University of Illinois

No. 6/May 1, 1998

Status of Alfalfa Weevils

The cool weather has not advanced alfalfa weevil activity very much during the past couple of weeks. However, as you will note in Figure 2, heat units continue to accumulate at a relatively steady pace. In southern Illinois (anywhere south of I-70), about 120 to 150 heat units above a base temperature of 48 degrees F have accumulated since April 13. Based upon historical average spring temperatures, another 100 to 200 heat units will accumulate in northern and southern Illinois, respectively, during the next 2weeks (Figure 3).

Figure 2. Actual heat-unit accumulations (base 48 degrees F) from January 1 to April 27, 1998.

Figure 3. Projected heat-unit accumulations (base 48 degrees F) from January 1 to May 11, 1998.

To repeat some information presented in a previous issue of this Bulletin, I remind you that hatching of overwintering eggs usually occurs when 200 heat units accumulate, and we suggest that scouting should begin when 300 heat units accumulate. An early peak of third-stage larvae from overwintering eggs occurs after an accumulation of 325 heat units; a second major peak of third-stage larvae from spring-deposited eggs occurs after an accumulation of 575 heat units. With this information at hand, you can determine from Figure 2 that hatching of overwintering eggs likely has taken place throughout Illinois, scouting should be under way in virtually all counties, and alfalfa weevil larval activity could be evident anywhere south of I-80. In addition, a second peak of third-stage larvae is right around the corner in the southern tip of the state.

We have received very little information about alfalfa weevil activity, and we are not aware of any insecticide applications taking place. However, that does not mean control activities are not under way in some fields in southern Illinois. We would appreciate any information you might have about the presence or absence of alfalfa weevils in your neck of the woods and whether any growers in the area have sprayed or are considering spraying alfalfa fields. Insecticides suggested for control of alfalfa weevils were listed in issue no. 4 (April 17, 1998) of this Bulletin.

One more thought about alfalfa weevils. Conditions right now could promote the establishment and spread of a fungal organism, Zoophthora phytonomi, that occasionally has a major impact on populations of alfalfa weevils. We have witnessed this organism causing weevil population "crashes" over a 3- to 4-day period. Consequently, as you scout for alfalfa weevils, keep your senses tuned to the presence of diseased alfalfa weevil larvae, which usually first appear off color (slightly yellow) and then turn brown as they die. Dead weevil larvae often are observed curled around leaflets on the plants. If you observe these characteristic signs of disease infection, think twice before you make a decision to treat the field. Sometimes Mother Nature does a pretty good job of "controlling" alfalfa weevils.

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