University of Illinois

No. 4/April 18, 1997

All's Quiet on the Weevil Front

Although alfalfa weevil activity should be evident in alfalfa fields anywhere south of a line from Pike County in western Illinois to Crawford County on the eastern side of the state, we have received no reports of injury caused by the larvae. Alfalfa weevil eggs hatch when about 200 heat units above a base temperature of 48°F have accumulated since January 1. Figure 1 (map provided by Steve Hollinger of the Illinois State Water Survey) shows the accumulation of heat units (above a base temperature of 48°F) from January 1 to April 14, 1997. The warmer temperatures during the week of April 14 suggest that by the time you receive this Bulletin, egg hatch will be occurring in almost any field in the southern two-thirds of the state. If enough larvae are present, feeding injury caused by alfalfa weevils should be evident in southern Illinois. An early peak of third-stage larvae (325 heat units above 48°F) should have occurred by April 14 in the southern two tiers of counties.

 Figure 1. Actual heat-unit accumulation (base 48 degrees F) from January1 to April 14, 1997.

Figure 2 projects heat-unit accumulations above 48°F from January 1 to April 28, 1997. The projections suggest that alfalfa weevil eggs should be hatching by the end of the month as far north as Whiteside and Will counties. The first peak of third-stage larvae could occur as far north as Adams (west) and Clark (east) counties by April 28. The second major peak of third-stage larvae from spring-deposited eggs (575 heat units above 48°F) could occur by early May in extreme southern Illinois.

 Figure 2. Projected heat-unit accumulation (base 48 degrees F) fromJanuary 1 to April 28, 1997.

Scouting for alfalfa weevil larvae and signs of their feeding injury should be under way throughout the southern half of Illinois. Concentrate your initial efforts on fields with south-facing slopes or lighter soils that warm up more quickly in the spring. As indicated in a previous issue of this Bulletin, small, yellow-green larvae feeding in folded terminal leaves cause pinholes. As the larvae grow and consume more leaf tissue, they skeletonize the leaves. Because injured leaves dry quickly, severely injured areas of a field appear frosted when viewed from a distance. An older larva is bright green, with a white stripe along the center of the back and a black head capsule. A mature larva is about 3/8 inch long.

We suggest scouting in a U-shaped pattern that covers the entire field. Collect 30 alfalfa stems evenly distributed along your route. Select the stems randomly to avoid collecting only those with noticeable injury. Grab each stem at the base, gently break it off, and place it top down in a plastic bucket. After all stems have been collected, dislodge the weevil larvae by beating a few stems at a time into the bucket. Counting the larvae dislodged and dividing by 30 provides an estimate of the number of larvae per stem. Also, estimate the percent of defoliation for the plants sampled. An insecticide treatment may be warranted when you find an average of three or more larvae per stem and 25 to 50% skeletonization. The range in percent of skeletonization reflects that larger, more vigorous plants can compensate for more defoliation.

Suggested insecticides for control of alfalfa weevils include *Baythroid 2 at 1.6 to 2.8 oz per acre; *Furadan 4F at 1/2 to 1 pt per acre; Imidan 70WP at 1-1/3 lb per acre; Lorsban 4E at 1 to 2 pt per acre; *Penncap-M at 2 to 3 pt per acre; and *Pounce 3.2EC at 8 oz per acre. Use of products preceded with an asterisk is restricted to certified applicators only. Please follow directions and precautions on the respective labels; be aware of the harvest interval (number of days that must elapse between insecticide application and harvest) before selecting the insecticide and rate of application.

As alfalfa weevils continue to develop and we receive reports, we'll keep you updated about weevil densities, extent of damage, control activities, and the incidence of parasitoids and a fungal disease. This information should be useful for making decisions about managing alfalfa weevils this spring.

Kevin Steffey, Extension Entomology, (217)333-6652