|Van Driesche, Roy|
Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/23/2010
Publication Date: 10/1/2010
Citation: Duan, J.J., Ulyshen, M.D., Bauer, L.S., Gould, J., Van Driesche, R. 2010. Measuring the impact of biotic factors on populations of immature emerald ash borers (Coleoptera: Buprestidae). Environmental Entomology. 39:1513-1522. Interpretive Summary: The invasive emerald ash borer (EAB), Agrilus planipennis Fairmaire, has killed tens of millions of ash (Fraxinus spp.) trees in both managed and natural forests throughout the Midwestern and northeastern United States since its discovery in 2002 in Michigan and Ontario. In this study, EAB larvae were experimentally established on healthy ash trees in two wooded plots at each of three sites near Lansing, Michigan using two different methods: (1) laboratory-reared eggs were placed on the trunks of selected trees and (2) EAB females were caged on tree trunks and allowed to lay eggs. Two natural enemies (parasitic wasps) of EAB native to China (Spathius agrili and Tetrastichus planipennisi) were then released in one study plot at each of the three sites. The experimentally-established EAB, along with all naturally occurring EAB in the lower 2.5 m trunk section of each tree were observed in the spring and fall of the year following parasitoid release to determine survival and mortality from the following factors: host tree defense, disease, predation, and parasitism by either introduced or native parasitoids. Host tree defense was the most important mortality factor for both experimentally-established and naturally-occurring EAB in both spring and fall sampling, whereas woodpecker predation was the second most important factor. Mortality from microbial disease was low in both the spring and fall sample periods. There were no significant differences in mortality caused by host tree defense, woodpecker predation and microbial disease between the parasitoid release and non-release plots. While there were no statistically significant differences in mortality rates due to parasitism between parasitoid-release and control plots, one released parasitic wasp (T. planipennisi) was detected in each of the three release sites by the end of the study, but was not detected in the experimental cohorts or associated wild larvae in any of the three control plots. The relevance of these findings to EAB population dynamics and the potential for the introduce parasitoids to bring the pest under control in North America are discussed.
Technical Abstract: Cohorts of emerald ash borer (EAB) larvae, Agrilus planipennis Fairmaire, were experimentally established in July of 2008 on healthy green ash (Fraxinus pennsylvanica) trees in two wooded plots at each of three sites near Lansing, Michigan by caging gravid EAB females or placing laboratory-reared eggs on the trunks of selected trees. One plot at each site was randomly chosen for release of two introduced larval parasitoids, Tetrastichus planipennisi Yang and Spathius agrili Yang, whereas the other served as the control. The stage-specific mortality factors and rates were measured for all experimentally established cohorts and for associated wild (i.e., naturally occurring) EAB immature stages present in the defined sample zone of the cohort-bearing ash trees, which were destructively sampled, half in the spring and half in the fall of 2009. Host tree defense was the most important mortality factor for both the experimental cohorts and the wild EAB larvae in both spring and fall sampling. The mean mortality +/- SE (per tree) for host tree defense was 32.0% +/- 8.7 to 41.1% +/- 9.6 for 1st to 3rd instars (the oldest life stage in cohorts) in the experimental cohorts and 17.5% +/- 10.1 to 21.5% +/- 12.1 in naturally occurring EAB larvae, prepupae and pupae by spring 2009. In the fall 2009 sampling, mean mortality (+/- SE) caused by host tree defense was 16.1% +/- 8.8 to 29% +/- 12.7 for the remaining portion of the July 2008 EAB cohorts, and 9.9% +/- 8.9 to 11.8% +/- 3.9 for the naturally occurring wild immature EAB stages. Woodpecker predation was the second most important factor, inflicting no mortality in the experimental cohorts, but causing 5.0% +/- 3.1 to 5.6 % +/- 3.0 mortality to associated wild EAB stages by spring 2009 and 9.2% +/- 4.4 to 12.8 +/- 8.9 and 3.2% +/- 2.0 to 17.7% +/- 8.6, respectively, for experimental cohorts and associated wild EAB life stages by fall 2009. Mortality from disease of unknown causes (in both the experimental and wild cohorts) was low (<3%), in both the spring and fall sample periods. In the fall 2009 samples, approximately 1.5% +/- 1.5 of experimental cohorts and 0.8% +/- 0.5 of the associated wild EAB stages were parasitized by Tetrastichus planipennisi. While there were no statistically significant differences in mortality rates due to parasitism between parasitoid-release and control plots, T. planipennisi was detected in each of the three release sites by the end of the study, but was not detected in the experimental cohorts or associated wild larvae in any of the three control plots. The relevance of these findings to EAB population dynamics and the potential for S. agrili and T. planipennisi to bring the pest under control in North America are discussed.