No. 2/April 4, 1997
Alfalfa Weevil Season Begins
Last year, Gail Snowdon, decision data specialist with Information Services in the College of ACES, sent a questionnaire about this Bulletin to 403 randomly selected subscribers. We were pleased to receive 301 responses (a response rate of 74%), and we appreciated the positive and constructive comments. However, one individual offered a comment that I'd like to share with you: "Less material on alfalfa weevils. From reading the newsletter, one might think they are the single greatest insect pest in existence." I had to chuckle because the observation is somewhat correct. From issue no. 1 (March 22) through issue no. 11 (June 11), we published 11 articles about alfalfa weevils. For alfalfa growers and those who service alfalfa growers, this information is important and timely. However, for many of the rest of you, the amount of information published may grow a bit tiresome. Because alfalfa weevils are the first field crop insects to become active every spring, I guess we entomologists are trying to release our pent-up energy for the beginning of the growing season. This year we will try to avoid overkill yet still provide useful information for the management of this key pest of alfalfa.
Did alfalfa weevils survive this past winter? Most of you know that alfalfa weevils overwinter in alfalfa fields both as eggs in alfalfa stems and as adults under debris in southern Illinois. The females begin depositing their eggs in the fall and occasionally on warm days during the winter. In northern Illinois, where egg laying usually does not begin until the spring, alfalfa weevils overwinter only as adults. Consequently, our first observations of weevil activity begin in the southern counties.
Steve Roberts, research entomologist with the Illinois Natural History Survey, looked for alfalfa weevil eggs in samples of alfalfa stems he collected from the Belleville and Carlyle areas in St. Clair and Clinton counties, respectively. What he found was somewhat promising. He found 5 eggs per square foot and 7 to 8 eggs per square foot from small plots near Belleville and Carlyle, respectively. These numbers are quite low, suggesting that alfalfa weevils may not have survived the winter very well or may not have been present in large numbers last fall when egg laying commenced. Whatever the explanation, we are beginning this season with relatively low numbers of alfalfa weevils. Keep in mind, however, that these samples were taken from small plots in only two areas. Densities elsewhere could be quite different. If more information becomes available, we'll keep you informed.
What alfalfa weevil activity can we expect soon? As you know, alfalfa weevils, like all insects, are coldblooded, so we can keep track of temperature accumulations to predict biological activity. Alfalfa weevils resume development at 48 degrees F, so we can accumulate heat units above 48 degrees F to predict their development. Figure 3 shows the accumulated heat units (above the base development temperature of 48 degrees F) from January 1 through March 31, 1997. Alfalfa weevil eggs usually begin hatching when about 200 heat units have accumulated since January 1. Consequently, egg hatch likely was under way in late March in the southern one-third of Illinois.
Figure 4 shows projected heat units from January 1 through April 15, 1997. By mid-April, egg hatch could be under way anywhere in the southern half of Illinois. An early peak of thirdstage larvae from overwintering eggs usually occurs after an accumulation of 325 heat units. A second major peak of third-stage larvae from spring-deposited eggs occurs after an accumulation of 575 heat units. Consequently, defoliation injury by alfalfa weevil larvae could be evident in counties south of St. Louis by mid-April.
Last year we offered some information from a research article published by some folks from the University of Nebraska. They correlated calendar dates with heat-unit accumulations to predict alfalfa weevil egg hatch. For example, for the three tiers of southern counties in Illinois, there is a 75% probability that egg hatch will occur between April 5 (200 heat units above 48 degrees F after January 1) and April 17 (300 heat units). For counties south of I70 and above the southern three tiers, there is a 75% probability that egg hatch will occur between April 13 (200 heat units) and April 23 (300 heat units). However, differences in fall egg-laying patterns in different areas lead to differences in the timing of egg hatch and development of larval populations. Fall egg laying in Illinois is dependent upon climatic conditions.
Begin scouting now in southern Illinois. It's time to start looking for evidence of small alfalfa weevil larvae feeding on tip leaves right now in counties south of St. Louis. The first-stage larvae are pale yellow-green and can be found within folded terminal leaves. The larva lacks legs, and its head is black. Evidence of their feeding is small, round holes (pinholes) in the tip leaves. Although this feeding is not economic, it indicates the presence of weevils. As the larvae grow, they consume more foliate and cause characteristic skeletonization of the leaves.
We'll provide information about control of alfalfa weevils in next week's issue of the Bulletin. If you encounter any alfalfa weevil activity as you scout the fields, please let us know. If a phone call is inconvenient, drop us a message via e-mail (firstname.lastname@example.org).
Kevin Steffey, Extension Entomology, (217)333-6652