Dr. Jian Duan, a Research Entomologist at USDA, is working on sustainable ways to manage the invasive emerald ash borer (EAB) through introduction and establishment of natural enemies (stingless wasps) from the pest’s native range. This approach, also termed as classical biological control in the literature, can lead to permanent or sustainable reductions of pest populations. Dr. Duan explained that there are four stages in the invasion of new environment by a non-native species: (1) Transport (Introduction), (2) Establishment (or colonization), (3) Spread, and (4) Impact. There are two possible outcomes at each stage that can lead to either failure or success in progression.
The cryptic life cycle/high fecundity (ability to produce large number of offspring) of the EAB is striking (Fig. 3). Of note, the EAB’s life cycle is semi- (one generation of offspring every two years) or uni-voltine (one generation of offspring per year). More than 90% of EAB life cycle – all immature stages (eggs, larvae, pupae) – live and develop in the cambium tissue under the ash bark. It may take 1 or 2 years to complete a generation, depending on location, temperature, and/or host tree condition. A complex of natural enemies in its native range (Northeast Asia) are good at finding, locating, and attacking its immature stages. Some of the specialized natural enemies (stingless parasitic wasp) can thus be introduced to the invaded regions (U.S. and Canada) as biocontrol agents against EAB.
Dr. Duan is currently conducting long-term field research to address two important questions on the current EAB biocontrol program: (1) the degree to which the introduced natural enemies reduced the density of EAB pests and (2) whether biocontrol has contributed to ash tree survival/regrowth. Using the life-table analysis, Dr. Duan found that one of the introduced EAB parasitoids (Tetrastichus planipennisi) played a dominant role in reducing EAB population growth on small ash trees/saplings in Michigan between 2008 – 2015. The newly released Spathius galinae appears to be promising with strong early recovery and spread and effectiveness in protecting large ash trees. On a national level, the EAB will be likely to reach all areas with ash trees from California to Florida, with the West having special concerns over loss of rare ash species. Furthermore, there is a need for new parasitoid collections matched to the climates of the West and South of the USA since only one of the four parasitoids collected in Asia occur at latitudes lower than 40oN, suggesting that the parasitoids may not be successful in these regions of the USA.
Finally, Dr. Duan mentioned that a lot is left to be done. For instance, future studies should expand releases of Spathius galinae to Northeastern and Midwestern U.S. and assess its role in protecting large ash trees. Further, there is still a need for foreign exploration of EAB parasitoids in southern and western areas of the pest’s native range (China), and more work is needed to uncover the role of egg parasitoids (Oobius agrili and O. primorskyensis) in controlling EAB populations for protection of all sizes of ash trees.
- Duan, J.J., Bauer, L.S., Abell, K.J., Ulyshen, M.D. and Van Driesche, R.G. (2015), Population dynamics of an invasive forest insect and associated natural enemies in the aftermath of invasion: implications for biological control. J Appl Ecol, 52: 1246-1254. doi:10.1111/1365-2664.12485
- Duan, J.J., Bauer, L.S., Van Driesche, R.G. (2017), Emerald ash borer biocontrol in ash saplings: The potential for early stage recovery of North American ash trees. For. Ecol. Man., 394: 64-72. https://doi.org/10.1016/j.foreco.2017.03.024
Mike Nan PhD student in the St. Leger lab studying how circadian rhythms affect Metarhizium infection of Drosophila.