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Alfalfa Weevils Are Still Busy

April 27, 2001
In last week's issue of the Bulletin (issue no. 4, April 20, 2001), I indicated that some people were applying insecticides to control economic infestations of alfalfa weevil larvae in southern Illinois. According to a report from Kevin Black (Growmark) and Alan Mosler (agronomist; Twin County Service Company; Perry, Franklin, Jackson, and Williamson counties), population densities of weevils have "exploded" in many fields, with economic damage obvious. However, so many producers are busy planting corn and so many applicators are spraying herbicides that the alfalfa might be ignored.

This is fair warning for alfalfa producers and others in central and northern Illinois. We often learn our earliest lessons in southern counties, providing ample opportunity to be prepared elsewhere. Economic infestations of alfalfa weevils are likely in central Illinois soon. John Shaw, coordinator of the Insect Management and Insecticide Evaluation Program, Illinois Natural History Survey, reported finding a mixture of second- and third-instar alfalfa weevils in a plot area in Champaign County on April 23. He also found adults that are still laying eggs, so the density of larvae in the plot area will increase. Damage to the alfalfa was already evident, so John intends to apply insecticides for his efficacy trial soon.

The topsy-turvy weather is not allowing alfalfa weevils to develop at the same rate as they would if temperatures were consistently warm. Figure 2 shows actual degree-days (base 48°F) that have accumulated from January 1 through April 22. Approximately 100 degree-days have accumulated within the past week in the southern half of the state, but only about 50 degree-days accumulated during the same period in northern Illinois. Following are the numbers of degree-days above 48°F required for an alfalfa weevil to complete development through each of its four instars: 71 degree-days for first instar, 67 degree-days for second instar, 66 degree-days for third instar, and 91 degree-days for fourth instar. From egg hatch to pupation requires approximately 295 degree-days.

Figure 3 shows the projected accumulation of degree-days (base 48°F) from January 1 through May 6, 2001 (actual data from January 1 through April 22, projected data from April 23 through May 6). If temperatures between now and May 6 are equivalent to the 40-year averages, we should expect the peak of third instars to have occurred throughout most of the southern third of the state. Damage to the alfalfa will be evident and weevil densities could be near economic levels in northern Illinois by May 6.

Insecticides suggested for control of alfalfa weevils were listed in issue no. 3 (April 13, 2001) of the Bulletin. With insecticide applications ongoing, it's appropriate at this time to provide harvest intervals for these products. Table 1 shows the products suggested for control of alfalfa weevil larvae, recommended rates of application, and numbers of days between application of a given product and harvest of hay. When alfalfa weevils reach economic levels, always keep harvest intervals in mind before selecting the product to be applied.

Here's another reminder to look for discolored alfalfa weevil larvae when scouting alfalfa fields. Larger larvae that appear yellow probably are infected with the fungus Zoophthora phytonomi. As the infection takes hold, the larvae turn brown and die. Spores from dead larvae are "showered" from the cadavers, and these spores infect other larvae. If the infection rate is high and environmental conditions are ideal, the disease spreads through the population rapidly and can reduce weevil densities to below economic levels within a matter of days. The presence of many infected and dead larvae may offset the need for an insecticide.

For several reasons, scouting alfalfa fields early in the season can pay major dividends. Avoid surprises, but don't treat fields in which alfalfa weevils are causing only light to moderate injury.--Kevin Steffey

Author: Kevin Steffey

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

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