When Wasps Win

Parasitic predator dines on soybean aphids.

A parasitic wasp approaches an unsuspecting soybean aphid, Image by Matt Kaiser

If you have a hankering for a sci-fi horror flick this summer, skip the theater and head straight to your soybean field.

There, tiny parasitic wasps called Aphelinus certus (A. certus) will likely be flitting among the rows, piercing soybean aphids with their stingers and inserting their eggs inside the aphid’s body. When the eggs hatch, the larvae will live off the aphid’s innards before they burst out of their host as winged adult wasps.

SURPRISE VISITS. Parasitic wasps like A. certus are so adept at killing soybean aphids that USDA has been testing different species during the past decade to see which ones it can safely release as biological controls of this growing soybean pest.

The agency actually tested A. certus and decided it wasn’t a picky enough wasp species for its purposes, says Robert Koch, University of Minnesota entomologist. “It’s a generalist feeder, which means it will feed on multiple hosts, in this case, a lot of aphid species.”

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The agency discarded it as a candidate for release. But, the wasp had other ideas. “It somehow made its way here on its own from Asia,” Koch says. “We don’t know how it got here.”

Scientists first found A. certus in North America in 2005. It quickly spread around the U.S. and landed in Minnesota in 2011, Koch says. The University of Minnesota and the Minnesota Department of Agriculture have been surveying its populations around the state since 2015.

Recently, farmers have reported field failures of pyrethroid applications against aphids in the Dakotas, Iowa, Minnesota and Manitoba, Canada. A. certus could prove an important ally in the fight against these growing populations of insecticide-resistant aphids, Koch says.

DELICATE BALANCE. Overuse of insecticides could be limiting the potential of these wasps, Koch notes. “If people were a bit more judicious with their insecticide use, we could help promote these populations more,” he says.

“In my Extension program, we’re really trying to stress the importance of waiting until you have 250 aphids per plant to spray, so you can give these good insects--parasitic wasps and lady beetles--a chance to do their thing,” Koch adds. “And, if they can’t, and the aphid population surpasses 250 per plant, then you can come in with an insecticide.”

The wasps may suffer long before growers set foot in a sprayer, however. A 2014 study by University of Guelph scientists found that insecticide seed treatments can threaten the effectiveness of parasitic wasps. When they fed on aphids on neonicotinoid-treated soybeans, the parasitism rates of A. certus plummeted 68 to 88% compared to untreated beans.

These neonicotinoid seed treatments are used on the majority of soybean seed planted in the U.S., but most Midwest and Southeastern entomologists have concluded they are not always necessary for soybean growers in those regions.

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