Alfalfa weevil life cycle
Most alfalfa weevils overwinter as adults, become active as temperatures increase and lay eggs. Some eggs may be laid in the stem during fall and successfully overwinter if the winter is not severe.

These eggs will hatch earlier than those laid in spring. This is most likely to occur in southern counties.

The seasonal occurrence of alfalfa weevils does not fit this general pattern in some areas in Nebraska. In the Panhandle and northern tier of counties there may be two flushes of weevil larvae in the spring.

In the last few years some areas of the state have received damage to regrowth after the first cutting due to a combination of late larval feeding and adult feeding. This is something to be aware of after the first cutting.

Two strains of alfalfa weevils exist in Nebraska (eastern and western). Larval numbers of the western strain peak one to three weeks after the eastern strain.

Historically, these two strains have overlapped in central Nebraska; however, recent data indicates a rapid eastern movement/detection of the western strain within the U.S. (as far east as Pennsylvania) and a potential displacement of the eastern strain.

While alfalfa weevil damage has been spotty in much of Nebraska over the past few years, the potential for damage always exists.

Even with the pressure of planting row crops, producers growing high-quality alfalfa hay should make time to monitor fields for weevils over the next few weeks.

Clover leaf weevils
Clover leaf weevils (CLW) are occasionally a problem, but are vulnerable to fungus disease. These pests haven’t been a problem since the late '80s to early '90s when spring rains were rare.

Dry conditions over the past several years in western Nebraska may have helped populations increase, although recent rains may have knocked populations down.

To scout for clover leaf weevil, look in the debris around the crowns during day. 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. (See Figure 1 and Table 1 for more information distinguishing the alfalfa weevil and the clover leaf weevil.

Both and alfalfa weevil 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.

Scouting
It is essential that fields be monitored for alfalfa weevil feeding now. Damage consists of small holes and interveinal feeding on the newest leaflets near the stem tips.

The larvae are a small (1/16 to 3/8 inch in length), 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.

Once the alfalfa is about 4 to 6 inches or so in height, take a net sample to establish whether or not weevils are present.

If they are, carefully cut some stems at ground level (30 to 50 per field, from various spots in the field) and shake the stems against the side of a 5 gallon bucket.

This will dislodge the weevils and make it easy to average the number of weevil larvae per stem.

Treatment thresholds
Charts and/or tables have been developed (see Figure 2) to aid decision-making on whether to control alfalfa weevils. These charts are guidelines based on available research and can fluctuate depending on growing conditions and variety.

Notice that these charts have lower price levels than what alfalfa is currently selling in Nebraska.

With prices varying considerable depending on quality and type of bales, start by using the $105 chart and adjust the levels somewhat according to the health, variety, and value of the crop, and when you expect to use or sell it.

At current prices counts averaging over 1 per stem will likely make it profitable to treat for alfalfa weevils.

Each chart has been developed for a different alfalfa value. To treat or re-sample depends on the average number of weevils per stem, the stem length, and the value of the alfalfa.

When alfalfa reaches a certain height, it may be more profitable to cut the alfalfa early rather than to treat, but cutting early also depends upon excellent drying conditions for curing hay.

Insecticides
A number of insecticides are registered to control alfalfa weevil larvae. They differ in their modes of action as well as pre-harvest interval.

Highly effective insecticides for alfalfa weevil control include those that are pyrethroids (active ingredient ends in ‘thrin’) and Steward. Pyrethrioid insecticides also can have detrimental effects on any beneficial insects present.  FG

—Excerpts from University of Nebraska CropWatch newsletter, April 2011

Keith Jarvi
Michael Rethwisch
Extension Educators
University of Nebraska
kjarvi1@unl.edu
mrethwisch2@unl.edu