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Why is emerald ash borer a problem here in North America?

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.

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

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[1,23]. 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.

Climate models suggest that about half of the natural ash range in North America is climatically suitable for the beetle, so, it is expected to continue to expand in range through much of North America[24]. The beetle is able to tolerate winters by producing an anti-freeze in its body that allows it to tolerate very cold temperatures[25]. There is evidence that certain extreme winter temperature fluctuations could kill some EAB larvae, but this may have minimal overall effect[26]. Research on overwinter mortality of the beetle in northern regions is being conducted.

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

In Asia EAB has many natural enemies, both parasitoid wasps and predators, that kill EAB eggs, larvae and adults, but there are fewer of these attacking the beetle in North America. In China the effect of parasitoids is highly variable but ranges up to 90% beetle mortality[27]. There are parasitoids in North America that that attack EAB, some are native parasitoids of other closely related beetles species and others are introduced[27]. However, the occurrence of these parasitoids is patchy at present and the parasitism rate is often quite low (e.g. <2%)[27]. Currently in urban forests natural enemies are not causing EAB mortality or limiting EAB populations[53]. 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 beetle populations.

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 are planned for 2014.

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

Woodpeckers and other insects (e.g. ants) are important predators of EAB but their impact varies from tree to tree. Studies show that woodpeckers kill about 4% of EAB larvae overall (up to 26% per tree) in China, and between 9-95% per tree in North America; hairy, downy and red-bellied woodpeckers are known to feed on EAB larvae[28-30].

See a less detailed version of EAB population factors.


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