University of Nebraska-Lincoln Extension, Institute of Agriculture and Natural Resources

Alfalfa weevil adults hinder regrowth

Alfalfa weevil adults have prevented regrowth in alfalfa fields in northeast Nebraska.
Alfalfa weevil adults have prevented regrowth in a number of alfalfa fields in the Hartington area in Cedar County and needed to be sprayed. On Thursday, June 8, I scouted seven alfalfa fields with a local farmer. The fields were lacking moisture so the combination of lack of rain and weevil feeding kept a large proportion of three fields brown. Other than causing moisture stress on the plants, the lack of rainfall reduces the incidence of a fungal disease that often decimates weevil populations. Almost all regrowth problems after first cutting alfalfa are associated with below normal rainfall periods. Weevil feeding may still occur for seven or more days.

After harvest and hay removal, examine the stubble in several areas to determine if adults are present and feeding. Adult weevils are about 3/8 inch long, light brown with a darker brown stripe halfway down the back. They have a rigid, curved snout with tiny chewing mouthparts. Generally, if greenup isn=t underway in three to five days, especially following rain, there is a good chance insects may be involved. Variegated cutworm and clover leaf weevils also may be present and contribute to feeding damage. Once adult weevils are finished feeding, they enter a summer “estivation” period where they become inactive and do not cause further alfalfa damage.

Any one of several insecticides can be used to control of alfalfa weevil adults when treatment is justified. Some of the insecticides registered to control alfalfa weevil adults include: Baythroid, Furadan, Lorsban, Mustang Max, Penncap M, Proaxis and Warrior. Always read insecticide product labels thoroughly and follow all instructions, restrictions and precautions.

Chemical control may be adversely influenced by weather (hot, windy weather will cut down residual, heavy rainfall shortly after application may wash off the insecticide) non-uniform or inadequate coverage, and poor choice of insecticides. To evaluate the effectiveness of chemical controls, carefully examine new growth several days after treatment. If the new growth is not damaged, the control probably was effective.

Keith Jarvi
Extension Integrated Pest Management
Northeast REC, Norfolk

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Published by University of Nebraska-Lincoln Extension in the Institute of Agriculture and Natural Resources Cooperating with the counties and the U.S. Department of Agriculture.
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