University of Illinois

No. 7/May 8, 1998

Heat-Unit Information for Alfalfa Weevils

Thus far, we have heard virtually nothing about alfalfa weevil activityaround the state. Although a few folks have told us about a little bitof feeding injury here and there, we are not aware of any widespread infestations.Nor are we aware of any insecticide applications being made, even in southernIllinois. Nevertheless, alfalfa growers and their advisers should continueto monitor for alfalfa weevil larvae as the spring progresses.

 Descriptions of alfalfa weevil larvae and the injury they causeand discussions about scouting techniques, economic thresholds, and suggestedinsecticides have been provided in previous issues of this Bulletin,so we won't use up additional space in this issue. Heat-unit updates areprovided in Figure 2 (actual heat-unit accumulation, base 48 degreesF, January 1 to May 4, 1998) and Figure 3 (projected heat-unit accumulation,base 48 degrees F, January 1 to May 18, 1998). Remember, hatching of overwinteringeggs usually occurs when 200 heat units accumulate, and we suggest thatscouting should begin when 300 heat units accumulate. An early peak ofthird-stage larvae from overwintering eggs occurs after an accumulationof 325 heat units; a second major peak of thirdstage larvae from spring-depositedeggs occurs after an accumulation of 575 heat units.

 Figure 2. Actual heat-unit accumulations (base 48 degrees F)from January 1 to May 4, 1998.


Figure 3. Projected heat unit accumulations (base 48 degrees F) fromJanuary 1 to May 18, 1998.


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