University of Illinois

No. 8/May 15, 1998

Current Insect Activity in Alfalfa Fields

Although we have received little input regarding insect activity in alfalfa fields this spring, a few reports have begun to trickle in. John Shaw, senior research specialist with the Center for Economic Entomology in the Illinois Natural History Survey, traveled to Belleville (St. Clair County) in southwestern Illinois to put out an alfalfa weevil insecticide trial. The field-plot area had an average of about two alfalfa weevil larvae per stem, and tip-feeding damage was evident. He also mentioned that all stages of larvae were present, and he believed that the second egg hatch from spring-deposited eggs had occurred. Figure 5 shows accumulated heat units (base 48 degrees F) from January 1 to May 11; and Figure 6shows projected heat-unit accumulations from January 1 to May 25. Use these maps with the critical heat-unit accumulations printed in several previous issues of this Bulletin to determine what alfalfa weevils might be doing in your area.

Figure 5. Actual heat-unit accumulations (base 48 degrees F) from January 1 to May 11, 1998.

Figure 6. Projected heat-unit accumulations (base 48 degrees F) from January 1 to May 25, 1998.

Rick Reed, an aerial applicator based in Mattoon (Coles County) in east-central Illinois, told me he has sprayed a few fields of alfalfa for control of alfalfa weevils this year, but not many. He also mentioned that the first cutting might be delayed due to the excessive rainfall we have experienced, so alfalfa could be exposed to weevil injury for a longer period of time. Several individuals in northwestern Illinois have told us that although alfalfa weevil larvae and a little injury are evident, fields with extensive damage are not common.

Maybe the most important recent report regarding alfalfa insects came from Matt Montgomery, unit crop systems educator in the Sangamon/Menard Unit. After inspecting several alfalfa fields in his area (west-southwest), Matt reported that he could find pea aphid "mummies" and alfalfa weevils that looked "sick." Both findings suggest that natural biological control is at work. Aphid "mummies" usually are light-colored or tan, globular "shells" of aphids that have been killed by a parasitic wasp. You usually can find a hole near the rear of the aphid where the parasitic wasp has emerged after developing inside the aphid as a larva.

More importantly, the "sick" alfalfa weevil larvae might be infected with the fungus Zoophthora phytonomi. Matt has sent us a sample of these weevil larvae, but we have not had time to have our insect pathologist verify that they are infected with the fungus. If the larvae are infected, the news is good. Zoophthora phytonomi is capable of causing epizootics in alfalfa weevil populations that cause weevil numbers to "crash" in a matter of 3 to 4 days. The most favored weather for these epizootics is cool and wet, just what we have experienced within the past few weeks. We will provide more information about this natural control agent next week and let you know if the sample was positive for fungal infection.

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