Cooperative
Extension
Service


University of Illinois
at
Urbana-Champaign


No. 4/April 17, 1998

Scout for Alfalfa Weevils in the Southern Half of Illinois

As of April 13, at least 200 heat units had accumulated from January 1 above a base temperature of 48 degrees F (Figure 2) in all counties south of a line from southern Adams County on the west side of the state to southern Vermilion County on the east side of the state. An early peak of third-stage larvae from overwintering eggs should have occurred in any area where accumulated heat units have exceeded 325 (probably anywhere south of Salem in Marion County, Figure 2). By April 27, enough heat units will have accumulated from April 13 to exceed 200 statewide (Figure 3). Figure 3 projects accumulated heat units (above 48 degrees F) from April 13 to April 27, 1998, not total heat units accumulated from January 1. To derive the total heat-unit accumulation from January 1, add the numbers in Figure 3 to the numbers in Figure 2.

Figure 2. Actual heat-unit accumulation (base 48 degrees F) January 1 to April 13, 1998.

Figure 3. Projected heat-unit accumlation (base 48 degrees F) April 13 to April 27, 1998.

Reports from entomologists at the University of Kentucky and at Purdue University (southern Indiana) indicate that feeding injury by alfalfa weevil larvae is evident. Reports from Pioneer Hi-Bred International agronomists in southern Illinois also indicate that alfalfa weevils are active in southern counties. Thus far, the level of injury observed has been quite low.

If you have not already done so, scouting for this key pest in alfalfa should commence immediately in any alfalfa field south of I-70. We suggest scouting in a U- or M-shaped pattern, or any other pattern that allows you to obtain a representative sample from the field. To avoid biasing what you find, select alfalfa stems randomly when you are looking for injury and larvae. Selecting only injured plants will not provide an accurate representation of the average level of injury in the field.

Select at least 30 stems along your sampling route, measure the height of the stems, and count the number of alfalfa weevil larvae per stem. An easy way to do this is to break off the selected stems at their base and place them tipside down in a plastic bucket. After all stems have been collected, the weevil larvae can be shaken free by gently beating the stems against the sides of the bucket. Also, a subsample of 10 of the 30 stems can be measured. An insecticide application might be justified if 25 to 50 percent of the tips are being skeletonized and there are three or more larvae per stem. The range in percentage skeletonization is based upon stem height; taller, more rapidly growing alfalfa usually can tolerate more weevil injury than shorter plants with less foliage. A threshold created by a former researcher at Purdue University suggests that a long-residual insecticide would be warranted when 400 heat units have accumulated (from January 1, above a base of 48 degrees F), the alfalfa is 9 or more inches tall, and 50 percent of the stem tips have been fed upon.

Insecticides suggested for control of alfalfa weevil larvae include *Ambush 2E at 12.8 oz per acre, *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, *Pounce 3.2EC at 8 oz per acre, and *Warrior 1EC (recently labeled for use on alfalfa) at 2.56 to 3.84 oz per acre. Products preceded with an asterisk are restricted-use insecticides. In our efficacy trial in 1997, Baythroid, Furadan, and Warrior all provided at least 90 percent control for 14 days. If you decide an insecticide is necessary for control of alfalfa weevils, please carefully abide by all label directions and precautions.

Kevin Steffey, (ksteffey@uiuc.edu), Extension Entomology, (217)333-6652