Status of Alfalfa Weevil and Presence of Natural Enemies

May 12, 2000
As is often the case, alfalfa weevil infestations are not necessarily consistent. Several Extension educators in northern counties have reported that some fields are heavily infested, with numbers of larvae per stem well above threshold, and other fields have low numbers of larvae. For the most part, the incidence of economic infestations of alfalfa weevils is higher this year than in recent years. Insecticides have been warranted in many fields this year, and now producers are considering cutting a bit early in lieu of applying an insecticide. As long as yield is not significantly compromised, I strongly recommend early cutting as a viable substitute for an insecticide application. However, be certain that you watch for mature larvae and adults feeding on regrowing buds after harvest. When infestations are severe, this type of feeding injury can retard "green-up" in a field. Insecticide application to stubble alfalfa after the first cutting may be warranted when larvae and adults are feeding on more than 50% of the crowns and regrowth is prevented for 3 to 6 days. Insecticides registered for control of alfalfa weevil larvae were published in issue no. 1 (March 17, 2000) of this Bulletin. However, among this group, only four of them are labeled for control of adults: *Furadan 4F at 1 to 2 pt per acre; Imidan 70W at 1 to 1-1/3 lb per acre; Lorsban 4E at 1 to 2 pt per acre; and *Penncap-M at 2 to 3 pt per acre. (Use of products preceded with an asterisk is restricted to certified applicators.)

The ever-vigilant Matt Montgomery, Extension unit assistant in Sangamon County, has observed alfalfa weevil larvae infected with the fungus Zoophthora phytonomi, and he has found cocoons of the parasitic wasp Bathyplectes species. As I have stated previously, these natural enemies, especially the fungus, can have a dramatic influence on populations of alfalfa weevils. The recent rains have created an environment in which the fungus can develop quickly, possibly resulting in epizootics that wipe alfalfa weevils out within days. If you are assessing the situation in fields in the northern half of the state, watch for infected weevil larvae and the presence of parasitoids.

Alfalfa weevil larvae infected with the fungus Zoophthora phytonomi. (Photo courtesy of Matt Montgomery, University of Illinois Extension.)

Infection of alfalfa weevil larvae by Zoophthora phytonomi is favored by wet weather or high humidity. Infected larvae curl around leaves at the tops of plants and then turn brown. The authors of Pest Management of Alfalfa Insects in the Upper Midwest make an interesting point: "Studies have shown that outbreaks of this fungus can be initiated by early harvest of first cutting alfalfa. By cutting early, weevil larvae are concentrated within windrows of hay for several days where environmental conditions are ideal for the development and spread of the fungus."

Cocoons of the parasitic wasp Bathyplectes species. (Photo courtesy of Matt Montgomery, University of Illinois Extension.)

The two species of parasitic wasps that attack alfalfa weevil larvae throughout much of the Midwest are Bathyplectes anurus and B. curculionis. In a study conducted in Illinois in 1990 and 1991, we found a higher rate of parasitism by B. anurus than by B. curculionis. Rates of parasitism were higher in western counties than in eastern counties, ranging from 32.7 to 58.6% in western Illinois and 5.8 to 26.7% in eastern Illinois.

Both parasitic wasps are so small (1/8 inch) that they are difficult to find. The wasps deposit eggs in weevil larvae. Bathyplectes curculionis attacks first and second instars; B. anurus attacks second and third instars. The egg hatches and the parasitoid larva feeds in the weevil larva, killing it after the weevil completes its silken cocoon. The parasitoid then constructs a hard, brown, football-shaped cocoon (about 3.5 mm long) that has a raised, white, equatorial band. You may find the cocoons of the parasitoids inside weevil cocoons. The equatorial band of the B. anurus cocoon is raised, and the cocoon jumps several centimeters when disturbed, like a "Mexican jumping bean."

Choices for managing alfalfa weevils now include early cutting, application of an insecticide, or waiting for the natural enemies to do their thing. Keep in mind that broad-spectrum insecticides will reduce the impact of the natural enemies. Weigh the benefits and limitations before you decide to treat an alfalfa field to control weevils.--Kevin Steffey

Author: Kevin Steffey