A determined soybean aphid natural predator from Asia is controlling soybean aphids in Canadian fields, and may expand its range into the northern tier of the Midwest. This Asian stingless wasp, Aphelinus certus, is smaller than the period at the end of this sentence and arrived in North America at the same time as soybean aphids or shortly after that.
Ontario scientists found aphids killed by A. certus in 80% of the soybean fields they sampled in 2007, three years after they noted its arrival there. A. certus quickly teamed up with lady beetles, minute pirate bugs and other natural soybean aphid enemies to protect Ontario soybeans.
But A. certus has moved South across the border into Pennsylvania in 2007 and later into all of Minnesota’s soybean-growing areas.
“Seeing them two years in a row is a big deal because it indicates they can overwinter here,” says University of Minnesota Entomologist George Heimpel. “We haven’t found them parasitizing (killing) aphids at very high rates yet, but that’s to be expected at this point of their move into the state.”
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With support from the soybean checkoff, Heimpel’s worked for 10 years to introduce beneficial insects to destroy soybean aphids. Most beneficial insect introductions require several attempts, and a species he introduced in 2007 didn’t survive a Minnesota winter.
A. certus, which showed up by accident, attacked every aphid USDA scientists showed it. However, A. certus may eventually be found to have a negative ecological impact, he says.
Approved bug immigrants
Unlike A. certus, Aphelinus glycinis feeds only on soybean and cotton aphids and passed all the official hoops for approval. Heimpel introduced it on a limited basis in Minnesota last fall.
“We are going to release hundreds of thousands of them in Minnesota this summer in replicated field trials to see if we can significantly control soybean aphids,” says Keith Hopper, research entomologist, USDA Beneficial Insects Introduction Research Unit. If A. glycinis fails to be the soybean aphid killer, Hopper says another stingless wasp called Aphelinus rhamni is near approval, and still another, Aphelinus coreae, is waiting in the wings.
Heimpel is optimistic. “The volunteer A. certus and introduced stingless wasps could provide the one-two punch that we need to really help soybean growers. Farmers won’t need to change what they are doing as long as they follow the Integrated Pest Management (IPM) guideline of 250 aphids/plant as the threshold before spraying insecticides.”
Aphid App, a pilot smartphone application developed by University of Guelph Entomologist Rebecca Hallet, already factors aphid and natural enemy numbers into spraying decisions.
Spraying early and often could destroy any hope of natural control by killing the good insects along with the bad. “If you are trigger-happy, you kill the natural enemies, which could cause the aphids to surge again if the temperatures and field conditions are right,” says Tracey Baute, entomologist with the Ontario Ministry of Agriculture, where the predator wasps have successfully controlled soybean aphids.
USDA’s Hopper is confident that natural enemies will turn the tide in the battle against soybean aphids. “A successful narrow host parasitoid like we are studying will make soybean aphids an occasional problem here.”
Researchers are still learning about how these insects interact. Here are some of their observations and speculations about aphid and wasp lifecycles, and how they came to U.S. fields.
Aphids under attack. A. certus, A. glycinis and other stingless wasps called parasitoids kill soybean aphids by laying eggs inside them. The larvae hatch and eat the aphids’ internal organs. Predators like lady bugs attack and devour the aphids.A. certus can build numbers while feeding on wheat aphids before the soybean aphids arrive in a field. They also are more active in hot weather than soybean aphids.
Wasp free ride. Soybean aphids are strong flyers. Aphelinus certus is not. This has led to speculation that aphids inadvertently carry the certus eggs or larvae to new areas.
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