Slow Down for Alfalfa Weevils

April 16, 1999
If you looked at our map of actual heat-unit accumulations (base 48°F) in last week's Bulletin (issue no. 3, April 9, 1999) and thought, "We sure are accumulating heat units rapidly this spring," you weren't alone. We just discovered that last week's map was generated using a base temperature of 44°F. Consequently, the accumulated heat units depicted in the map were much more advanced than they would have been had we used the appropriate base temperature of 48°F. This week's map (Figure 1) shows the actual heat-unit accumulations, base 48°F, from January 1 through April 12, 1999. We regret the error.

Some second-instar alfalfa weevils have been found at a couple of sites in southern Illinois. Omar Koester, Unit Assistant/Crop Systems in Randolph County, reported finding second instars on April 6 in the southwestern area of the county. Feeding was evident but not serious. Second instars also have been found at the Southern Illinois University research station in Belleville in St. Clair County. These reports are in accord with the accumulation of heat units in southern Illinois. Based on Figure 1, we should expect that people will find third instars very soon anywhere south of the line from Madison County on the western side of the state to Gallatin County on the eastern border. Scouting should be under way anywhere south of a line from Henderson County in the west to Crawford County in the east.

Figure 2 shows the projected heat-unit accumulations (base 48°F) from January 1 through April 26, 1999. Remember, these projections are based on historic weather data; therefore, cooler-than-"normal" temperatures
will slow down the accumulation of heat units. It is likely that by the time we get to April 26, the actual accumulation of heat units will be less than what is projected in Figure 2. Nevertheless, by the end of April, we suspect that alfalfa growers throughout the state should be alert to the potential for alfalfa weevils. In southern Illinois, if densities of alfalfa weevils are high enough, insecticide applications could be ongoing by the end of the month. Refer to last week's Bulletin (issue no. 3, April 9, 1999) for a list of insecticides suggested for control of alfalfa weevils.

Regarding potential for the occurrence of economic infestations of alfalfa weevils this year, entomologists from the University of Kentucky indicated in an earlier issue of their newsletter that numbers of overwintering eggs in their state are quite low. Although egg laying has resumed this spring, a low density of overwintering eggs usually means that the potential for economic infestations early in the year is low. It is possible that by the time alfalfa weevils that develop from spring-deposited eggs reach peak levels (575 heat units above a base temperature of 48°F), the alfalfa will be tall enough to tolerate feeding injury.

I indicated that I would include information about the natural enemies of alfalfa weevils in this issue of the Bulletin. However, because cooler temperatures have slowed down weevil development, I will postpone discussion of these important biological control agents to coincide with their likely occurrence in the fields. In the not-too-distant future, look for more details about the parasitic wasps Bathyplectes curculionis and B. anurus and the fungal organism Zoophthora phytonomi. In the meantime, keep us posted.--Kevin Steffey

Author: Kevin Steffey