Soybean Aphid No-Show?

July 21, 2004

WOOSTER, Ohio — The soybean aphid, an insect pest whose quirky behavior has made it more famous than the actual damage it causes, is doing something this season that Midwest entomologists may have successfully predicted: absolutely nothing.

The insect, which can level soybean fields with its voracious appetite if in high enough populations, is MIA so far this season — not only in Ohio, but also throughout the Midwest.

"It's very difficult to find the soybean aphid in Ohio. Most people who go out looking for it probably won't find it," said Ron Hammond, an Ohio State University research entomologist with the Ohio Agricultural Research and Development Center. "Not only is nothing happening in Ohio, but nothing much is happening in the Midwest. Reports from northern states indicate researchers aren't finding many of the insects up there. For example, in Minnesota, only six fields were identified last week with the aphid. This time last year, the state was spraying hundreds of fields."

Researchers are patting themselves on the back for what is turning out to be an accurate prediction of soybean aphid populations so far this season. Hammond said that if the prediction holds true, the aphid populations could be falling into a cycle of "low" one year and "high" the next.

"Last year, the soybean aphid was just awful, with high populations throughout the Midwest," said Hammond. "If populations stay low this year, we expect insects to overwinter this fall and explode next year in high numbers." When the insect was discovered throughout the region in 2001, masses of winged aphids sent people running for cover as far north as Canada. The following year, aphid populations were, at the time, uncharacteristically low. Populations rose again in 2003.

Hammond said that researchers speculate the level of soybean aphid populations may be tied to the population of the multicolored Asian ladybeetle, a known predator. Put simply, when soybean aphid numbers are high, ladybeetle numbers are also high. Although controlling the aphids during the summer months is light, the ladybeetles may prevent the aphids from overwintering by actively feeding on them in the fall. Ladybeetle populations are expected to be low this year because the aphid numbers are also low, which would account for researchers' assumptions that soybean aphid populations will be high come next season.

"We expect to see some aphid population build up at the end of this summer, but how much we don't know yet," said Hammond. "But just because soybean aphid populations are low right now doesn't mean growers should be forgetting about the insect. They need to be out in their fields scouting for the insect as usual. We will know that our prediction of low aphid populations is accurate only at the end of the season. Time will tell."

Good advice to adhere to now that other insects are emerging, and at a time when soybean plants are most sensitive to insect defoliation. Bean leaf beetles, Mexican bean beetles, Japanese beetles and grasshoppers are likely to be making an appearance soon, if not already emerging.

"Insect defoliation is an issue growers face every summer, and all we can do is recommend to growers to pay attention to any defoliation and take action if necessary," said Hammond. "It's normally not one insect that will cause significant damage, but when you add up all these insects, then you can start reaching thresholds."

Hammond suggests that growers pay special attention to the bean leaf beetle, whose first generation brood not only feeds on plants, but whose second generation can also feed on the pods.

"Use first generation populations as an indication of how high the second generation will be," said Hammond. "If first generation populations are high, then second generation populations, which emerge in August and September, might also be high."

Hammond also recommends a technique to help reduce the additional spread of bean pod mottle virus, based on Iowa State University research.

"For those growers who sprayed to control the overwinter populations to lower the transmission of bean pod mottle virus, research has shown that another treatment targeted at the first generation helps to slow the feeding of bean leaf beetles and, hence, slows the continued spread of the virus," he said.

Author(s): 
Candace Pollock
Source(s): 
Ron Hammond