Scouting advised for alfalfa weevil

By Robert WrightExtension Entomologist | Julie Peterson - Extension Entomologist | Jeff Bradshaw - Professor of Entomology

Alfalfa weevil eggs have started hatching in southern Nebraska. As temperatures warm up, expect to see alfalfa weevil larvae throughout southern Nebraska and slightly later, in northern Nebraska.

The larvae of alfalfa weevils feed on first cutting alfalfa as larvae, and adults (and sometimes larvae) feed on the regrowth after the first cutting.

Even with the pressure of planting row crops, it is essential that producers growing high quality alfalfa hay make time to monitor fields for weevils now and over the next few weeks. In the Panhandle and in the northern tier of counties, there may be two flushes of weevil larvae this spring, leading to regrowth damage after the first cutting. Observations indicate the cause may be due to significant survival of both adult and larval weevils.

Life Cycle

Most alfalfa weevils overwinter as adults, become active as temperatures increase and lay eggs. Some may lay eggs in the stem during fall and, if winter is not too severe, will successfully overwinter. These eggs will hatch earlier than those laid in spring. This is most likely to occur in southern counties.

In some areas of Nebraska, alfalfa weevils are not following this seasonal pattern. In the Panhandle and in the 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.

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, it is essential that producers growing high quality alfalfa hay make time to monitor fields for weevils over the next few weeks.

Scouting

Alfalfa weevil 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 (Figure 1). The alfalfa weevil larvae spend nearly all their time on the plant. They curl into a C-shape when disturbed. Once the alfalfa is high enough to use a sweep net, take a sample to establish whether weevils are present. If they are, 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 into a deep-sided bucket. Count the larvae and determine the average number of larvae per stem. Make sure to check for small larvae that may be enclosed in new, folded leaflets at the tips of the stems. Measure stem lengths and determine the average stem height. Use these averages in Table 1 to determine the appropriate action.

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