Alfalfa Weevil Problems the Most Serious in Several Years

May 7, 1999

· Alfalfa weevil damage is severe in southwestern, south-central, and central counties.

· Densities of alfalfa weevil larvae are well above thresholds, and some spraying has been warranted.

· Alfalfa weevil larvae in southern counties are from both fall- and spring-deposited eggs; the larvae in central Illinois are probably from fall-deposited eggs.

· Maps of heat-unit accumulations and projected heat-unit accumulations have been updated.

Reports over the past two weeks suggest that population densities of alfalfa weevils are higher than they have been in several years. We have not had many difficulties with alfalfa weevils for quite some time, especially in northern counties, and I and other entomologists in Illinois have speculated that natural enemies (parasitic wasps, fungus disease) have suppressed weevil populations below economic levels. However, despite this speculation, alfalfa weevils are making their presence known this spring, at least in southwestern, south-central, and central counties.

Omar Koester, Extension Unit Assistant/Crop Systems in Randolph County, has been reporting that growers and applicators are spraying alfalfa fields to control alfalfa weevils in southwestern counties. Bill Brink, Extension Educator/Crop Systems at the Springfield Extension Center, has observed "tremendous numbers of alfalfa weevils, the worst [he's] seen in a long time." Bill has found as many as six to seven larvae per plant with evidence of defoliation on 50 to 75 percent of the plants. This intensity of alfalfa weevil damage obviously is well above economic levels. These reports suggest that if you have not already scouted for weevils in alfalfa, you better get to it.

Heat-unit accumulations to date (Figure 2) suggest that these alfalfa weevils are from eggs that were deposited last fall (peak larval activity from fall-deposited eggs occurs when 325 heat units accumulate above the base temperature of 48°F). I learned last week that some folks in southwestern counties were finding larvae of all sizes, suggesting that larvae also had hatched from spring-deposited eggs. Their peak activity will occur when about 575 heat units accumulate above the base temperature of 48°F, which likely will occur in southwestern Illinois at least by the beginning of the week of May 10. Figure 3 shows the projected heat-unit accumulations (base 48°F) from January 1 to May 16, 1999. Obviously by mid-May, most alfalfa weevil activity will have run its course in southern counties, but we'll still have plenty of fun with them in central and northern counties.

We are not certain how widespread the weevil problems are in alfalfa, so we'd like to hear from you. Any information you can provide from your neck of the woods would be most useful for everyone.

Dynamic economic thresholds for alfalfa weevils were printed in Table 1 of issue no. 5 (April 23, 1999) of the Bulletin. Insecticides suggested for control of alfalfa weevils and the harvest intervals associated with these insecticides were printed in last week's Bulletin (issue no. 6, April 30, 1999). Keep this information with you as you scout for and make decisions about whether to treat for alfalfa weevils. Also, keep a watchful eye on alfalfa weevil larvae that might be diseased with the fungus Zoophthora phytonomi (see issue no. 5 of the Bulletin). Natural enemies don't seem to be slowing weevils down this spring, but the impact of natural enemies, especially the fungus, can be realized in short periods of time.--Kevin Steffey

Author: Kevin Steffey