Scout Now For Alfalfa Weevils In Ohio

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Unusually warm weather is pushing up the start of the alfalfa-weevil scouting season in parts of Ohio – to now, says Ron Hammond, entomologist with Ohio State University (OSU) Extension.

Ordinarily, Ohio growers wouldn’t need to be concerned about alfalfa weevils for another 10 days to three weeks, says Hammond. This year, though, heat-unit accumulations for weevils were above 250 in the southern part of the state and 200 in central Ohio as of early last week.

He recommends the following scouting procedure:

• Collect a series of three, 10-stem samples selected randomly from various locations in a field and place the stem tips in a bucket.
• Shake the bucket vigorously to dislodge the late third- and fourth-instar larvae, which cause most of the foliar injury. Inspect the stem tips closely to detect the early first- and second-instar larvae.
• Count the larvae in the bucket and record the height of the alfalfa.

The economic threshold for treatment is based on the number of larvae per stem, their size and the alfalfa’s height. A rescue treatment is recommended if you find one or more large larvae/stem on alfalfa less than 12” tall. For alfalfa 12-16” tall, the action threshold is two to four larvae/stem (depending on the crop’s vigor). At 16” or taller, early harvest is recommended at four or more larvae/stem.

For more details, see this fact sheet from OSU Extension.