Update on Alfalfa Weevils

April 14, 2000
Some reports from Kentucky, Indiana, and southern Illinois suggest that alfal-fa weevil damage is more obvious this year than it has been in recent years. We have heard that application of in-secticides to control alfalfa weevil lar-vae is ongoing in some fields in Kentucky. Numbers of alfalfa weevils and amounts of damage have exceeded economic levels in some fields in southern Illinois and southern Indiana.

The up-and-down temperatures we have experienced recently have slowed down the accumulation of heat units for alfalfa weevil development. Actual heat-unit accumulations from January 1 through April 10 are shown in Figure 3, which reveals that approximately only 50 heat units have accumulated since March 31 in most areas of Illinois. Figure 4 shows the projected heat-unit accumulations from January 1 through April 21. The information in this figure suggests that, if temperatures remain relatively "average," alfalfa weevils will be active throughout the state by the end of the month, and the peak of third-stage larvae from spring-deposited eggs (575 heat units) will be imminent in southern counties.

Last year, a dynamic decision-making table was published in Integrated Pest Management of Alfalfa Insects in the Upper Midwest, written by entomologists and agronomists from Oklahoma State University, Iowa State University, the University of Wisconsin, and the University of Minnesota. The eco-nomic thresholds for alfalfa weevil are based on numbers of larvae per stem, plant height, and value of the alfalfa; I have reproduced the table in Table 1. This table is more informative than the rule-of-thumb threshold of two to three alfalfa weevil larvae per stem and 25% to 50% skeletonization.

Everyone in southern counties also should begin watching for signs of natural control of alfalfa weevil larvae. A fungus, Zoophthora phytonomi, and parasitic wasps in the genus Bathyplectes can suppress alfalfa weevil populations when environmental conditions are favorable. In a near-future issue of the Bulletin, we will offer more information about these important natural enemies. Their combined impact on alfalfa weevils occasionally precludes the need to spray insecticides.--Kevin Steffey

Author: Kevin Steffey