Ohio State Entomologist: Warmer Weather Brings Need to Scout for Alfalfa Weevil in Ohio

WOOSTER, Ohio – With the warmer weather that’s finally hit the region, growers in southern and central Ohio need to start scouting for alfalfa weevil at the end of the week (May 2), while field crop growers in northern Ohio should prepare to start scouting for the pest later next month, said an entomologist with Ohio State University’s College of Food, Agricultural, and Environmental Sciences.

While the extended cold conditions felt in the region this winter may have had some negative impact on the alfalfa weevil, growers still need to prepare for scouting their fields for this insect, said Andy Michel, an Ohio State University Extension pest expert.

The pest, known to cause significant alfalfa damage in its larval stages, typically starts showing up in southern Ohio first, slowly progressing its way north, said Michel, who also has an appointment with the Ohio Agricultural Research and Development Center.

OSU Extension and OARDC are the statewide outreach and research arms, respectively, of the college.

“It’s hard to tell how the cold will impact overwintering insects such as alfalfa weevil,” he said. “The extended cold weather does make a difference with other types of insects, but we don’t know if there is a similar trend with alfalfa weevil.

“Normally alfalfa weevil is controlled by parasitic wasps as well as fungal pathogens, which typically thrive in the wet conditions that we have now. So things may be slowed down this year, plus alfalfa may not be as vigorous as typical because of the colder weather, and its ability to withstand feeding may be compromised.”

But, Michel said, “Growers still need to be out there scouting because we don’t know how well the weevil fared during the winter.”

The major concern for this pest is that adult alfalfa weevils can lay large quantities of eggs in the plant stems. The hatched larvae then start feeding within the folded leaves at the growing tips. Plants that experience heavy feeding can develop a frosted look and can result in yield reductions because of stunted plants.

Growers can scout with the bucket sampling method, in which a series of 10-stem samples are randomly collected from various locations in a field. Each stem should be carefully picked off at the base and placed top-down in a bucket and vigorously shaken, so growers can count the number of larvae collected, Michel said.

The shaking will dislodge the late third and fourth instar larvae, which cause most of the foliar injury. Growers who find one or more larvae per stem on alfalfa that is 12 inches or less in height can use a rescue treatment, he said.

“Where alfalfa is between 12 and 16 inches in height, the action threshold should be increased to two to four larvae per stem depending on the vigor of alfalfa growth,” Michel said. “When alfalfa is 16 inches in height and there are more than four larvae per stem, early harvest is recommended.”

More information on alfalfa weevil can be found in an OSU Extension fact sheet at http://ohioline.osu.edu/ent-fact/pdf/0032.pdf. Information on insecticides that can be used for alfalfa weevil can be found at http://go.osu.edu/alfalfaweevil.