Growing soybeans? Watch for insects

WOOSTER, Ohio – Soybean growers may be looking to get their crop in the ground, but they especially need to keep in mind two insect pests this growing season.

The soybean aphid and bean leaf beetle are problematic for soybean growers and can cause economic losses either from feeding or through the transmission of viruses.

Ohio State University Extension specialists recommend that growers scout their fields for the presence of the insects as the growing season wears on and take appropriate management actions as deemed necessary.

Aphid population. The soybean aphid, the first known aphid to colonize soybeans, is a relatively new insect that has, in just two years time, spread throughout the midwest region of the country. States that reported soybean aphid populations last year included Ohio, Michigan, Illinois, Minnesota, North Dakota, South Dakota, Indiana, Kentucky, Iowa and Wisconsin. The insect has also been reported in Virginia, West Virginia, New York and Ontario, Canada.

Researchers know very little about the soybean aphid – what exactly is its overwintering host, whether or not it’s a vector for disease, and why larger insect populations tend to be found in more northern states. But what they do know is that high insect populations can cause significant yield loss to the crop.

Ron Hammond, an Ohio State Extension entomologist with the Ohio Agricultural Research and Development Center, said some states reported an average six to eight bushel loss per acre last year due to soybean aphid feeding.

“Some fields had as high as a 12 to 15 bushel loss per acre. There were reports in Ohio that indicated loss ranged from five to 10 bushels per acre. When you get into numbers like that, that’s considered an economic loss,” he said.

Yield losses. Researchers in Ohio and other states speculate that yield losses are coming from the insect feeding, rather than from soybean mosaic virus that the insect is able to transmit.

“The soybean aphid is a sapsucker, sucking the juice out of the plants. It’s an indirect injury, but it’s enough to cause stress to the plant and cause pod abortion or a lack of pod set,” said Hammond.

“Researchers in other states speculate the damage is caused solely from the insect itself. Soybean mosaic virus was not prevalent in other states last year and in Ohio we haven’t seen any big increase in the virus due to the presence of the aphid.”

Hammond said that the best way to control the soybean aphid is to apply insecticides, but timing is crucial in order for the applications to have any impact on insect populations.

“Based on data that we are receiving from other states, we seem to be zeroing in on the critical application time to be around flowering and immediately thereafter. That’s when populations are beginning to increase and injure the crop,” said Hammond.

Spraying the crop. He said that spraying before flowering is ineffective because the insect reproduces so rapidly that once the insecticide wears off, the populations could build up again to high levels during the most critical stage of crop development.

Spraying a few weeks after flowering might be ineffective as well, because damage caused by feeding has already occurred and the insects might have taken flight because of overcrowding. Hammond also believes that spraying during times other than flowering may kill any beneficial insects that may be helping to keep soybean aphid populations in check.

“The longer a grower goes away from the flowering stage with sprays, the less positive impact those sprays will have. Our recommendations are for growers to monitor fields beginning in June, but not to spray,” said Hammond.

“We will also be developing population thresholds based on data from other states to determine how large aphid populations must be on plants to warrant treatment.”

Another bug, virus. The second insect growers should keep their eye on is the bean leaf beetle, a defoliator and pod feeder that is also a vector of bean pod mottle virus. The virus causes yield reduction in seed quality and yield, which is a problem for food-grade soybean growers who have little luck is getting a premium price for “mottled” seed.

One of the main symptoms of bean pod mottle virus is green-stem syndrome, where the plant stem remains green right up to harvest and pods are malformed.

“We are getting reports now that the bean leaf beetle is emerging, which may be of a concern for those growers who have already planted their soybeans,” said Hammond.

“With all the rain we’ve been having, planting has been very spotty and in those areas with few soybean fields, they will act as trap crops and attract all the beetles in the area.”

Hammond recommends that growers who already have their soybean crop in the ground to pay close attention to their fields for bean leaf beetle populations.

“The first line of defense in controlling bean leaf beetle is to plant late, but there has been some interest in early-season insecticide applications to help prevent the transmission of the disease,” said Hammond.

“We don’t recommend that right now because we feel there isn’t enough data to suggest that it actually works. Also, it’s a preventive tactic that must be done even before it’s known if bean leaf beetle populations will be high. And that’s something we definitely can’t tell right now.”

Scout early. Bean leaf beetle populations were relatively low in Ohio soybean fields last year. Overwintering populations were also low, which many speculate may result in low populations during the growing season again this year.

However, with the mild winter, those beetles that were present probably overwintered very well, said Hammond. Hence, the need for early scouting.

Regular updates on soybean aphid and bean leaf beetle will be posted to Ohio State Extension’s Crop Observation and Recommendation Network newsletter found at http://corn.osu.edu/.

Information on the soybean aphid can be found on Ohio State Extension’s Integrated Pest Management Web site at http://ohioline.osu.edu/~ipm/.