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Alfalfa Weevils Creating Headaches in Many Fields

May 3, 2002
By now it's no secret that alfalfa weevils have caused more damage to alfalfa this year than in several recent years. They survived the mild winter conditions quite well, and they seemed to come on early and strong in most areas. Alfalfa weevils have been active in southern Illinois for a few weeks. During the week of April 22, alfalfa weevils captured a great deal of attention as far north as I-80. Entomologists at Purdue University reported high densities of alfalfa weevils in west-central and northwestern Indiana during the same week.

On April 24, Kevin Black, with Growmark, reported that 50 to 100% of alfalfa plants were infested with alfalfa weevils in Ford County. Kevin Foreman, with Crop Production Services, reported that spraying for alfalfa weevil control began in Henderson County on April 25. It's apparent from these reports that scouting for alfalfa weevils in central counties should have been initiated well before now. So, I strongly encourage people in northern counties to look for weevil activity in their alfalfa fields.

The maps of actual degree-day accumulations (Figure 1) and projected degree-day accumulations (Figure 2) pretty much tell the story. Based on actual degree-day accumulations, alfalfa weevils should be evident throughout the state, although the larvae should be rather small in northern counties. In southern Illinois, the second peak of larvae from spring-deposited eggs (about 575 degree-days [base 48 deg F] from January 1) should be occurring. Omar Koester, Monroe/Randolph Extension unit assistant in crop systems, observed newly hatched larvae on April 30.

Omar Koester also reported that, in alfalfa fields that were sprayed too late (after the weevils had caused extensive damage) or not sprayed at all, the first cutting is virtually lost. In these fields, he noticed new growth from the crowns. The absence of foliage and the penetration of sunlight have initiated this new growth. In fields with little foliage left, chickweed has "taken over" if herbicides were not used.

Shortly after I had submitted my article about alfalfa weevils to the editor last week, I received a report (April 24) from Doug Kirkbride, M & J Fertilizer, that alfalfa weevil larvae were dying in some alfalfa fields in Christian County. He indicated that the larvae were brown and that the population was "crashing." It's probable that the larvae Doug found were infected by the fungus Zoophthora phytonomi, which I discussed in last week's issue (no. 5, April 26, 2002) of the Bulletin. Although I have not received other, similar reports about Z. phytonomi, I encourage you to keep your eyes peeled. The cool, wet weather is ideal for this pathogen, and an epizootic can overcome alfalfa weevil populations very quickly.

Scouting tips, thresholds, and suggested insecticides have been discussed thoroughly in previous issues of the Bulletin. However, I haven't mentioned that an alternative to applying insecticides is early harvest, which is almost as effective at reducing numbers of alfalfa weevil larvae as some insecticides. If a producer intends to harvest early to avoid using an insecticide, make certain that yield and quality of the alfalfa are not compromised.--Kevin Steffey

Author: Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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