Soybean aphid numbers are climbing in some growing areas, but entomologists at Ohio State University and Purdue University say farmers may want to think twice about spraying the pests.

"Over the past few weeks, the numbers of soybean aphids has increased," said Ron Hammond, OSU entomologist. "But we are trying to be cautious when it comes to treatment because we still don't know what the true threshold is before aphids start to damage the plants.

"We still believe it takes thousands of aphids per plant to cause damage, and there has to be a lot of plants infested with aphids in a field."

Field observations indicate soybean aphid infestations may have reached their peak and numbers could begin dropping in the weeks ahead, Hammond said.

"Some of the hot spots we've been observing have not increased in aphid populations," he said.

"There could be several reasons for this: winged aphids are dispersing and reducing numbers, there is a lot of predation from beneficial insects, or there might be a pathogen at work. I'm not finding as many aphids in some fields as I did a week ago."

Growers concerned about the aphid should randomly pick 10-15 sites in a field, observe the number of aphids found and decide whether to spray, Hammond said.

"If a grower is finding thousands of aphids per plant throughout his field, then he may need to spray," he said. "We don't want to discourage the grower from treating fields, but we don't want to encourage it, either, if it's not warranted. It's just as frustrating for us not to know what to do as it is for the farmer."

Lorsban and dimethoate are insecticides that have proved effective in lowering soybean aphid numbers.

Some soybean plants are yellowing, a characteristic related to soybean aphid damage. But entomologists are unsure whether the discoloration is being caused by the aphid or by other factors, such as lack of rain.

"The yellowing associated with aphids is often caused by a potassium deficiency," Hammond said. "The question is if one is causing the other -- whether aphid damage is increasing existing potassium deficiencies. There is some stunting of plants too, but we attribute much of that to lack of rain. Some of these fields haven't gotten sufficient rain since early June."

John Obermeyer, Purdue entomologist, said he is discouraging treatment as much as possible. "We're very concerned that should a producer treat today, in all likelihood they'll be treating again in a couple of weeks.

"When you treat fields with insecticide you're going to kill much more of the predator population than the aphid population," Obermeyer noted. "Aphids have a tremendous potential to repopulate, whereas the predators are going to be slow coming back, in comparison."

Additional soybean aphid information is available on the Internet at:

www.ag.ohio-state.edu/~ipm/soyaphid.