Alien Invaders

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.

Dr. Kris Abell, one of the UMD Department of Entomology’s newest post-doctoral associates, has investigated biological control efforts to combat two invasive forest pests, the elongate hemlock scale and the emerald ash borer. Biological control methods for controlling a pest involve using predators and parasites from the pest’s native range to create a natural population balance in its introduced range.

During his time as a PhD student at the University of Massachusetts, Amherst, Dr. Abell followed up Dr. Mark McClure’s work on biological control of Elongate Hemlock Scale (EHS), Fiorinia externa Ferris. EHS is an invasive insect pest from Japan, which attacks hemlock trees. Feeding by EHS scales damages the hemlock’s needles, turning them from green to yellow.

Elongate hemlock scale on hemlock needles. The oblong brown insects are the adult female scales, the smaller round yellow insects are 1st instar scales. Photo credit: Kris Abell

Encarsia citrina adult wasp measuring less than 1 mm in length. Photo credit: Kris Abell

The best way to understand a pest is to observe it in its native range, so with that, McClure went to Japan to study EHS. McClure’s research found that one species of wasp, Encarsia citrina, is a parasitoid, an insect that develops in and eventually kills its host, of EHS in both Japan and in its new range in North America.  Wasps lay eggs in the 2nd instar of EHS, and wasp larvae hatch and feed on the scale, which kills the scale insect and produces more E. citrina. McClure’s research indicated that EHS is less abundant and has much higher parasitism rates by E. citrina in Japan than the EHS at his study sites in Connecticut. He hypothesized that differences in climate between the two locations, Japan

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. 

After Dr. Abell finished his work on EHS, he moved to Michigan State University where he studied another forest pest, the emerald ash borer (EAB), Agrilus planipennis Fairmaire, an invasive beetle species introduced to the United States from Asia. The adult females lay their eggs in bark of ash trees (Fraxinus spp.) and the larvae burrow under the bark where they feed and develop. Feeding damage results in girdling and ultimately the killing of the trees. There are a few parasitoid wasps that are known to attack EAB at different stages of its life cycle in Asia. Two of these parasitoids that have been introduced to the U.S. are Tetrastichus planipennisi, a wasp that attacks EAB in its larval stage, and Oobius agrili

Emerald ash borer adult: David Cappaert, Michigan State University, Bugwood

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.

Under the bark of this ash tree we see the aftermath of developing EAB larvae: Serpentine galleries of EAB larvae that have fed on this tree. All of this damage results in girdling. Photo credit: Kris Abell

Once biological control agents like these wasps are released, it is important to continue monitoring them.  Dr. Abell helped determine the best methods to monitor the establishment and range of the tiny egg parasitoid, O. agrili.  Logs infested with sentinel EAB eggs were set out in the field to detect O. agrili. This method, while effective, did not accurately represent the parasitism, taking place on wild trees. Dr. Abell tested two other methods; visual searching and bark sifting. Visual searching is a labor-intensive method by which observers flake off pieces of ash bark to reveal EAB eggs. Once parasitized by O. agrili the eggs turn black, therefore any black eggs were brought back to the lab and were further analyzed to confirm parasitism by O. agrili. Bark sifting entailed shaving the outer bark off of the ash trees and sifting out the smallest pieces in the lab to look for parasitized EAB eggs.

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.