Alfalfa Weevils

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Alfalfa weevils were produced in unusually large numbers in many alfalfa fields last year. The mild winter meant additional egg laying was likely during warm days and few fall- laid eggs died during the winter. All this has meant an unusually large number of Alfalfa Weevils are in our alfalfa fields this year.

Although weevil damage can be very severe, treatment is not required in all fields even during a heavy weevil damage year. The only sure way to determine if & when a field requires treatment is to sample the weevil population during the time that damage is occurring.

STEM COUNT DECISION METHOD:

Break off 30 to 50 alfalfa stems at ground level, take care not to dislodge the feeding larvae. Select the stems at random from across the field and shake individually into a deep- sided bucket. Count the stems, determine the average height, and count the larvae so the average number of larvae per stem can be calculated.

• If the alfalfa is 5 inches or shorter and there is less than 1 larvae per stem, no spray is needed and the field should be resampled in 7 days. If there are more than 2 weevils per stem and less than 5 inches tall, spray as soon as possible.
• If there are between 1 and 2 weevils per stem, resample in 3 to 5 days. You may need to spray.
• If the alfalfa is 17 inches tall, and there are 3 weevils per stem, you need to spray. If between 1 ½ and 3 weevils per stem resample in 3 to 5 days. If 17 inches tall and less than 1 ½ weevils per stem, no spray is needed at this time, keep watch.
• If the alfalfa is 20 inches tall and there are 3 or more weevils per stem, it may be best to cut the hay early instead of spraying, and then consider spraying the stubble to protect the regrowth.

If your alfalfa condition calls for a spray program go to "Alfalfa Weevil Control" for recommended controls.

Also see: MU XPLOR Guidesheets:  Crop Puplications (Forages)

Leon Stites, HLStites@oznet.ksu.edu
County Extension Agent, Agriculture
Leavenworth County, Kansas
Kansas State University Research and Extension