Alfalfa Weevils Develop Slowly, Steadily

April 30, 1999

· The cool temperatures have slowed alfalfa weevil development throughout much of the state.

· Densities of alfalfa weevil larvae have exceeded economic levels in southwestern and western counties.

· Some observers have found alfalfa weevil larvae that appear to be "sick."

· Suggested insecticides are provided, as well as some restrictions.

A comparison of heat-unit accumulations (base 48°F) to April 26, 1999 (Figure 2), with heat-unit accumulations on April 19, 1999 (Figure 1, Bulletin No. 5, April 23, 1999), reveals that we picked up only 100 heat units in southwestern counties and about 50 heat units elsewhere in the state. Under these cool conditions, alfalfa weevils won't develop quickly.

Nevertheless, the warmer temperatures predicted will move things along quite nicely. Figure 3 shows projected heat units (base 48°F) to May 10, 1999. By then, we should be able to observe alfalfa weevil activity in northern Illinois, and we'll be in the thick of it in central counties. Remember: alfalfa weevil larvae that develop from spring-deposited eggs reach peak levels when 575 heat units accumulate above the base temperature of 48°F.

Omar Koester, Extension Unit Assistant/Crop Systems in Randolph County, verified that some spraying of insecticides for control of alfalfa weevil larvae has been justified in southwestern Illinois. He found as many as six larvae per stem in one field, well above most economic thresholds that have been published. Mike Roegge, Extension Unit Educator/Crop Systems in Adams and Brown counties, sampled an average of about 3.5 weevil larvae per stem in a field in Adams County. Most of the larvae were small, 1/8 to 1/4 inch long. Jimmy Finger, Senior Technical Supportive Scientist with the Illinois Natural History Survey, examined some alfalfa at the Southern Illinois University Belleville Research Center on April 27; he found only 11 larvae per 20 stems, but he indicated that the amount of injury to the alfalfa seemed to be more than what could be accounted for by the small number of larvae he found. Both Mike Roegge and Omar Koester observed some larvae that appeared to be "sick," that is, rather yellow in appearance and somewhat slow moving. It is possible that some of the larvae are infected with Zoophthora phytonomi.

Be on the lookout for diseased alfalfa weevils. Refer to last week's issue of the Bulletin for a more complete description of alfalfa weevil larvae infected with the fungus Zoophthora phytonomi. During cool, wet conditions, this natural biological control agent could cause weevil populations to "crash" relatively quickly. If diseased weevil larvae are evident, don't rush to a decision to apply an insecticide. However, you should also remember that young alfalfa weevil larvae are not as bright green as more mature larvae; healthy, young larvae may appear slightly yellow. So don't rush to judgement if the larvae look "funny."

Refer to Table 1 in last week's issue of the Bulletin (no. 5, April 23, 1999) for modern economic thresholds based on alfalfa weevil larvae per stem, plant height, and value of the hay. The table was reprinted from Integrated Pest Management of Alfalfa Insects in the Upper Midwest, available from the Leopold Center for Sustainable Agriculture at Iowa State University. If the conditions are met, an insecticide should prevent economic loss. Table 2 shows a list of insecticides suggested for control of alfalfa weevil larvae.

If you make a decision to apply an insecticide to alfalfa, you should know the harvest intervals. Table 3 shows the number of days that should elapse between application of the selected insecticide and harvest or grazing. Ranges in these harvest intervals reveal different rates of applications or different uses for the alfalfa.

Refer to product labels for guidelines for application and precautions regarding use. Never spray blooming alfalfa. And during this busy time of year, take every precaution to avoid drift of the insecticide.--Kevin Steffey

Author: Kevin Steffey