If you’re a woodlot owner, you’ve probably heard about the destructive forest pest emerald ash borer (if not, you can get introduced here). This invasive green beetle has spread over half the US, killed more than 25 million trees, and threatens ash trees with extinction.
Though smaller than a penny, the emerald ash borer is devastating. Its larvae kill trees by eating the tubes ashes use to transport nutrients throughout the tree.
Photo credits: Howard Russell, Michigan State University, Bugwood.org (adult); Pennsylvania Department of Conservation and Natural Resources – Forestry, Bugwood.org (larva), both CC BY v.3.0.
Until recently, the best way to deal with emerald ash borer was to slow its spread by never moving firewood to or from your woodlot (still a good idea, by the way). But in the past few years, another method for dealing with emerald ash borer has gained new promise: biocontrol. Biocontrol is the introduction and establishment of specialist predators from the native range of an invasive pest. I’ve been fortunate to contribute research to this control technique, and I wanted to share our work with you.
Biocontrol starts with a visit to the ash borer’s native range in Asia to see what animals keep it in check there. Researchers found several specialized predators, called parasitoids.
Parasitoids are amazing insects. They’re small, stingless wasps about 1/4" long. When a female parasitoid finds an ash, she taps the bark with her antennae to feel for vibrations of moving ash borer larvae. When she locates a larva, she uses her ovipositor (a modified stinger) to drill through the bark and paralyze the larva. She then lays eggs on the larva, and when those eggs hatch, the wasp larvae eat the borer larva.
Spathius agrili, one of the parasitoid wasps being researched to control emerald ash borer.
Photo credit: Jennifer Ayer, Bugwood.org
In Asia, parasitoids kill up to 50% of emerald ash borers in infested trees. The hope is that if we bring these parasitoids to the US, they could have the same impact in controlling ash borers here.
But before we can release the parasitoids in North America, we have to make sure they don’t attack native insects. To do that, we first test them against native insects similar to emerald ash borer inside a closed lab. If the parasitoids don’t attack the other insects, they’re considered suitable for release.
Based on that research, ash borer parasitoids are indeed specific to ash borers. When we gave them a choice between the ash borer and a native insect, they attacked the ash borer.
With that success in hand, we received permission to release small numbers of the parasitoids in New York, and other researchers have released parasitoids in 22 of the 25 states with known infestations. So far the parasitoids are establishing at almost every site, but it will still be a few years before we’ll know if they can impact ash borer populations. Still, our initial results have been promising, and the parasitoids’ establishment is a great first sign of hope that we can save ash from extinction.
Author Bio: Michael Jones is a PhD student in Forest Entomology at the State University of New York – College of Environmental Science and Forestry. He worked for the US Forest Service, Forest Health Protection for three years in California as a field entomologist studying gold-spotted oak borer, a forest pest similar to emerald ash borer.