There are a number of factors that keep insect populations under control including the availability of food and habitat resources, weather and climate, competition, and natural enemies (predators, parasites, parasitoids and pathogens). Natural enemies often have a major role in controlling insect populations but insects introduced from other regions may have few natural enemies in their new range.

Checkered beetle (predator) feeding on adult emerald ash borer. Image: David Cappaert, Michigan State University, Bugwood.org

Food and habitat resources for emerald ash borer (EAB) are plentiful in its current range in North America. There is an abundance of ash trees in both urban and rural environments and North American ash species have little ability to resist the beetle. As ash is killed and the seed bank is depleted, there will be less and less food and habitat for the beetle, so it will disperse to look for new resources. This means that within an EAB-infested area, the beetle population will decrease substantially when ash becomes rare.

About half of the natural ash range in North America is climatically suitable for the beetle, so it will continue to expand its range. The beetle is able to tolerate winters by producing an anti-freeze in its body that allows it to tolerate very cold temperatures. There is evidence that certain extreme winter temperature fluctuations could kill some EAB larvae, but this may have minimal overall effect. Research on overwinter mortality of the beetle in northern regions is being conducted.

Parasitic wasp laying eggs in emerald ash borer larva. Image: Houping Liu, Michigan State University, Bugwood.org

In Asia EAB has many natural enemies, both parasitoid wasps and predators, that kill EAB eggs, larvae and adults. However, currently there are fewer of these attacking the beetle in North America, and none in urban forests. Parasitoid populations may increase in response to high EAB populations if environmental conditions are suitable for them, however, there will be a delay before the natural enemy population increases enough to have an impact on the beetle.

Research to increase the role of parasitoids in controlling EAB is ongoing in Canada and the U.S. The focus of this research is identifying native parasitoids that can be reared in a lab and released to augment the current parasitoid population, as well as identifying, rearing and releasing Asian EAB parasitoids. A species of parasitoid from Asia was released in southwestern Ontario in 2013, and more releases are planned for 2014.

Downy woodpecker (predator) on a dead ash tree. Image: David Cappaert, Michigan State University, Bugwood.org

Woodpeckers are important predators of EAB. In North America hairy, downy and red-bellied woodpeckers are known to feed on EAB larvae.

See a more detailed version of EAB population factors.