The invasive emerald ash borer (Agrilus planipennis) is a small but destructive pest in much of the eastern half of the United States. A key part of management efforts have focused on biological control via parasitoid wasp species. But an extreme cold-weather period in the U.S. in the winter of 2019 showed researchers that extreme fluctuations in temperature driven by climate change could complicate the success of such biological control efforts, as significant numbers of the parasitoids did not survive the extreme cold. (Photo by Howard Russell, Michigan State University, Bugwood.org)

By Ed Ricciuti

Ping-ponging winter temperature extremes of severe cold and unseasonable warmth due to an increasingly fickle jet stream may make it necessary to rethink the deployment of certain parasitoids used against the damaging emerald ash borer (Agrilus planipennis), according to a paper in the Journal of Economic Entomology.

Ed Ricciuti

The study’s findings demonstrate that developing biological controls for harmful insects can be much trickier than simply identifying a potential predator or parasite of the species involved. A host of environmental factors could also impact the success of the effort.

Sudden cold waves may be lethal to the overwintering larvae of two wasps—Spathius galinae and Tetrastichus planipennisi—that that share the range of the emerald ash borer (EAB) in Northeast Asia and have been released across the United States’ northern tier to control it. EAB larvae, on the other hand, seem to more easily weather extreme cold, giving the borer an edge over its enemies.

EAB overwinter as larvae ranging from first to mature fourth instars, whereas its parasitoids overwinter as mature last-instar larvae inside the feeding galleries of parasitized larvae between the bark and sapwood.

Arctic cold at a study site in southern Michigan during 2019 killed up to half of S. galinae larvae, and 30 percent of T. planipennisi. Those at the other site used in the study, in central Connecticut, made it through the winter relatively unscathed.

Jian Duan, Ph.D., of the U.S. Department of Agriculture, de-barks a tree at a study site at William M. Burchfield Park in Ingham County, Michigan. (Photo courtesy of Jian Duan, Ph.D.)

Given time, many insects can adapt gradually to bitter cold. They become supercooled, a process in which chemicals that serve as natural antifreeze prevent ice crystals from forming in cells while the circulatory fluid around them freezes. Overwintering EAB supercool at -28 to -35 Celsius, while the supercooling point of the two wasps seems to be a few degrees higher.

Insects like EAB and its parasitoids can thus tolerate low temperatures, but cold tolerance develops gradually, as winter progresses. Say the researchers, “It takes time.”

The researchers who conducted the study noted that the parasitoids reared and released for borer control in North America came from populations in parts of China and Russia’s Vladivostok that enjoy a temperate coastal climate, like that of the Connecticut site. Michigan tends to have an inland, continental climate, subject to bouts of artic cold during winter when the polar vortex is destabilized, a phenomenon of increasing frequency with the warming of the Arctic.

“As a result of climate change, extreme low temperature events occur with increasing frequency and intensity due to a weakened jet stream in the Northern hemisphere,” said the research team, headed by Jian Duan, Ph.D., of the U.S. Department of Agriculture. “Therefore, we recommend expanding foreign exploration to more northerly and inland regions of Asia … for species and/or strains of potential EAB biocontrol agents with greater cold-hardiness, capable of surviving bouts of extreme low winter temperatures.” Fertile grounds for such control agents, says Duan, are cold inland regions of Northeast China and the Russian Far East.

The observations at the Michigan sites strongly suggest that climate change could reduce the effectiveness of the two wasps for managing EAB populations. “The very rapid cycling between warm and cold events in response to global climate change may lead to higher winter mortality [of the wasp larvae],” the paper notes, “despite the general warming of mean winter temperatures associated with global climate change.” At the same time, however, the researchers noted that studies are needed on how more typical winters impact the wasp larvae.

Says Duan, “Future foreign exploration for natural enemies against invasive pests (i.e., classical biological control) need to consider the possible impact of extreme weather due to climate change on the pest and natural enemy interactions.”

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.