University of Illinois

No. 5/April 25, 1997

Update on Alfalfa Insects

Very little alfalfa weevil activity has been reported this spring. Densities of the larvae in alfalfa fields have been quite low, and thus far we have received no reports of significant levels of injury. On April 15, Steve Roberts, research entomologist with the Illinois Natural History Survey, sampled a couple of alfalfa fields in St. Clair and Clinton counties and found an average of 0.27 and 0.4 larvae per sweep, respectively. Please note that these are numbers per sweep, not per stem. The treatment guideline is two to three larvae per stem, so the densities detected were far below an economically damaging level.

Growers, consultants, and dealers should continue to monitor alfalfa fields for signs of alfalfa weevil injury and the presence of larvae. Critical heat-unit accumulations (above a base temperature of 48 degrees F) from January 1 are as follows: 200--egg hatch; 325--early peak of third-stage larvae from overwintering eggs; and 575--second peak of third-stage larvae from spring-deposited eggs. Figure 1 shows the actual heat-unit accumulations from January 1 to April 21, and Figure 2 shows the projected heat-unit accumulations from January 1 to May 4. The limited activity in southern Illinois thus far this spring suggests that survival of overwintering eggs was not great. Consequently, alfalfa growers may want to be more aware of the time of occurrence of the peak of larvae from spring-laid eggs this year. If enough adults survived the winter and egg-laying conditions were suitable this spring, the peak of third-stage larvae from spring-deposited eggs could be more important. If this is the situation in southern Illinois, a slightly early harvest that does not sacrifice yield will likely be a more appropriate strategy than insecticides to control an economic infestation of alfalfa weevils. Because alfalfa weevils don't lay many eggs in the fall in northern Illinois, growers in northern counties typically contend with only one peak of third-stage larvae.

Figure 1. Actual heat-unit accumulation (base 48 degrees F) from January 1 to April 21, 1997.
Figure 2. Projected heat-unit accumulation (base 48 degrees F) from January 1 to May 4, 1997.

By the end of the first week in May, we should have a pretty good idea if alfalfa weevils will cause any economic problems in most of the state. Eggs should have hatched virtually everywhere in the state (see Figure 2), and time for the second peak of third-stage larvae will have occurred in the southern four tiers of counties. We'll keep you posted.

As an add-on note, Steve Roberts also found the first potato leafhopper of the season on April 15 in St. Clair County. He found only one leafhopper in 200 sweeps, but his find occurred on one of the earliest dates in recent years.

Kevin Steffey, Extension Entomology, (217)333-6652