Originally posted on May 15, 2015
Early efforts to eradicate emerald ash borer (EAB) from North America in 2002 eventually ended because there were no available tools for detecting or limiting its spread and the high number of new infestations being found each year suggested that EAB was already well established in North America. Thus, biological control came to the forefront as a potential long-term strategy for combating EAB.
Early surveys for EAB natural enemies in China during 2003 identified two parasitoid wasps: Tetrastichus sp. (Hymenoptera: Eulophidae) and Spathius sp. (Hym: Braconidae), later identified to species as the larval parasitoids, Tetrastichus planipennisi Yang and Spathius agrili Yang, respectively. Research conducted showed that T. planipennisi is an endoparasitoid, laying its eggs inside EAB larvae, whereas S. agrili is an ectoparasitoid, laying its eggs on the outside of EAB larvae. Both species of wasps are known to be gregarious (i.e. producing many offspring from just a single EAB host) and capable of parasitizing up to 50% of EAB larvae in the field (Liu et al. 2003). A solitary egg parasitoid, Oobius agrili Zhang and Huang (Hym: Encyrtidae), was also found attacking EAB in China, but unlike the former parasitoids, it produces only one offspring per EAB egg. Surprisingly, O. agrili has been shown to parasitize up to 61.5% of EAB in China, its home country (Liu et al. 2007).
Figure 1. A 4th-instar emerald ash borer (EAB) larva on a cut, debarked ash tree. EAB larvae feed in serpentine galleries in the phloem and cambium, usually scoring the outer xylem; this disrupts the flow of nutrients and water, leading to ash mortality. Photo by: Justin M. Gaudon
All three Chinese parasitoid wasps were first released in North America in 2007 following tests on their host ranges and rearing potential. They continue to be released in both the USA and Canada and their establishment monitored. Currently, North American parasitism of EAB by these wasps is lower than that observed in China, likely due to their recent introduction.
Similar surveys for native natural enemies were also conducted in North America during the early years, and several species have been found to attack and parasitize EAB. Although low rates of parasitism of EAB by native parasitoids are typical, high levels have been recorded, usually associated with older, more established EAB populations. Two native species that appear to be important mortality factors of EAB include the solitary larval ectoparasitoid, Atanycolus cappaerti Marsh & Strazanac (Hym: Braconidae) and the solitary larval endoparasitoid, Phasgonophora sulcata Westwood (Hym: Chalcididae). Cappaert and McCullough (2009) reported up to 71% parasitism of EAB by A. cappaerti, while Lyons (2010) calculated a parasitism rate of 40.7% for P. sulcata based on sticky trap catches. Both show strong potential to reduce populations and slow the spread of EAB, and this has inspired me to further investigate their ability as biocontrol agents against EAB.
Figure 2. Native Atanycolus sp. released into a Toronto field site by transporting ash logs cut from sites heavily infested by emerald ash borer (EAB) and native North American parasitoids. Photo by: Justin M. Gaudon
Part of my PhD research focuses on augmentative biological control of EAB, where the idea is to transport parasitoid-infested ash logs to new sites where they have the potential to increase native parasitoid populations in recently EAB-infested sites and where these native parasitoids have not yet been observed. This approach of introducing and augmenting native natural enemies in new areas may be a cost-effective and environmentally-sound approach to combating alien invasive species, such as EAB, and provide a more sustainable approach to management than traditional insecticide applications or introductions of non-native species in classical biological control.
Preliminary results of my work suggest that native parasitoid populations can be increased one year after transporting parasitoid-infested material and that augmented parasitoid populations can add to EAB-mortality in newly-colonized sites. If these initial observations remain and EAB are suppressed, then their populations will spread more slowly and overall ash tree mortality could be reduced. This, in turn, would allow for not only continued survival and reproduction of ash trees, but also the discovery of potential natural EAB resistance in ash populations. In addition, this will provide municipalities and other landowners with more time to prepare, budget, and take action against EAB by planting new tree species and creating a more diverse urban forest with increased resilience against the next invasive species to arrive.
EAB will likely be a "pain in our ash" for quite a bit longer, but I am certain that parasitoid wasps are one piece of the puzzle that may help us eventually slow the spread of EAB and “save our ash”.
Faculty of Forestry, University of Toronto