May 18, 2017
Check Alfalfa for Weevil Damage
While everyone has been concentrating on getting corn and soybeans planted, don’t forget about alfalfa. There have been a few scattered reports of alfalfa weevil in alfalfa fields. I haven’t heard any reports on clover leaf weevils.
Both the alfalfa and clover leaf weevils feed on first cutting alfalfa as larvae, and regrowth after the first cutting as adults and sometimes larvae. While research in northeast Nebraska has shown that clover leaf weevil larva feeding does not cause yield reduction to first cutting alfalfa, alfalfa weevil feeding can cause severe losses to yield and quality of the first cutting. This is why it's important to correctly identify the type of weevil feeding causing damage.
Here are some guidelines to help you distinguish between these two pests.
• Overwinter primarily as adults
• Adults are brown with a dark brown stripe halfway down the back, and 3/16 inch long
• Larvae prefer to feed on tips
• Larvae remain on the plant most of the time.
• Larvae have black heads
• Adults leave fields in June
Clover Leaf Weevils:
• Overwinter primarily as larvae
• Adults are dark brown, pitted light brown underneath, and over 1/4 inch long
• Larvae feed anywhere on plant
• Most larvae are in soil or debris during daytime hours
• Larvae have brown heads
• Adults may remain in fields
Clover leaf weevils are occasionally a problem, but are vulnerable to fungus disease. The wet (too wet) conditions this spring make it unlikely that they will be a problem this year. To scout for clover leaf weevil, look in debris around the crowns. Scratching in the soil around the crowns and counting the number of larvae found per crown will help give a better idea of clover leaf weevil infestation. Their brown heads will help distinguish them from the black-headed alfalfa weevil.
Most alfalfa weevils overwinter as adults, become active as temperatures increase, and lay eggs which hatch out as larvae and begin feeding in alfalfa. While alfalfa weevil damage has been spotty over the past few years, the potential for damage always exists.
It is essential to monitor fields for alfalfa weevil feeding now. Damage consists of small holes and interveinal feeding on the newest leaflets near the stem tips. The larvae are small (1/16 to 3/8 inch long) and pale yellowish green, becoming a darker green when larger. These legless worms have black heads and a white stripe the length of the back. The alfalfa weevil larvae spend nearly all their time on the plant. They curl into a C-shape when disturbed.
If larvae are present, randomly select at least five sampling sites from across the entire field. At each site, gently pick or cut at least 10 alfalfa stems at ground level. Shake the larvae off the stems by beating the stems against the side of a five-gallon bucket. A white plastic bucket makes it easier to see and count the larvae. Count the larvae and determine the average number of larvae per stem. An average of three larvae per stem would suggest you manage them now.
At this time and stage of growth, it is probably more economical to take an early cutting rather than to spray the field. Then monitor the regrowth carefully to see if feeding is holding back the regrowth. If it is, then treat the field to control larvae that survived harvest and exposure to the elements after the hay was removed. Often they will die from the exposed conditions after harvest.
For more information on alfalfa weevil and clover leaf weevil control, contact your local Nebraska Extension office.