It's a bug-eat-bug world out there. So far, predatory insects are winning the bean field wars against invading soybean aphids that flew into fields on a University of Missouri research farm.
Hosts of beneficial bugs — including five species of ladybugs — are devouring thousands of soybean aphids on research plots, said Wayne Bailey, MU Extension entomologist. “The hot weather slowed the aphids, but the beneficial insects are controlling the population.”
For Tom Clark, MU research entomologist, the beneficial bugs create a dilemma in his study of economic thresholds for applications of insecticides to control the soybean aphids.
“We had an explosion of soybean aphids, with over 6,000 aphids per plant,” Clark said. “Now it's difficult to find them. The beneficial insects are working.”
Soybean aphids that suck sap from growing soybean plants had almost disappeared after 10 days of attack by the beneficial bugs in the MU plots. Large populations of the aphids can reduce yields or kill the plants.
There is a lesson to be learned. “Don't spray insecticides too early. Give the biocontrols a chance to work,” Bailey said.
If growers spray insecticides too early in the season, the beneficial insects are killed, allowing aphids to invade unopposed.
Two species of ladybugs — the 12-spot and the Asian — and the insidious flower bug eat the aphids and provide most of the control. Others are helping, including the common pink ladybug and damsel bugs.
For the past four years, soybean aphids have been a problem for growers in northern states, where insecticides were often needed to control the aphid colonies on soybeans. Missouri soybean fields, for the most part, have avoided damage from the pest. Some controls were needed in counties along the Iowa line last year.
The pests, originally known as Chinese aphids, were accidentally introduced into the United States on buckthorns, ornamental plants imported from China.
“We may have a unique environmental advantage,” Clark said. “We have a large population of beneficial bugs that like to eat aphids. They just seem to come out of the woods.”
As part of the annual Crop Injury Diagnostic Clinic, Clark brought a wagonload of certified crop advisors to the field to show the aphids. The group, many of whom had never seen the pest, had a hard time finding even one in plots that had thousands of the pests earlier in the month.
In mid-July, soybean leaves had hundreds of aphids on the underside of each leaf. On clinic day, they had only one aphid, or none. To an untrained eye, the tiny translucent green aphids were difficult to see.
One of the crop advisors, who had found no aphids while scouting the plot, said, “I was looking for something a lot bigger.” The individual aphids are best seen with a 10X magnifying glass.
The soybean aphids have a distinct set of black “tailpipes,” Bailey said. He added that if you see a colony of aphids on soybeans, they are Asian soybean aphids.
Clark said it is easier to spot the ladybugs feeding on the aphids and ants that eat the honeydew that aphids create.
When aphids build colonies large enough to cause economic damage, they are easier to see. The leaves seem alive with the aphids.
Currently, MU Extension specialists recommend spraying insecticides when the population reaches 250 aphids per plant between flowering and pod-filling stages. Those control thresholds are adapted from research in Canada, Minnesota and Wisconsin.
Clark was hoping to start research to determine the appropriate threshold numbers for Missouri.
At the crop clinic, as Clark turned over a soybean leaf, the only aphid on the plant was being eaten by an Asian ladybug.