Since emerald ash borer was first detected in Michigan in 2002, the non-native invasive beetle has killed tens of millions of ash trees across the U.S., and continues to infest new regions.
Within its native range in Asia, emerald ash borer is attacked by a variety of predators including several species of parasitoid wasps that specialize on the beetle’s eggs or larvae. Because these wasps are expected to play a role in maintaining low emerald ash borer populations in Asia, three species have been introduced into North America as biocontrol agents. “There is great interest in knowing how effective these introductions have been in reducing the population growth rates of emerald ash borer in North America,” says U.S. Forest Service Southern Research Station (SRS) scientist Michael Ulyshen.
Ulyshen is a research entomologist at the SRS Insects, Diseases, and Invasive Plants unit, and one of the co-authors of a new study on population dynamics of emerald ash borers and the species that prey on it. The study was led by Jian Duan, a scientist at the Agricultural Research Service, and was published in the Journal of Applied Ecology. Other co-authors include Leah Bauer, a Forest Service research entomologist who played a pivotal role in early releases of the Chinese parasitoids and Roy Van Driesche, professor of entomology at University of Massachusetts Amherst.
In addition to the introduced biocontrol agents, some native wasps have also been shown to parasitize emerald ash borer larvae, and birds – especially woodpeckers – eat the larvae and pupae. From 2007 to 2010, the researchers released thousands of non-native predatory wasps in experimental release plots in forests of southern Michigan.
Researchers studied emerald ash borer populations for seven years to identify the causes of larval death. “We observed significant declines in the densities of live emerald ash borer larvae in infested ash trees,” says Jian. The study indicated that parasitism – by both the introduced wasps and native wasps – helped control emerald ash borer populations, especially after their numbers naturally declined when fewer ash trees were available for them to feed on.
Although native parasitic wasps are generalists – they do not prefer emerald ash borer larvae over other wood boring insect larvae – they were important predators, especially during the early years of the study when there were large numbers of emerald ash borers present. As the study progressed, the non-native wasp Tetrastichus planipennisi became an increasingly important predator. “Our findings strongly suggest that native, generalist enemies help control emerald ash borer in the initial phase of its invasion,” says Jian. “This may gradually shift towards introduced parasitic insects that prefer and actively seek out emerald ash borer larvae.”
Both native enemies and introduced parasitic wasps play important roles in suppressing emerald ash borer populations. Non-native parasitic wasps can help prevent widespread ash tree death in newly infested forests, and the scientists recommend that they be released as soon as the presence of emerald ash borer has been detected. The non-native wasps also keep emerald ash borer populations low in forests that have already been invaded. “Emerald ash borer has many native and non-native natural enemies,” says Ulyshen. “Maintaining stable populations of these predators can help ash trees survive emerald ash borer infestations.”
For more information, email Michael Ulyshen at email@example.com