It’s time to scout fields for alfalfa weevils

If you grow alfalfa, it’s time to begin scouting your fields for alfalfa weevils.

Ron Hammond, an Ohio State University research entomologist with the Ohio Agricultural Research and Development Center, said that southern Ohio has warmed up enough to where alfalfa weevil feeding will become more prevalent. Growers in central and northern Ohioshould begin scouting their fields next week.

“Alfalfa weevil feeding is tied to temperatures. The need for scouting is especially true in southern counties where heat unit accumulation has reached the 300 heat units needed for egg hatch and beginning feeding,” said Hammond. “Remember that fields that have a south-facing slope tend to warm up sooner and need to be checked for weevil earlier.

To effectively scout alfalfa fields, entomologists recommend that growers collect a series of three 10-stem randomly selected samples from various locations in a field. Place the stems in a bucket and vigorously shake them, counting the number of alfalfa larvae that fall into the bucket. In addition, the height of the alfalfa should be recorded.

“Economic threshold is based on the number of larvae per stem, the size of the larvae and the height of the alfalfa,” said Hammond. “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. 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.”

The adult alfalfa weevil is a small, brown, snout-nosed beetle with a dark stripe down its back. The alfalfa weevil larva is green with a black head and a white stripe down its back. The larvae develop through four stages, or instars. Larvae that are in their 3rd or 4th instar cause the most foliar injury. First cuttings of alfalfa are at the highest risk for defoliation damage.

“Fields that are severely defoliated are left with a brown or bronze appearance,” said Hammond. “Once you see it you don’t forget it.”

The alfalfa weevil is controlled naturally by parasitoids, beneficial species that prey on the weevil and help keep its populations in check. In cases of high populations, the alfalfa weevil can be controlled with insecticides.

For more information on the alfalfa weevil, how to scout for it and how to control it, consult Ohio State’s Ohioline fact sheet: http://ohioline.osu.edu/ent-fact/0032.html.

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