In last week's issue of the Bulletin (no. 2, April 6, 2001), I mentioned receiving a report about alfalfa weevil activity in southwestern Illinois. Alan Mosler, Twin County Service Company, and Kevin Black, Growmark, also observed first-instar alfalfa weevils and some tip feeding in Perry County on April 5. More recently on April 9, Matt Montgomery, Extension unit educatorcrop systems, Sangamon/Menard Extension Unit, also observed very young alfalfa weevil larvae and evidence of tip feeding. Undoubtedly the recent very warm weather has stirred things up in the world of alfalfa weevils.|
Alfalfa weevil adults begin laying eggs in the fall in southern Illinois, and both the eggs and adults overwinter. Therefore, larvae hatch relatively earlier in southern counties and begin feeding on the newly growing alfalfa before it attains much height. For this reason, economic infestations of alfalfa weevils occur more frequently in southern Illinois than in northern counties. However, during some years with "extended falls" (temperatures remain warmer than usual for a longer time), alfalfa weevil adults in central Illinois also lay eggs in the fall. Consequently, early-spring development of alfalfa weevil larvae in central counties is similar to development of alfalfa weevil larvae in southern counties.
Figure 3 shows degree-day accumulations (base 48°F) from January 1 through April 9, 2001. A comparison of degree-day accumulations this year with degree-day accumulations at a similar time last year (see issue no. 3, April 14, 2000) reveals that alfalfa weevil development this year is significantly behind the pace set last year.
Two distinct peaks of larval activity usually occur in southern Illinois, one from fall-deposited eggs and one from spring-deposited eggs. Hatching of overwintering eggs usually occurs when 200 degree-days (above a base temperature of 48°F) have accumulated beyond January 1, and we suggest that scouting should begin when between 250 and 300 degree-days have accumulated. An early peak of third-stage larvae from overwintering eggs occurs after an accumulation of 325 degree-days; a second major peak of third-stage larvae from spring-deposited eggs occurs after an accumulation of 575 degree-days. Although we're still a long way from 325 degree-days throughout most of Illinois, a continued "string" of warm days could get us there fast.
Figure 4 shows projected degree-day accumulations (base 48°F) from January 1 through April 23, 2001. This should give you some idea of where alfalfa weevil development might be within the next couple of weeks. Remember, however, that these forecasts are based on historical temperature records. Significant fluctuations from average temperatures may speed up or slow down the accumulation of degree-days.
People throughout southern and central Illinois should be scouting for alfalfa weevils now, looking for the small larvae in folded terminal leaves and for pinholes in the leaves, the first symptom of larval injury. As the small, yellowish first instars grow into the larger, greener second instars, and ultimately third instars, the amount of injury will increasepinholes in the leaves will be replaced with skeletonization of the leaves as the larvae consume more leaf material. The percentage of tip feeding often is used as an economic threshold for alfalfa weevils. Table 2 shows some economic thresholds for alfalfa weevils based on percentage of tip feeding associated with accumulated degree-days. Table 3 shows some economic thresholds based on numbers of alfalfa weevil larvae per stem at different alfalfa heights and values of alfalfa hay. These thresholds are published in Pest Management of Alfalfa Insects in the Upper Midwest, published in 1999 by the Leopold Center for Sustainable Agriculture, Iowa State University, Ames. Table 4 shows some insecticides suggested for control of alfalfa weevil larvae. Although we don't anticipate a lot of control activity any time soon in most areas of the state, the thresholds and suggested insecticides provide references for potential future use. In future issues of the Bulletin, we will discuss the impacts of natural enemies and insect pathogens and provide preharvest intervals for the insecticides.--Kevin Steffey