University of Illinois

No. 6/May 2, 1997

Alfalfa Weevils Still Relatively Quiet

Although the cool temperatures we have experienced recently are slowing down the development of alfalfa weevils, the alfalfa is growing nicely. Because alfalfa grows faster than alfalfa weevils at lower temperatures, harvest time may arrive before the development of economic infestations of weevils when cool temperatures prevail. However, the onset of warmer temperatures could speed up weevil development quickly.

Although the densities of alfalfa weevil larvae have increased in some fields in southwestern Illinois, we still have received very few reports of weevil activity. During the latter part of the week of April 21, I learned that some fields of alfalfa in Monroe and Randolph counties had already been sprayed for control of alfalfa weevils; some had been sprayed in early April. Although an economic infestation was verified in at least one of the fields, we have reason to believe that several fields were sprayed before an economic infestation developed. More on this later.

Steve Roberts, a research entomologist with the Illinois Natural History Survey, sampled a field trial in St. Clair County during the latter part of the week of April 21 and found a maximum of 0.8 larva per stem, well below the threshold of 3 larvae per stem. The average density in the plot area was even lower--0.5 larva per stem. The alfalfa was 11.5 inches tall. Steve also visited a production field south of Belleville and detected about 25% tip-feeding injury.

Figure 1 shows the heat-unit accumulations above a base temperature of 48 degrees F from January 1 to April 28, 1997. If you compare these actual accumulations with those projected for April 28 in issue no. 4 (April 18) of this Bulletin, you'll note that we have accumulated approximately 100 fewer heat units above 48 degrees F than projected. Remember that the projections are based upon average temperature data gathered over approximately 40 years. Therefore, cooler-than-normal temperatures cause actual heat-unit accumulations to lag behind projected heat-unit accumulations. That having been said, again we offer projected heat-unit accumulations from January 1 to May 12 (Figure 2). These forecasts allow you to prepare for weevils in advance, but nothing beats scouting fields. Refer to previous issues of this Bulletin for critical heat-unit accumulations, that is, those that indicate critical times during the development of alfalfa weevil populations.

Figure 1. Actual heat-unit accumulations (base 48 degrees F) from January 1 to April 28, 1997.Figure 2. Projected heat-unit accumulations (base 48 degrees F) from January 1 to May 12, 1997.

For the past few years, we have become aware that some applicators convince alfalfa growers that preventive applications of insecticides for control of alfalfa fields is worthwhile. The reasoning is that if a residual insecticide (some registered insecticides provide control for 2 to 3 weeks) is applied early enough, a grower won't have to worry about the development of an economic infestation of alfalfa weevils. I still find it difficult to believe that people think like this. Odds seem favorable for few economic infestations of alfalfa weevils to develop this spring, so why would anyone want to spend money needlessly? Our opinion of preventive applications of insecticides in the absence of information about insect densities will not waver--we will not support this approach. For alfalfa weevil management, still the best approach is to scout fields and apply an insecticide only when the density of larvae reaches or exceeds three per stem. End of story.

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