Saving Our Ash Trees: A Wasp-Powered Approach

Ash trees across the United States have been under attack since the early 2000s. The emerald ash borer, an invasive beetle, likely came to the country as a stowaway in wooden pallets built from trees in China. North American ash trees had no resistance to the Asian beetle, nor did the pest face any predators, so populations boomed. The bug was first identified in Detroit, where it killed more than 99% of ash trees within ten years. Attempts to stop the bug failed and it spread nearly nationwide. The emerald ash borer cost the U.S. more than $10 billion between 2009 and 2019, including the costs of control efforts and the loss of the trees themselves.

The borer lays its eggs in the deep crevices of the ash tree’s bark. Once hatched, the larvae begin eating. They prefer the nutrient-rich phloem, the tissue that transports the sugars made in the leaves during photosynthesis to the rest of the tree. With enough damage, the tree can no longer move nutrients from its leaves to its roots. Leaves turn yellow, then fall off completely, and the tree dies within five years.

Studies in eastern China showed that Asian ash trees had developed a resistance to the ash borer and could defend themselves from the pest. They also discovered a parasitic wasp which laid its eggs on the larvae of the emerald ash borer and reduced its population. Biologists wondered if that would work in the U.S., though knowing that the introduction of one species to control another one is risky. They spent several years studying Chinese wasps to find one that wouldn’t parasitize North American beetles. Several species came close to being effective but were sensitive to cold or too small to reach the beetle larvae in deep fissures of tree bark. After expanding their search, they found a wasp in Siberia which fit the bill. The first wasps were introduced in 2016 to infected trees in Connecticut, New York and Massachusetts.

Ash borer populations in these areas began to decline dramatically, with 30% to 70% of parasitism in their larvae. After that success, more wasps were introduced up and down the east coast. The addition to the ecosystem of a co-evolved natural enemy seems to be restoring a balance. And scientists are hopeful that this remedy will give North American ash trees enough time to develop a resistance to the bug as their counterparts in Asia have done. With luck, our grandchildren and great-grandchildren will still be able to witness the beauty of a wide-spreading ash tree.