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College of Food, Agricultural, and Environmental Sciences


Dry Weather Could Aggravate Insect Damage on Crops

June 15, 2007

WOOSTER, Ohio -- Pest pressure on Ohio field crops is not unusual, but with this season's abnormally dry conditions, any extensive feeding injury could put stressed plants in further jeopardy.


Ron Hammond, an Ohio State University Extension entomologist with the Ohio Agricultural Research and Development Center, said that in situations of extended dry conditions and heat, the economic thresholds of insects might be lower than usual.

"Injury from insects can make problems even worse for crops already stressed under severe environmental conditions. Sometimes those thresholds need to be lowered to help protect the crop," said Hammond. "A good example is the potato leafhopper on alfalfa.

"The rule of thumb in determining potato leafhopper thresholds is, if out of a 10-sweep sample, the number of leafhoppers present is greater than plant height, treatment is warranted. Under hot, dry conditions, that threshold might be lowered by a third because plant development might not be as far along as it should be."

Hammond recommends that growers scout their fields for pests and apply insecticide treatments if warranted.

Some pests making their appearance now include:

• The first brood of European corn borer. "Even though much of the corn in Ohio may contain the Bt corn borer gene, fields without the gene should be checked for first brood borers," said Hammond. "If whorl injury is apparent, then about 20 plants should be inspected at five locations to determine the percentage of stand exhibiting whorl injury. If the number of larvae found exceeds an average of one or more per plant and the larvae have not yet begun to burrow into the stalks, treatment may be warranted."

• Armyworms on corn. "Growers should be watching their corn fields adjacent to wheat fields or planted into grass cover crops, especially rye, for the presence of armyworm injury," said Hammond. "Detection of foliar feeding injury by armyworm on 15 to 20 percent of a stand should be regarded as an indicator of a potential problem, and treatment is warranted if infestation is greater than 50 percent and the larvae are not mature."

• Potato leafhopper on alfalfa. "Growers should now be sampling their fields for leafhoppers and paying attention to the threshold," said Hammond. "For potato leafhopper-resistant varieties of alfalfa, the economic threshold is three leafhoppers per inch of growth. In areas with little moisture and the alfalfa is beginning to show stress, we recommend lowering the threshold."

• Soybean aphid. Soybean aphids are beginning to appear throughout northwest Ohio, mostly likely migrating from populations already established in Michigan and Ontario, Canada. Hammond said that the insect is emerging in northern states several weeks earlier than normal, indicating that Ohio infestations could also occur earlier than the normal July or August emergence. "Ohio growers in more northern areas are advised to check their fields so as not to be caught off-guard by high aphid populations," said Hammond. "My colleagues to our north are surprised soybean aphid is showing up already. Infestations by mid-June are a bit unusual."

As the season progresses, soybean leaf defoliators such as bean leaf beetle, Japanese beetle and Mexican bean beetle will make an appearance. If hot, dry conditions continue these pests could also cause excessive injury to underdeveloped, stressed plants.

According to the National Drought Mitigation Center, most of Ohio is experiencing either abnormally dry conditions or a moderate drought.


Candace Pollock
Ron Hammond