2013 Annual Report
1a.Objectives (from AD-416):
The purpose of this agreement is to quantify the impact of introduced parasitoids on the survival of emerald ash borer (EAB), Agrilus planipennis Fairmaire, populations. The work will be done in the US, using one species of American ash (Fraxinus pennsylvanica). Evaluations will be based on life tables developed from marked cohorts of eggs and larvae at six field sites in natural forests where the parasitoids imported to the US to control EAB either have already been released or will be released in the first year of the study. Specific objectives will be:
1. Locate 3 control and 3 treatment sites. At the treatment sites, release the three EAB parasitoids imported from China (Oobius agrili, Tetrastichus planipennisi , Spathius agrili).
2. At all sites, assess the impact of parasitoids on cohorts of EAB eggs and larvae/pupae in green ash (F. pennsylvanica)
3. Use data from EAB cohort studies to develop life tables for EAB populations and compare stage specific survival rates and other life table parameters.
1b.Approach (from AD-416):
We will test whether parasitoids introduced from China will greatly reduce survival of emerald ash borer cohorts on green ash trees (F. pennsylvanica) in U.S. Work will be done in Michigan at three control and three treatment sites, the latter where three introduced EAB parasitoids either have been or will be released. Cohorts of EAB eggs and larvae/pupae will be created at each site, and used for construction of site-specific life tables for measuring the impact of released parasitoids.
Partial life-table analyses of data collected between 2009 and 2012 showed that both North American native and introduced parasitoids appeared to have begun significantly reducing the density of the emerald ash borer in Michigan. As of fall of 2012, approximately four years after field releases of the introduced parasitoid (Tetrastichus planipennisi) began, 92% of sampled emerald ash borer-infested ash trees (from six forest sites) contained at least one brood of this introduced biocontrol agent, indicating the widespread dispersal of this introduced parasitoid in Michigan following environmental releases. During that period, the mean number of T. planipennisi broods increased from about one brood per tree to three broods per tree, and emerald ash borer larval parasitism by T. planipennisi reached 21% in the release plots and 13% in the control plots.
Besides parasitism by the introduced parasitoid (T. planipennisi), emerald ash borer larvae also suffered heavy losses from other biotic factors including woodpeckers, putative plant resistance, diseases, and other larval parasitoids. Among the other mortality factors detected across our study sites in Michigan, woodpeckers were the most abundant during the entire study period and across different study sites, removing up to 57% of older immature emerald ash borer stages (4th instar to pupae) from feeding galleries and/or pupal chambers. Putative tree resistance killed up to 15% of younger larvae and unknown diseases caused similar levels of mortality of larger larvae throughout the study. Although less than 2% of emerald ash borer larvae across different study sites were parasitized by other parasitoids (primarily Atanycolus spp. and Phasgnophorus sulcata) before 2009, parasitism by these native parasitoids increased sharply to 18% in some of our plots by the following year. This increase in parasitism of emerald ash borer larvae by native parasitoids across our study sites was most likely the result of numerical responses of those native species to high emerald ash borer densities in our study sites.