Crops: A new ally for farmers in controlling soybean aphids

Tuesday, April 1, 2008

A diminutive wasp has joined the growing number of natural predators which are helping to control otherwise costly aphid infestations in the province's $194 million soybean crop

by MARY BAXTER

While scouting for soybean pests last year, Ed Kaiser noticed some black spots on the bottom of his plants' leaves. "They could have been dust, for all I knew. They were very tiny and didn't seem to be of any consequence," says the Napanee-area farmer.

Later in the season, a University of Guelph student, who was conducting a survey on aphids and their natural predators, arrived at his farm and told Kaiser the spots were mummified remains of aphids, victims of an unnamed parasitic wasp. Months later, in a southwestern Ontario lecture hall miles away, he would further learn that these one-millimetre insects eliminated a whopping quarter of the aphids the researcher found on his plants.

In 2003, a particularly bad year for aphids, provincial estimates put the combined cost of soybean aphid infestations to growers at $194 million.

With more than two million acres of soybeans grown in the province each year and their market value on the rise, this insect "puts even more dollars at risk," says Tracey Baute, the province's field crop entomologist. "And unfortunately we are at risk of experiencing threshold levels of soybean aphids every year here."

So it's no wonder that the province's research community is beginning to buzz about this diminutive wasp's potential to control aphid populations.

The wasp is among a number of natural aphid predators, which include two varieties of lady beetles, minute pirate bugs, lacewings, another small wasp called aphidius colemani (used in greenhouse pest management) and midge larvae. The two lady beetles might pose the greatest threat to aphids, but the new wasp "is certainly up there," says Rebecca Hallett, an environmental biologist at the University of Guelph and one of the leads in a three-year research project focused on finding out how to harness the aphid's natural enemies to control infestations.

Noticed around 2003, the wasp came to the attention of researchers at the University of Guelph's Ridgetown campus in 2005. "They actually had trouble keeping it out of cages where they were rearing soybean aphids (for study)," Hallett says. Results from a genetic analysis determining whether it is native to the area will be available in early spring.

Meanwhile, researchers do know that the wasp, which is of the aphelinus genus, is present in soybean fields if aphids are present. They also know that it can survive on some other forms of aphids and that, along with laying eggs in the aphid, the female will snack on its egg's unwitting hosts.

But there's a lot they don't know, including what role crop rotation might play in its survival during years when the soybean aphid population isn't high and what effect pesticides used to control aphids might have on their numbers and development.

Hallett notes that the wasp plays a bigger role in controlling populations in Ontario than in the United States, where it has also been spotted. Results from last summer's survey, which encompassed 25 counties in Ontario, indicate that the wasps parasitized anywhere from 10 to 25 per cent of the aphids on a plant.

The role natural predators play may eventually lead to establishing a new action threshold for applying insecticides to control aphid populations, and that is the thrust of the research project in which Hallett is involved.

Away from the labs and out in the field, is it feasible to expect natural critters such as these wasps to eventually save growers the cost of insecticide applications?

For Kaiser last year, the answer was yes. He first noticed aphids, well below threshold levels, while harvesting wheat at the end of July. By August, the populations "were building in some spots."

He debated spraying, but then noticed a significant population of ladybugs. He decided to let the ladybugs do the work for him.

Kaiser now knows that the wasp was the ladybug's ally in reducing aphid populations on his farm. He says they didn't change his perception of what he should do, but admits "they may have altered it."

Ironically, although these insects saved Kaiser the costs of between $15 and $20 per acre for a one-time insecticide application, they couldn't save his crop from the weather. Drought persisted through August and the beans' pods didn't fill properly. By harvest, Kaiser's average crop yield was a disappointing 28 bushels per acre. BF