Spraying for Control of Alfalfa Weevils in Central Illinois

April 28, 2000
According to Matt Montgomery, Extension unit assistant in Sangamon County, numbers of alfalfa weevil larvae in some alfalfa fields in central Illinois have exceeded economic levels, and insecticides are being applied to prevent more damage. Serious damage by alfalfa weevils is also being reported from counties in central Indiana. In many of the fields that Matt visited, the weevils' feeding activity was outpacing alfalfa growth, so early harvest of the alfalfa is not an option for control of the weevils. Accumulated heat units above a base temperature of 48°F from January 1 through April 24 (Figure 3) indicate that development of alfalfa weevils is somewhat ahead of schedule this year. A comparison of this year's accumulated heat units with last year's accumulated heat units at about this same time of year (Figure 2, issue no. 6, April 30, 1999) reveals that 50 to 100 more heat units have accumulated this year. Consequently, alfalfa growers throughout Illinois should be watching for alfalfa weevils.

Projected heat-unit accumulations from January 1 through May 8, 2000, are shown in Figure 4. Keep in mind that these projections are based on average temperatures from 40 years of data. If temperatures are above average during the next couple of weeks, the accumulation of heat units will occur at a faster pace. The bottom line is that everyone must remain watchful for early signs of injury by alfalfa weevils (pinholes in the upper leaves) and be prepared for more significant injury (skeletonization of the leaves).

Skeletonization of alfalfa leaves by alfalfa weevil larvae.

Dynamic economic thresholds for alfalfa weevils were printed in Table 1 in issue no. 3 (April 14, 2000) of the Bulletin. As harvest approaches for some growers in southern Illinois and for others soon throughout the rest of the state, one must choose an insecticide, if needed, based on the harvest interval, that is, the number of days between application of an insecticide and harvest. Table 2 indicates harvest intervals for the insecticides suggested for control of alfalfa weevils.

We have received no reports of natural control of alfalfa weevil larvae by the fungus Zoophthora phytonomi or the Bathyplectes parasitoids. Recent rains in some areas increase the likelihood for fungal infection, so stay alert for sickly-looking alfalfa weevil larvae. As we have indicated in the past, epizootics of this disease organism can wipe out a population of alfalfa weevils within 3 to 4 days. If you encounter infection by the fungus, let us know so that we can share the information with others.--Kevin Steffey

Author: Kevin Steffey