The fate of Virginia’s stately ash trees might rest on the wings of a tiny wasp.
For more than a decade, ash trees (Fraxinus genus) have been under threat from an invasive insect pest, the emerald ash borer (Agrilus planipennis) – EAB, for short. The larvae of this beetle feed on the phloem of ash trees, disrupting nutrient transfer. Native ash trees didn’t evolve with EAB, and its natural enemies aren’t here to control it. As a result, EAB has steadily decimated ash trees across much of the state, and an estimated 99% of our ash trees will die without some intervention.
A finger points to an adult emerald ash borer beetle on a piece of ash cambium. The wood is inscribed with “galleries” created by feeding ash borer larvae.
Efforts to protect individual valuable ash trees often involve treating them with systemic insecticides. But a landscape-scale approach is needed to protect forest trees. Enter the wasps – specifically, the tiny parasitic wasps that keep EAB in check in its native range – also known as “parasitoids”. There’s no need to worry about stings from these wasps. They use their ovipositors to lay eggs in or on EAB eggs or larvae – not to sting. Plus, they’re tiny. In fact, you might mistake them for gnats!
Three species of parasitoids have been intensively researched by USDA and approved for release in Virginia. They attack only EAB, so the hope is that they will establish populations here and keep the EAB population in check, allowing ash seedlings to survive and the ash population to regenerate in the future.
Release of parasitoids is a form of biological control, also known as “biocontrol.” In Cumberland and Whitney State Forests, DOF has released thousands of wasps: Oobius agrili, Spathius agrili and Tetrastichus planipennisi. So far, a survey of felled and debarked trees from Cumberland State Forest has revealed that some Spathius wasps have parasitized EAB larvae. (Incidentally, nearly a third of the EAB larval tunnels studied had been terminated by woodpecker feeding – an unintentional form of biocontrol!)
It will take time to verify how well the wasps are controlling EAB populations. At Cumberland State Forest, they have made a start, and DOF will assist land managers at Grayson Highlands State Park and Montpelier to monitor the impact on their properties.
Note: The parasitoids were produced and supplied by the USDAEABParasitoid Rearing Facility in Brighton, MI.
Watch: Emerald Ash Borer: Fighting Back with Biocontrol
This video shows how The Virginia Department of Forestry (DOF) is using biological control agents—tiny parasitoid wasps from Asia—to combat the emerald ash borer threatening Virginia’s ash trees.
The emerald ash borer is an invasive insect that is native to Asia and it was first discovered in the U.S. in 2002 in Michigan and then it reached Virginia in 2008. So it’s been slowly spreading throughout the state since then and currently it’s found in over 50 Virginia counties, so that’s over half of the state. And it will infest and kill any ash tree that is native to Virginia. So today we are releasing biocontrol agents to try to prevent the spread of emerald ash borer. In addition to biological control there are also control methods for the emerald ash borer using insecticides. We’re choosing to use biological control at Cumberland State Forest because there are plentiful ash and it’s not really cost-effective for us to go out and treat every individual tree with insecticide so this is kind of a more long-term sustainable approach if you will. So a bio control agent is a predator of the pest in its natural environment. So the emerald ash borer is native to Asia and in Asia there are predators and parasitoids that control the emerald ash borer population. We don’t have those here in Virginia since the emerald ash borer is not native to Virginia, so what we’re doing is we’re releasing some of these biocontrol agents that are native to Asia here in Virginia. But all of these biocontrol agents have been tested in quarantine facilities for years prior to them being allowed to be released here. And they’ve been tested against any other possible hosts that they may come into contact so by the time that that we’re allowed to release them here we are very certain that they will not attack anything other than that emerald ash borer. We’re releasing our first biological control agent which is a Oobius wasp sp. parasitoid. So this is a tiny little wasp that is native to China and what it does is it lays eggs inside the eggs of the emerald ash borer. And then when those parasitoids eggs hatch they end up killing the emerald ash borer eggs. These wasps are in their pupae form so when they emerge from the pupae as adults they’ll fly away from the container and hopefully parasitize emerald ash borer eggs that are on this tree and surrounding trees in the area. The second biological control agent that we’re releasing out here is Tetrastichus wasp sp. and it is a larval parasitoid of the emerald ash borer so similar to when Lori was speaking about Oobius where the parasitoid will actually lay her eggs inside of the eggs, and Tetrastichus will lay her eggs inside of the larvae that are inside feeding on the ash trees. And so by releasing these releasing adults both male and female of the species and weres releasing them out there and they will sense when there are larvae inside of the tree and she will lay her eggs through into the larvae itself and her developing young will feed on and subsequently kill the emerald ash borer larvae and stop them from feeding while they’re in the tree right there. So we’ll definitely continue to monitor this site for the next few years to see if these parasitoids are able to establish a population here. And there’s a number of ways to do that; You can try to trap for the adult wasps to see if they’re still in at this site. If you have the means you can also fell a tree, debark it, find the emerald ash borer larvae, and dissect them looking for the eggs that were laid by the parasitoids. So one of those methods we’ll use to continue to monitor the site for the next few years.
