Alfalfa Weevil Situation

April 21, 2000
The continuing cool weather has slowed down development of alfalfa weevil larvae. According to Figure 1, only 50 heat units above a base temperature of 48°F accumulated from April 10 (Figure 3 in last week's issue of the Bulletin, no. 3, April 14, 2000) through April 16, 2000. The heat-unit accumulations presented in Figure 1 suggest that alfalfa weevil larvae should be found in fields in northern Illinois right now. However, reports of weevil activity from northern counties have been few and far between thus far. On the other hand, we are aware that weevil densities in alfalfa fields in southern Illinois, southern Indiana, and Missouri have exceeded economic thresholds, and some insecticide applications have occurred. Control soon may be warranted in alfalfa fields in central Illinois. Refer to Table 1 in issue no. 1, March 17, 2000, of the Bulletin for a list of insecticides suggested for control of alfalfa weevil larvae.

Projected heat-unit accumulations through April 30, 2000, are presented in Figure 2. Again, because of the recent cool weather, projected heat-unit accumulations changed by only 50 heat units. By the end of the month, peak levels of alfalfa weevils from spring-deposited eggs will be present in the southern quarter of the state.

Healthy alfalfa weevil larva (center) and larvae infected with Zoophthora phytonomi (note discoloration).

Here's another reminder to watch for alfalfa weevil larvae infected with the fungus Zoophthora phytonomi. Recent rains might encourage some epizootics of this disease, which can wipe out an alfalfa weevil population within a field in a relatively short time. Look for slow-moving larvae that do not have the bright green color of healthy alfalfa weevils. Weevil larvae infected with the fungus are yellow or tan. When they die, they turn dark brown. If enough larvae are infected within a field, a dying population of alfalfa weevils might not need an insecticide application.--Kevin Steffey

Author: Kevin Steffey