent in urban areas than in forests. Many scientists have sought to understand this phenomenon with hypotheses on enemy release and abiotic influences. Enemy release relies upon the absence of natural enemies such as predators and parasitoids of pest insects in urban areas. In the resulting absence of enemies, populations of pests are allowed to build to extraordinarily high numbers. Abiotic influences, on the other hand, are aspects of the physical environment such as temperature that lead to increased population sizes. Dr. Steve Frank of North Carolina State University argues that the main driver in the high occurrence of pests in urban areas is not natural enemies but increasing temperatures in cities. Dr. Frank and his lab work with scale insects in Raleigh, North Carolina to elucidate the importance of temperature on pests, how pests impact tree health, and predicting effects of global warming.
Does temperature affect the insect pests?
Gloomy Scale, Melanaspis tenebricosa, is an excellent model for studying the importance of temperature on pest insects as it has a long history of localized high abundance in Raleigh. The earliest report on gloomy scale by Zeno Metcalf in 1912 heralded this insect as being the most important shade tree pest in North Carolina. Since that report, it has remained in high numbers in Raleigh but not in neighboring forest areas. Looking at heat maps of the area, Dr. Frank’s lab surveyed red maple trees within a gradient of temperatures in Raleigh and observed that gloomy scale abundance increased with temperature (Dale and Frank. 2014b). The question of why they were seeing this trend spurred additional investigation.
(Left) Gloomy scales covering the tree’s branch making the bark appear rough and bumpy. Photo by Adam Dale. (Right) Zoomed in on the branch, you can see that each bump is an individual scale with its brown armor blending into the branch. The red dot in the middle is scale insect with its armor removed revealing the soft insect underneath. Photo by Matt Bertone
The lab found that body size as well as egg production increased with temperature (Dale and Frank. 2014b). With increased egg production there is a corresponding increase in population growth, which helps the gloomy scale reach high densities. In short, because temperatures are higher in cities, gloomy scale is able to respond with increased body size, reproduction, and growth rate.
Does temperature and pest abundance affect tree health?
Dr. Frank’s lab wanted to investigate the effects of a possible interaction between warming and pests on tree health. To test this, they assessed the condition of all the red maple trees studied previously and then expanded the study to analyze 8462 trees inventoried by the city of Raleigh (Dale and Frank 2014a). Results showed that trees in hot locations, where scale abundance was therefore higher, were more than twice as likely to be in poor condition than trees in cold locations. The lab measured the water potential of the trees to help explain the physiological link between temperature and tree condition. Water potential in trees is linked to the ability to move water from the roots to the shoots. Dr. Frank’s lab found that water potential decreased with increasing site temperature, meaning that the trees were under higher drought stress in hotter areas (Dale and Frank 2014a). Linking temperature effects to both pest insects and tree health gives a holistic picture of the health and functioning of trees in urban landscapes.
Can we predict the effects of global warming?
As urban areas are hot patches in the broad landscape, perhaps they can be used as proxies to look at the future of climate change. To tackle this idea, Dr. Frank’s lab used historical samples of red maple from herbarium collections, dating as far back as 1895. These samples were collected from rural areas around the southeast, and scales remain visible on them. The researchers found that the change in gloomy scale abundance with temperature variation was congruent across rural historical and modern urban samples (Youngsteadt et al. 2014). They also resampled trees at some of the historical sites and found that in most cases, the rise in temperature over the last several decades has led to an increase in gloomy scale abundance (Youngsteadt et al. 2014). Thus, it seems plausible that the relationships and patterns between scale insects and temperatures in cities could be used as an informative tool to predict potential outcomes of global climate change.
What can we do today to help manage effects of temperature in cities?
Urban areas around the world are becoming larger and hotter. As Dr. Frank’s lab has shown, the increase in temperature leads to more pest outbreaks, and these pest outbreaks have negative consequences for ecosystem services such as carbon sequestration and air purification. As temperature increases, cities will need to consider how to manage the effects on urban trees. For example, pest outbreaks may start occurring earlier in the season (Meineke et al. 2014), so scouting for pests should be increased. Cities should also try to plant more tree species and varieties that are resistant to the effects of increasing temperature on pest outbreaks and water stress. This may mean breeding or genetically modifying new varieties for cultivation.
In a broader sense, the decrease in urban forest canopy cover will have many negative effects both on wildlife and on humans. Studying urban trees will help identify problems, and potentially elucidate future effects of climate change. These effects threaten not only urban trees but also natural areas; therefore, understanding the impact of temperature on urban trees can help predict and prepare for the future of climate change across temperature gradients in landscapes.
Dale A., S. Frank. The Effect of Urban Warming on Herbivore Abundance and Street Tree Condition. Plos One. 2014a.
Dale A., S. Frank. Urban warming trumps natural enemy regulation of herbivorous pests. Ecological Applications. 2014b.
Meineke E., R. Dunn, S. Frank. Early pest development and loos of biological control are associated with urban warming. Biology Letters. 2014.
Youngsteadt E., A. Dale, A. Terando, R. Dunn, and S. Frank. Do cities simulate climate change? A comparison of herbivore response to urban and global warming. Global Change Biology. 2014.
About the Authors:
Jessica Grant is a master’s student currently working on kudzu bug (Megacopta cribraria) pest management. For more information on her work see this site mdkudzubug.org
Aditi Dubey is a Ph.D. student looking at the effects of neonicotinoid seed treatment in a three-year crop rotation.
Stalk-eyed flies reveal more than meets the eye - investigating sexual selection, speciation & genomic conflict
Stalk-eyed flies are another one of nature’s curious and fascinating examples of biodiversity and an extreme result of sexual selective forces. These insects are true flies (Diptera) in the family Diospidae, typically distinguished by their unique eye stalks, from which they get their name. Dr. Gerald Wilkinson of the biology department at the University of Maryland has been conducting research on how genetic mechanisms may influence the outcome of evolution, specifically sexual selection, speciation and genomic conflict in stalk-eyed flies.
All diospid flies have eye stalks, although some males display extremely long and seemingly disadvantageous stalks. When a species is sexually dimorphic, there is a physical difference between females and males, such as the difference in plumage between peacocks. In this case, the eye stalks are dimorphic with males having the longer eye stalks, and females having short eye stalks. While not all species of stalk-eyed flies are sexually dimorphic, sexual dimorphism has evolved independently several times in this genus. Based on Dr. Wilkinson’s research, the development of these intriguing eye stalks appears to be a product of sexual selection. The female flies favor mating with males with larger relative eye stalks to body size, pushing for directional selection for longer and longer eye stalks over evolutionary time. To test some predictions, Dr. Wilkinson artificially selected for both smaller eye stalk populations and larger eye stalk populations in the lab using Teleopsis dalmanni. Intriguingly, he found that after 20 generations, males from short eye stalk lines were producing only daughters and some males from longer eye stalk lines were producing predominantly sons. Dr. Wilkinson had stumbled onto X drive, an example of genomic conflict. Genomic conflict arises when certain genes selfishly try to push themselves into the next generation even at the expense of the host/population. In the populations of short stalk eyed flies, X drive influences spermatid elongation. Those that elongate properly contain the selfish driving element on the X chromosome (X) which in turn leads to a female biased population and spread of XD. Unchecked, this would lead to the eventual demise of the population.
The discovery of X drive in T. dalmanni through artificial selection on male eye span, is a convincing catalyst for sexual selection. Because females that mate with XD carriers can lead to population extinction, there is pressure for females to avoid mating with these short stalk-eyed males. A preference for males with long eye-stalks can develop because 1) the population survives and 2) long eye-stalks are a good advertisement for the lack of XD. Because of this, Dr. Wilkinson predicted and found that eye span genes are X-linked and are in linkage disequilibrium with the driver element (i.e. short eye stalks genes stay close together with the driver element on XD). Sexual selection should eliminate drive except if recombination or imprecise female choice occasionally occurs.
Having unraveled some of the secrets to mating in stalk-eyed flies, Dr. Wilkinson turned his curiosity to another aspect. He was interested in identifying mechanisms for reproductive isolation. Which mechanism is more important, prezygotic isolation (e.g. differences in mating, sperm transfer, sperm survival, or fertilization) or postzygotic isolation (e.g. hybrid inviability or hybrid sterility) for the creation of new species? Using very similar species of southeast Asian stalk-eyed flies (cryptic species), Dr. Wilkinson and his lab determined that reproductive isolation increases with genetic distance and that those populations evolve postzygotic isolation rapidly, but prezygotic isolation relatively slowly over evolutionary time. In particular, hybrid males between closely related species were sterile. Dr. Wilkinson hypothesized this may be due to cryptic drive. This is where drive is hidden because another genetic element (a suppressor) is counteracting its effects. If the suppressor is removed the drive is suddenly active. Through breeding experiments he ultimately found the presence of both cryptic X and Y drive. Their suppressors were found on the non-sex chromosomes and were in the same genomic regions as factors that caused male hybrid sterility.
So while stalk-eyed flies are a natural wonder to behold, it appears they have more to offer than meets the eye. From them we have learned that genomic conflict can influence sexually selected traits and that reproductive isolation can evolve rapidly due to post-mating sexual selection and X drive. Much of this work is ongoing and there is more to learn about genomic conflict and drive system using diospids as model organisms.
Baker, R.H., Narechania, A., DeSalle, R., Johns, P.M., Reinhardt, J.A. and Wilkinson, G.S.
2015. Spermatogenesis drives rapid gene creation and masculinization of the X chromosome in stalk-eyed flies (Diopsidae). Genome Biology and Evolution (in review)
Baker, R.H. and Wilkinson, G.S. 2010. Comparative genomic hybridization (CGH) reveals a
neo-X chromosome and biased gene movement in stalk-eyed flies (genus Teleopsis).
PLoS Genetics 6(9): e1001121
Christianson, S.J., Brand, C.L., and Wilkinson, G.S. 2011. Reduced polymorphism
associated with X chromosome meiotic drive in the stalk-eyed fly, Teleopsis dalmanni.
PLoS ONE 6:e27254.
Christianson, S.J., Swallow, J.G., and Wilkinson, G.S. 2005. Rapid evolution of postzygotic
reproductive isolation in stalk-eyed flies. Evolution 59(4):849-857
Reinhardt, J.A., Brand, C.L., Paczolt, K.A., Johns, P.M., Baker, R.H. and Wilkinson, G.S.
2014. Meiotic drive impacts expression and evolution of X-linked genes in stalk-eyed
flies. PLoS Genetics 10(5):e1004362
Rose, E.G., Brand, C.L., and Wilkinson, G.S. 2014. Rapid evolution of asymmetric
reproductive incompatibilities in stalk-eyed flies. Evolution 68:384-396
Wilkinson, G.S., Johns, P.M., Metheny, J.D., and Baker, R.H. 2013. Sex-biased gene
expression during head development in a sexually dimorphic stalk-eyed fly. PLoS ONE
Wilkinson, G.S., Christianson, S.J., Brand, C.L., Shell, W. and Ru, G. 2014. Haldane's rule is
linked to extraordinary sex ratios and sperm length in stalk-eyed fles. Genetics
Wilkinson, G. S., Presgraves, D.C. and Crymes, L. 1998. Male eye span in stalk-eyed flies
indicates genetic quality by meiotic drive suppresion. Nature 391:276-279
gAbout the authors:
Jackie Hoban is a first year Master's student in Paula Shrewsbury’s lab, researching biological control of the invasive, wood-boring pest, emerald ash borer, Agrilus planipennis (Coleoptera: Buprestidae).
Jonathan Wang is a PhD student in Raymond St. Leger’s lab. He is studying Drosophila immunity and fungal pathogenesis.
Mengyao Chen is a Master’s student in Dr. Leslie Pick’s Lab. Her research focuses on segmentation genes in Brown Marmorated Stink Bug (BMSB, Halyomorpha halys). Her current work is looking for orthologs of pair-rule genes in BMSB, and studying their expression and functions using RNA in situ hybridization and RNAi.
Evolution Occurring Before Our Very Eyes: Parasitoids Diverging Based on Host’s Food Source
Parasitic wasps, or parasitoids, lay their eggs inside other insects or animals, and their larvae feed on that insect or animal once they hatch. Many parasitoids lay their eggs in the larvae of other insects, for example caterpillars. Caterpillars often have adapted to feed on specific plants, called “host plants,” that may have chemical defensive properties (allelochemicals) that the caterpillars are able to cope with and sometimes incorporate in to their own chemical defenses. When parasitoids use these caterpillars as hosts, they too must adapt to the host plant’s toxins. These phenomena create a very interesting relationship between host plant, host insect, and parasitoid. The plant tries to defend against the host using toxins, the caterpillar incorporates and becomes immune to those toxins, and finally the parasitoids must adapt to the toxins present in the caterpillar host to guarantee its larvae can survive inside the host and eventually use it as food. Dr. Karen Kester received her PhD from the University of Maryland under the guidance of Dr. Pedro Barbosa researching the ability of parasitic wasps to adapt to the particular food plants and plant toxins of their insect hosts
Parasitic wasps are a diverse group containing over 750,000 species, but the mechanisms of the group’s diversification are relatively unknown. Parasitic wasps are associated with a number of life history traits that can promote rapid adaptation to new hosts with specialized diets. These traits may have led to their incredible diversity. To test her hypotheses, Dr. Kester chose to focus on Cotesia congregata, a larval parasitoid of sphingid moths, including Manduca sexta caterpillars. These caterpillars have the ability to incorporate plant toxins into their own defense, for example the nicotine found in tobacco. Using field sites in multiple farms, and caterpillars feeding on either tomato or tobacco plants, she found that wasps localized to tomato-feeding caterpillars did poorly when made to parasitize tobacco-feeding caterpillars. On the other hand, wasps localized to tobacco fields had adapted to the nicotine toxins in the plants and the caterpillars and were able to cope with the toxicity. Dr. Kester determined that parasitoids can become genetically adapted to parasitize hosts found on locally abundant food plants (Barbosa et al, 1991; Kester & Barbosa, 1994).
Dr. Kester went on to become a professor of Biology at the Virginia Commonwealth University where she has furthered her research on tritrophic interactions, first exploring Cotesia wasp adaptations to host – food plant complexes using both Manduca sexta and Ceratomia catalpae, important pests of tobacco and catalpa trees, respectively. Tobacco plants and catalpa trees differ in the types of chemicals they contain, and it was of interest to determine if these chemicals had any effect on the parasitoids of the two caterpillars. Dr. Kester determined that the wasps associated with tobacco-feeding caterpillars and catalpa-feeding caterpillars were genetically different, with a 2% sequence divergence in their mtDNA (Jensen et al, 2002; Kester et al, 2015). In fact, the genetic data revealed that the Cotesia wasps specializing on the two caterpillar types comprised two separate lineages with limited gene flow.
This discovery raised additional questions regarding whether the two types of Cotesia could be considered different species, and led to another series of experiments. The primary question was if the differences were due to factors occurring before fertilization (prezygotic) or factors occurring after fertilization (postzygotic). Some prezygotic factors measured were the male response to female pheromones and male acoustic signals. Differential adaptations to host and food plants, as well as hybrid inviability are examples of some postzygotic factors that were explored. Hybrid inviability would occur if the two types Cotesia mated and had offspring that died before reaching adulthood. This would indicate that the two Cotesia wasps were so different that they could not successfully reproduce; this is called “reproductive isolation” and is an important part of the evolution of separate species. Results showed that fewer males responded to the pheromone from the “other” wasps compared to their own females. For example, wasps found to parasitize caterpillars feeding on tobacco were less likely to respond to pheromones of those parasitizing catalpa-feeding caterpillars (Bredlau, 2011; Bredlau & Kester, 2015). Dr. Kester also showed that the courtship songs between the two wasps also differed in duration and frequency, although no significant differences were detected in mating success (Bredlau et al, 2013).
Although there were no differences in the success of different wasps mating, first-generation sibling crosses between progenies of two different wasps were discovered to be inviable. Dr. Kester’s current work investigates the underlying mechanisms causing this inviability. Recent discoveries have led her lab to believe the inviability in the first generation cross is related to the absence of polydnaviruses in the inviable offspring. Polydnaviruses are obligate symbiotic viruses that have coevolved with a subfamily of a group of Braconid parasitic wasps, including Dr. Kester’s target species Cotesia congregata (Albrecht et al, 1994; Savary et al, 1999). Ongoing research by Dr. Kester is exploring the evolutionary role of polydnaviruses as an underlying mechanism of speciation in wasps relative to changes in their host immune response. For instance, these viruses can lead to lateral gene transfer between wasp and host species, which may play an important role in the evolution of nicotine tolerance in the wasps associated with tobacco-feeding caterpillars. With the addition of viruses into this complex system, Dr. Kester now considers these species interactions to be “multi-trophic”.
Dr. Kester’s final message was that, “once you know your system well, the questions never end.” She intends to further research differences in acoustic signals between different parasitic wasps, as well as look at other host – food plant complexes of Cotesia congregata. Dr. Kester’s work reminds us that evolution is occurring all around us and that new species are diverging right before our very eyes.
Albrecht, U., Wyler, T., Pfister-Wilhelm, R., Gruber, A., Stettler, P., Heiniger, P., ... & Lanzrein, B. (1994). Polydnavirus of the parasitic wasp Chelonus inanitus (Braconidae): characterization, genome organization and time point of replication. The Journal of general virology, 75, 3353-3363.
Barbosa, P., Gross, P., & Kemper, J. (1991). Influence of plant allelochemicals on the tobacco hornworm and its parasitoid, Cotesia congregata. Ecology, 1567-1575.
Bredlau, J. (2011). Investigation of pre-and post-zygotic reproductive barriers between two host-plant complex races of the parasitic wasp Cotesia congregata (Say)[Hymenoptera: Braconidae].
Bredlau, J. P., Mohajer, Y. J., Cameron, T. M., Kester, K. M., & Fine, M. L. (2013). Characterization and generation of male courtship song in Cotesia congregata (Hymenoptera: Braconidae). PloS one, 8(4), e62051.
Bredlau, J. P., & Kester, K. M. (2015). Pre-and Postzygotic Barriers to Reproduction Between Two Host-Foodplant Complex Sources of the Parasitic Wasp, Cotesia congregata (Hymenoptera: Braconidae). Annals of the Entomological Society of America, sav089.
Jensen, K. M., Kester, K. M., Kankare, M., Brown, B. L. (2002) Characterization of microsatellite loci in the parasitoid, Cotesia congregata (Say) (Hymenoptera Braconidae). Molecular Ecology Resources. 2(3): 346-348.
Kester, K. M., & Barbosa, P. (1994). Behavioral responses to host foodplants of two populations of the insect parasitoid Cotesia congregata (Say). Oecologia, 99(1-2), 151-157.
Kester, K. M., Eldeib, G. M., & Brown, B. L. (2015). Genetic Differentiation of Two Host–Foodplant Complex Sources of Cotesia congregata (Hymenoptera: Braconidae). Annals of the Entomological Society of America, sav088.
Savary, S., Drezen, J. M., Tan, F., Beckage, N. E., & Periquet, G. (1999). The excision of polydnavirus sequences from the genome of the wasp Cotesia congregata (Braconidae, Microgastrinae) is developmentally regulated but not strictly restricted to the ovaries in the adult. Insect molecular biology, 8(3), 319-327.
About the Authors:
Grace Anderson is a Master’s student in the Sultz lab at UMD and works on the evolution of Harvestmen. She is currently exploring the morphological differences between closely related species that vary greatly in mating behavior.
Veronica Johnson is a Master's student in the Hooks lab working with genetically modified corn. She is currently investigating the effects of different post-harvest practices on the persistence of toxic Bt proteins in senesced corn debris.
What impact do pesticides have on Honey bees?
Honey bee health is impacted by a variety of factors (such as pests, disease, and nutritional stress) and a multitude of interactions between these factors, making the solution to our nation’s bee health problem a difficult one to find. However, when the topic of honey bee health comes up in the media, the focus is almost always on pesticides. While pesticides are most certainly an issue for honey bees, it is important to realize that real-world exposure levels must be considered when looking at how pesticides can affect bee health. Dr. Reed Johnson of Ohio State University has done just this. As the colloquium guest of the UMD Entomology graduate students, Dr. Johnson discussed results of studies performed by his team on real-world agricultural field pesticide exposures for honey bee colonies.
Pesticides need an EPA Risk Assessment in order to be registered and authorized for use. The risk to bees is judged through a calculated Risk Quotient. The criteria of acceptable risks have also become stricter: it is not enough anymore for a pesticide to just not kill bees, to register a product, applicants need to prove that they do not harm any pollination services, biodiversity and hive products. The new regulations made overt and massive bee kills rarer, but they still present some limitations. For one, the Risk Quotients are based on testing done on individual adult bees. But honey bees do not live as individual bees; they are part of a superorganism: the colony. Testing the impact of pesticides on whole colonies is a challenging task. One colony can contain up to 50,000 individuals in a variety of reproductive castes and life stages.
With this in mind, Dr. Johnson chose to conduct an in-hive based study in an attempt to solve a problem faced by a select group of California beekeepers (Johnson & Percel 2013). Almonds are a major pollinator crop in the US. With over 800,000 acres of almonds in the Central Valley of California, and about 2 honey bee colonies required per acre, almonds pollination contracts are a major source of revenue for US beekeepers. Additionally, some beekeepers in this region of California also specialize in queen rearing. Each year over 1 million queens are produced in the region. A few years back, those queen producers started complaining that about 80% of their queens were dying after the almond bloom. Was a product used during the bloom responsible? Several pesticides, labelled “bee safe” were indeed applied while flowers were in bloom: fungicides, herbicides and a few insecticides, none of them acutely toxic to bees according to standard testing. One of them in particular, received a lot of blame from beekeepers: the fungicide Pristine.
In 2012, Dr. Johnson performed a full hive study to test the effect of Pristine on queen development using field realistic levels of exposure (Johnson & Percel 2013). The colonies were given frames of pollen contaminated with different concentration of Pristine. For comparison, the experiment also provided a negative control (non-contaminated pollen, which should yield no effect on development) and a positive control (contaminated-pollen with a chemical known to impact developing bees: the Insect Growth Regulator Diflubenzeron). The positive control is an essential part of a good scientific study: if no effect can be seen in the positive control, the conclusion should be that the something in the method of application was deficient rather than that the product had no effect. Each colony in the study was used to rear queens and the survival of those queens was followed throughout their development, from egg until emergence. The results showed that the method was appropriate: the positive control was associated with a high level of kill in the queens, as expected. But the different concentration of Pristine did not seem to have an effect on queen survival: the levels were similar to the negative control. So, if Pristine was not killing the queens in CA almonds, what was?
The solution to this question was found in the CA Pesticide Database. California is the only US state in which every pesticide application has to be recorded in an official database. This mine of information provides a comprehensive documentation of all pesticide applications since the 1990’s. The database revealed that a certain insecticide was regularly sprayed during bloom on almonds, to control for the Peach Twig Borer. The name of this insecticide: Diflubenzuron. Does that ring a bell? This was the Insect Growth Regulator that was used as a positive control in Johnson’s study.
Dr. Johnson performed another study, with the same design, to test different concentration of Diflubenzuron and found a clear dose-response relation: at high concentrations, the product would kill all queens; at moderate concentrations, the product had a marked effect. Interestingly, that product used to be labelled “bee safe”, because when tested on workers, the product did not seem to have such an effect.
Pesticide Risk Assessments should represent complete assessments of risk and the examples provided by Dr. Johnson pointed to the limitations of the current standard methods required by regulations. By focusing on individual worker adults, and ignoring the impact on the other castes (such as the queens), other life stages, and whole colony impacts, current risk quotients are under-evaluating the real risk of pesticides to beneficial insects, such as honey bees. Another limitation of standard methods is how they focus on single pesticide exposure. We know that bees are never exposed to only one product at a time: 96% of pesticide applications are performed as “tank mixes” where several products are mixed together.
Despite the confusion surrounding Colony Collapse Disorder, honey bee population trends, and mortalities, one thing can be certain: beekeepers in the US are losing an unacceptable number of colonies every year. So far, beekeepers have been able to keep up with their losses by splitting colonies, but at great cost, both in labor and financially. Beekeeping is harder today than it was in the past. As a result, managed bees have become more valuable, and the level of loss that a beekeeper would deem acceptable reduced. While not the only problem for honey bees, pesticide exposure is certainly a noteworthy one. Click here to see more of Dr. Johnson’s work on the effects of pesticides on honey bees (Johnson et al., 2010).
Johnson, R. M., Ellis, M. D., Mullin, C. A., & Frazier, M. (2010). Pesticides and honey bee toxicity - USA. Apidologie, 41(3), 312-331. doi:10.1051/apido/2010018
Johnson, R. M., & Percel, E. G. (2013). Effect of a Fungicide and Spray Adjuvant on Queen-Rearing Success in Honey Bees (Hymenoptera: Apidae). Journal of Economic Entomology, 106(5), 1952-1957. doi:10.1603/ec13199
About the Authors:
Nathalie Steinhauer is a PhD student working in Dennis vanEngelsdorp’s Lab. She links the risk factors associated with beekeeping management to colony mortality.
Andrew Garavito is a Master’s student in Dennis vanEngelsdorp’s Lab. He is studying honey bees; with a focus on the diversity of pollen types brought in by foragers, and the health effects of different pollen diets.
Meghan McConnell is a Master’s student in Dennis vanEngelsdorp’s Lab. She is studying mechanical methods of control for the honey bee parasite Varroa destructor.