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College of Food, Agricultural, and Environmental Sciences


Cold Snap May Slow Crop Pests, But Scouting Still Essential

April 18, 2007

WOOSTER, Ohio -- Farmers shouldn't count on the recent spring cold snap to stop the development of field crop insects, only to slightly slow them down. Several of these pests could potentially pose problems this growing season.

Ron Hammond, an Ohio State University Extension entomologist with the Ohio Agricultural Research and Development Center, said that farmers should continue to scout their fields for insects when the time is right and make preparations to control them.

"For most of the insects we deal with, we don't think that the cold temperatures would have had any major mortality effect. We are assuming that populations will only develop a little more slowly," said Hammond. "Most of these insects have developed means to withstand fluctuations in temperatures."

One insect that remains on track based on entomologists' predictions is the soybean aphid, which is expected to show up in soybean fields in high numbers this year.

"We know that most of Ohio's aphids don't come from overwintering, but migrate from northern states," said Hammond. "However, we found soybean aphids hatching from buckthorn in the Columbus area, which indicates that they survived the winter very well."

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 also become high. Although the ability to control the aphids during the summer months is low, the ladybeetles may prevent the aphids from overwintering by actively feeding on them in the fall. Ladybeetle populations were low last year because the aphid numbers were also low, which would account for researchers' assumptions that soybean aphid populations will be high this growing season.

For growers, the best way to manage the soybean aphid is to educate themselves on the insect, know when to scout, and to carefully time foliar insecticide applications if treatments are warranted.

The survival of winter annual weeds may set the stage for the appearance of other insects, such as the black cutworm.

"We know that there's been concerns with controlling weeds this winter, and how that may impact insect development," said Hammond. "We have placed pheromone traps in fields to determine if and when such insects arrive in large numbers and if the potential is there for any problems."

The black cutworm moth, a corn pest, tends to be attracted to fields with significant ground cover of winter annuals, such as chickweed, at the time of peak migratory flights. Adults lay eggs on the ground cover and hatched larvae move over to emerging corn plants, causing extensive feeding injury after weeds are killed.

Cover crops and other grassy areas are also a habitat for the common armyworm, another corn pest, which can cause devastation if infestations are severe enough.

"Most farmers will use herbicides to kill off the ground cover, but in some cases the material will be tilled under. In those situations, you can run into seedcorn maggots, which are attracted to the decaying residue and can impact seedling emergence," said Hammond. "Preventive maintenance is all about being aware of the situations in your field."

A resident insect that farmers should monitor for now is the alfalfa weevil.

"Larvae hatch from eggs and begin feeding in the spring," said Hammond. "Scouting recommendations are based on temperature, and we do know that in southern Ohio we have reached the right number of heat units that larvae are now becoming active. Although central and northern Ohio is behind this accumulation of heat units, growers in those areas should begin their scouting over the next one to two weeks."

Alfalfa weevil scouting is accomplished by collecting a series of three 10-stem samples randomly selected from various locations in a field. After 10 stems have been collected, the stems should be vigorously shaken in the bucket and the number of larvae in the bucket counted. Economic threshold is based on the number of larvae per stem, the size of the larvae and the height of the alfalfa. The detection of one or more large larvae per stem on alfalfa that is 12 inches or less in height indicates a need for rescue treatment.

For more information on field crop insect management, log on to the Ohio State Agronomic Crops Team Web site at

The Agronomic Crops Team is a group of Ohio State University Extension educators and Ohio Agricultural Research and Development Center researchers who provide practical and timely information and educational opportunities that address the most pressing needs of Ohio's agronomic crop industry. Information is widely distributed through the Crop Observation and Recommendation Network (C.O.R.N.), a free statewide electronic newsletter available on the team's Web site.

Candace Pollock
Ron Hammond