Earlier this year, the U.S. Department of Agriculture decided to stop regulating the movement of living ash trees and wood products as a way to control the spread of one of North America’s most devastating invasive insects, the emerald ash borer. The strategy, which had been in place for over a decade, has proven no match for the insect which has now spread to 35 states and is considered the most destructive and costly invasive wood-boring insect in U.S. history. Now, the USDA is pursuing alternative strategies that include parasitic wasps to fight the insects and save North American forests.
As with most invasive species, the emerald ash borer (EAB) doesn’t have any native predators in North America. The pests, which likely came from China, were first identified in Michigan in 2002 after ash trees in the state began dying off for no apparent reason. EAB larvae burrow under ash tree bark to feed, eventually weakening and killing trees. Despite intense quarantine efforts enacted by the USDA’s Animal and Plant Health Inspection Service (APHIS), the wood-boring beetles have killed tens of millions of trees across the U.S., mostly in the East and Midwest. They’ve also infested southern Canada and continue to creep ever further in all directions. That includes toward the West, where forests are already tinderboxes due to years of drought combined with uncontrolled outbreaks of native tree-killing insects.
That steady expansion made the USDA’s decision to drop quarantines extremely controversial with many stake holders. However, the USDA believes the quarantine efforts are a waste of resources. Adult emerald ash borers can fly up to 12 miles per day and can go undetected for years, which many argue makes quarantines ineffective. APHIS first proposed ending the quarantine back in 2018, acknowledging the efforts had failed to stop the emerald ash borers spread.
USDA is now looking at “more effective and less intrusive methods” to fight the borer, with an emphasis on biological controls, meaning the use of natural enemies. USDA’s Plant Protection and Quarantine (PPQ) program initiated a search for biological control agents soon after discovering emerald ash borer in the U.S. Scientists have identified four species of parasitic wasps native to Asia that kill the borers early in their life cycle.
The wasps themselves do not sting. Instead, they hunt by feeling vibrations from EAB larvae feeding under the bark. They use their ovipositor to pierce through the bark and lay multiple eggs inside the larvae or eggs. When the wasp eggs hatch, they feed on and eventually kill the EAB as the wasps complete their lifecycle. The adults then bore out of the tree, ready to attack EAB larvae.
Since the start of the biocontrol program, PPQ has released about 8 million wasps in 30 EAB-infested States. PPQ says they have successfully recovered wasp offspring in 22 States, demonstrating that the wasps are reproducing, becoming established in the areas where they were released, and—more importantly—attacking and killing emerald ash borers. They have proven especially effective in protecting young ash saplings. USDA scientists are now trying to raise massive numbers of the wasps so they can establish self-sustaining populations in nature.
Ash trees have significant value ecologically, economically, and culturally. They provide habitat and food for wildlife; reduce pollution and stormwater runoff; shade and beautify our landscapes; and are vital to our forests. Ash tree wood is one of the most popular hardwoods used to produce various wood products. The wood is of high value before it is infected, but once infected, the wood becomes brittle and dangerous to remove. Several species of as trees are now at risk of extinction in North America. (Sources: USDA, Science.org, New York Times)
