What’s Eating You, Emerald Ash Borer?

By Roderick Mackler

Most of you have at least a passing familiarity with the emerald ash borer (Agrilus planipennis). The bright green beetle is decimating the ash trees on the eastern half of North America, from Ontario to Virginia. Females lay their eggs in the crevices in the bark of ash trees (several species of genus Fraxinus) and the larvae eat the layer between the bark and the hardwood, eventually killing the tree. 

Americans have tried to control the spread of the invasive emerald ash borer through insecticides and “biocontrols,” which the U.S. Department of Agriculture (USDA) and its Animal and Plant Health Inspection Service (APHIS) define as the use of natural enemies such as parasitoids, predators, pathogens, or antagonists to reduce plant pests or diseases. Scientists from USDA, in cooperation with counterparts in the beetle’s native China, have found three tiny parasitic (non-stinging) wasps (and later one in Russia) that prey on the emerald ash borer. Just as importantly, these wasps feed only on the emerald ash borer. If there are none of these invasive beetles available, the wasps die out. APHIS has studied the biocontrols. Before USDA clears the parasitic wasps for release, APHIS will have determined that the wasps do no further harm in the environment. 

For the past two years, Arlington County Parks & Recreation has introduced one of the parasitic wasps, Oobius argili, into several parks that have been infested with the emerald ash borer. This is a very tiny wasp—about the size of a period at the end of a sentence. The female Oobius lays her eggs inside the egg of the emerald ash borer, and when the wasp larvae hatch, they eat and kill the beetle’s egg. 

This past summer, ARMN members have been monitoring yellow pan traps attached to infested ash trees to see if the population of Oobius has reached self-sustaining levels. Yellow plastic pans are attached to infested ash trees. Each pan is filled with propylene glycol, which should attract and kill the Oobius. Once a week, an ARMN volunteer visits each trap, pours the old propylene glycol into a filter, and refills the trap. 

The collected insects and debris from the traps are taken to the Arlington/Alexandria Cooperative Extension Office where they are examined through a microscope. The specks that look like Oobius are sorted out, put in small vials, and sent to the USDA APHIS lab in Michigan for confirmation. 

We do not yet know whether the biocontrols have caught on. In any case, Arlington Parks & Rec plans to introduce more of the parasitic wasps in 2026 and 2027. This is because Parks & Rec is not sure whether the initial period has been sufficient for the wasp population to become self-sustaining. However, given the favorable experience of neighboring jurisdictions, particularly in Fairfax County, they feel it is worth the effort. They plan to introduce more Oobius for the next two years, then monitor the yellow pan traps again.  

We all hope the biocontrols will slow the infestation of Arlington County’s emerald ash borer and can provide beneficial information to other communities with EAB infestations.