Submitted to: Suppressing Over-Abundant Invasive Plants and Insects in Natural Areas by Use of Their Specialized Natural Enemies
Publication Type: Book / Chapter
Publication Acceptance Date: 3/30/2017
Publication Date: 4/28/2017
Citation: Duan, J.J. 2017. Can ash communities and their dependent species be partially protected through biological control of emerald ash borer? In Van Driexche, Roy G. and Reardon, Richard C. Suppressing Over-Abundant Invasive Plants and Insects in Natural Areas by Use of Their Specialized Natural Enemies. United States Department of Agriculture. pp. 41-47.
Technical Abstract: Ash trees were once relatively free of serious, major diseases and insect pests in North America until the arrival of the emerald ash borer (EAB), Agrilus planipennis Fairmaire, which was first detected in North America in Michigan in 2002 and has been detected in 32 U.S. states and two Canadian provinces, killing millions of ash trees. The economic cost of the EAB invasion is estimated to be $1 billion per year for the next decade; however the ecological impact from the EAB invasion on North American forest ecosystems may be more severe and widespread. Therefore, taking no action against it is not a sensible option. Initial efforts to contain EAB damage focused on eradication of incipient populations from newly detected areas by destroying all infested trees within these areas, while at the same time restricting the movements of EAB-infested live trees or firewood in or out of the eradication area. However, the eradication effort was abandoned by 2009 because EAB populations in many infested area (such as Michigan, Ohio, and Maryland) were already too high and too widespread. Subsequently, efforts were shifted towards slowing the spread of EAB into new areas by regulatory restriction of movement of EAB-infested wood or plant materials, insecticide treatment of susceptible trees (either artificially girdled or naturally stressed) as trap trees, and biological control via introduction and releases of natural enemies collected from EAB’s native range. Because it is impractical to treat all susceptible trees with insecticides in natural forests, classical biological control via establishment of introduced (non-native) natural enemies is needed to protect ash resources against EAB in North America. The EAB biocontrol program, started nearly a decade ago via introduction and establishment of co-evolved natural enemies from the pest’s native range, appears to hold promise to protect North American ash communities. To date, this program has successfully established two Chinese natural enemies, the egg parasitoid Oobius agrili and the larval parasitoid Tetrastichus planipennisi, in most of the areas in the U.S. where these species have been released. While the role of O. agrili in reducing EAB population growth requires continued evaluation, the larval parasitoid T. planipennisi currently plays a significantly suppressive role in saplings and smaller trees (DBH <12 cm) in aftermath forests in Michigan. This suppressive effect is likely to spread geographically as populations of O. agrili and T. planipennis increase in density in more EAB-infested areas where they have been released, protecting saplings and small trees. To protect larger trees, however, establishment of more effective egg parasitoids and larval parasitoids with long ovipositors is needed. The most recent introduction of Spathius galinae from the Russian Far East, which has much longer ovipositor (4-6 mm) than T. planipennisi, is one such species. Initial recoveries in New York and Connecticut have been made at promising levels. As EAB densities decline, future studies will be required on (1) growth and survival rates of ash in aftermath forests with self-sustaining and increasing biocontrol agent populations, (2) persistence of key ash-dependent invertebrates, and (3) repetition of basic population work in new regions as EAB invades southern and western areas in the United States, which will be ecologically distinct from the north central and northeastern US areas on which this discussion is based.