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written by: Jillian Stewart There’s something smelly down in Texas. These odors are produced by plants under attack by insects. Plants react to their insect attackers by producing specific blends of odor compounds. These responses to pests, and how they differ between plants was the topic of Dr. Emily Russavage’s Doctoral thesis, which she presented at UMD recently. She tested the reaction of different cultivars of sorghum when the sorghum aphid -a major, destructive pest- arrived and started sucking their juices. What is sorghum and why should we care about it? Sorghum is commonly used for making human food (e.g., multigrain Cheerios), animal feed, and biofuel, making it the fifth most important cereal grain crop in the world. In 2012, sorghum produced $1.8 billion in revenue in the US alone. However, the sorghum aphid started causing damage in Texas and Louisiana in 2014, where it caused $31 million in damage in just one year! Insecticide applications are the major pest control technique used against sorghum aphids. But insecticides come with a lot of drawbacks, not least of which is the price of buying, applying, and reapplying them. Insecticides kill the pest at first, but their power weakens as the pest evolves resistance. You end up needing more and more insecticide to control the same pest. Wouldn’t it be nice if the plant could resist the pest without constant human intervention? Such plant defenses exist in the form of host plant resistance (HPR). People have been selectively breeding crops for pest resistance for years. Today, resistant plants are part of a system called integrated pest management (IPM) that aims to de-emphasize pesticide use and highlight other, more sustainable ways to control pests. One of the key principles of IPM is an idea called an action threshold. An action threshold is when you hit a certain number of pests per plant that has been scientifically determined to be the point at which the pests cause economic harm. Only when pests reach that threshold do growers need to intervene with pesticides or other control measures. Thresholds help prevent unnecessary and wasteful pesticide use which can lead to resistance in pests and harm beneficial insects like honey bees. There are two kinds of HPR. One is direct, in which plants affect an herbivore’s physiology or behavior. Examples would include trichomes or toxins. Direct defenses are commonly bred for when people are developing resistant cultivars of crops. Think of corn: it makes multiple toxins to fend off the corn earworm. The other type of defense is indirect. An indirect defense relies on natural enemies, like lady beetles and wasps, to come eat the herbivores (Image 1). A major example of indirect defense would be herbivore-induced plant volatiles (HIPVs). HIPVs can serve many purposes, ranging from a warning to neighboring plants (Ninkovic et al, 2021) to a simple byproduct of a change in the plant’s metabolism. Some HIPVs can help predators and parasitoids locate their prey. Sorghum has been bred for multiple direct defenses, but no one had studied its potential for indirect defenses.  Figure 1. Three methods by which a plant uses indirect defenses to attract natural enemies of a pest. Extrafloral nectaries are sources of nectar not in the flowers. Insects like ants will set up shop on a plant with these nectaries and protect the plant from other insects that want to eat it. Domatia are places of shelter (dorms) that friendly insects can live in. HIPVs, herbivory-induced plant volatiles, are chemical compounds that a plant produces when something starts eating it. (Russavage et al, 2024). In Texas, Dr. Emily Russavage did her Ph.D. thesis in Dr. Micky Eubanks’ lab on HPR in sorghum. Emily was interested in identifying sorghum cultivars that attracted natural enemies and determining what HIPVs may play a role in this behavior. She collected volatiles using a technique called dynamic headspace sampling and identified and quantified compounds using gas chromatography-mass spectrometry. To see how insects behave in response to plant odors, Dr. Russavage set up a two choice experiment (Figure 2).  Figure 2. Dr. Russavage’s experimental setup to determine which cultivars of sorghum produced HIPVs to attract a parasitoid wasp known to attack aphids. (Russavage et al, 2024). This test allowed her to determine which plants produce HIPVs that attract natural enemies of aphids. If a parasitoid chose the aphid-infested plant, this indicated that it did produce attractive HIPVs. This could be abundantly useful to apply in pest management: we can breed plants that attract more beneficial insects to kill pests!
Of all the cultivars Dr. Russavage tested, only two attracted the parasitoid wasp. She identified a volatile called methyl salicylate, a known insect attractor, as a key compound that those two cultivars produced more of when aphids arrived. Methyl salicylate does not act alone either: work by earlier scientists had found that a mixture of volatiles was more attractive to natural enemies than any single chemical (Kaplan, 2012). In the future, the whole volatile profile of sorghum could be analyzed for its role in attracting natural enemies, signalling danger, and otherwise protecting itself. Farmers might even be able to do a sniff check (via app) to find out what pests their crops have! Dr. Russavage is interested in discovering the genetics underlying indirect defenses. She conducted a two-year field experiment in Texas using these sorghum cultivars to determine if the ‘attractive’ cultivar could recruit higher densities of natural enemies. She also asked if these cultivars, by attracting more insect ‘killers’, would have lower densities of aphids, thereby preventing damage to plants. Dr. Russavage tracked natural enemy and aphid densities throughout the growing season to answer these questions, and intended to determine the effect of indirect defenses on crop yield. However, wild pigs and a rogue cow ate some of her sorghum plots, foiling her plans to answer the latter. Sounds like she needed some direct defenses! Dr. Russavage is currently working in Dr. Shrewsbury’s lab at the University of Maryland where they study plant and insect communities. Her current project is focused on biological control of the spotted lanternfly using entomopathogens. Dr. Russavage presented on her spotted lanternfly work in November, at the Entomological Society of America’s annual meeting in Arizona. Here is a link to Dr. Russavage’s full paper if you want to give it a read! https://link.springer.com/article/10.1007/s10886-024-01493-y Cited: https://www.nass.usda.gov/Publications/Highlights/2015/Sorghum_Farming.pdf https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3493419/ Kaplan I (2012) Attracting carnivorous arthropods with plant volatiles: the future of biocontrol or playing with fire? Biol Control 60:77– 89. https://doi.org/10.1016/j.biocontrol.2011.10.017 Russavage, E.M., Hewlett, J.A., Grunseich, J.M. et al. Aphid-Induced Volatiles and Subsequent Attraction of Natural Enemies Varies among Sorghum Cultivars. J Chem Ecol 50, 262–275 (2024). https://doi.org/10.1007/s10886-024-01493-y Ninkovic V, Markovic D, Rensing M. Plant volatiles as cues and signals in plant communication. Plant Cell Environ. 2021; 44: 1030–1043. https://doi.org/10.1111/pce.13910 Comments are closed.
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