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Status of Alfalfa Weevil Development

April 20, 2001
The recent cool weather slowed the development of alfalfa weevils, but it didn't stop them. Omar Koester, unit assistantcrop systems in Monroe and Randolph counties, and Dale Burmester, Gateway FS in Red Bud, have observed significant damage caused by alfalfa weevil larvae in Randolph County. The amount of skeletonization and numbers of larvae have exceeded "rule-of-thumb" thresholds (25% to 50% tip feeding and three or more larvae per stem) in some fields, and those fields have been treated with insecticides to control the infestation. Omar also reports that the hatch of larvae from spring-deposited eggs has begun; he has found first and third instars in the same fields.

Figure 1 shows accumulated degree-days (base 48°F) from January 1 through April 15, 2001. The cool temperatures slowed down the accumulation of degree-days, especially in central and northern Illinois. However, a comparison of accumulated degree-days through April 15 with those reported in last week's Bulletin (issue no. 3, April 13, 2001) through April 9 reveals that 75 to 100 degree-days accumulated in southern counties during the intervening week, enough to keep the weevils active and growing.

As we have stated at least a couple of times already, two distinct peaks of larval activity occur in southern Illinois, one from fall-deposited eggs and one from spring-deposited eggs. Hatching of overwintering eggs usually occurs when 200 degree-days (above a base temperature of 48°F) have accumulated beyond January 1. An early peak of third-stage larvae from overwintering eggs occurs after an accumulation of 325 degree-days, which has already occurred throughout the southern third of the state. A second major peak of third-stage larvae from spring-deposited eggs occurs after an accumulation of 575 degree-days, still some time away. Figure 2 shows projected accumulated degree-days (base 48°F) from January 1 through April 29 (actual data from January 1 through April 15, projected data from April 16 through April 29). Assuming relatively average temperatures within the next 2 weeks, we should expect alfalfa weevil activity throughout most of the state by then.

Experience thus far in southern counties suggests that alfalfa growers need to be very alert for alfalfa weevils right now. Assuming we are mostly done with cool temperatures, the larvae will start developing rapidly. An alfalfa weevil larva develops into the injurious third instar when about 140 degree-days have accumulated after hatching. Only 66 degree-days are required to complete the third instar. This development happens quickly when temperatures are warm.

Dale Burmester also observed some discolored alfalfa weevil larvae, which may represent the beginning of a disease epizootic in some fields. Alfalfa weevils infected with the fungus Zoophthora phytonomi become slightly yellow and then brown as they die. As you scout fields, keep your eyes peeled for discolored alfalfa weevil larvae. Under the right environmental conditions (usually cool and damp), the fungus can spread rapidly through the weevil population and cause it to "crash" within 3 to 4 days. A crashing alfalfa weevil population may indicate that an insecticide spray is not necessary. Refer to last week's Bulletin (issue no. 3, April 13, 2001) for tables of dynamic economic thresholds and a list of insecticides suggested for control of alfalfa weevils.

Keep us posted about alfalfa weevil activity in your area. Reports from the field help us to spread the word to others.--Kevin Steffey

Author: Kevin Steffey

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

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