Alfalfa Weevils Get an Early Start This Year

March 17, 2000
The warm weather in late February and early March gave us an unusually early look at alfalfa weevils this year. Mike Plummer, Extension Educator in Natural Resources at the Carbondale Extension Center, reported that alfalfa weevil larvae were hatching and beginning to feed on alfalfa foliage by late February in some fields in extreme southern counties. Reports of larger alfalfa weevil larvae and more significant damage followed shortly thereafter. Omar Koester, Extension Unit Assistant in Crop Systems in Randolph County, recognized alfalfa weevil larvae and injury in fields in his area on March 7. That same week, Ria Barrido and Alan Mosler, with Growmark, observed as many as three to four alfalfa weevil larvae per stem in fields of 7- to 8-inch alfalfa in Williamson County. Although the damage had not turned the fields white, the feeding injury was significant. Ms. Barrido sent us some larvae, ranging in size from first to late-second instars.

Compared with alfalfa weevil development during most years, alfalfa weevil activity this year is very early. As you know, alfalfa weevil adults begin laying eggs in the fall in southern Illinois, and both the eggs and adults overwinter. Adults also lay eggs on relatively warm days during the winter, and they resume laying eggs in the spring. Therefore larvae hatch early in southern counties, and they 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.

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) accumulate beyond January 1, and we suggest that scouting should begin when between 250 and 300 degree-days accumulate. 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.

At the time this article was prepared for this issue of the Bulletin, accumulated degree-days were not available from the Illinois Climate Network run by the Illinois State Water Survey. However, we are well past 300 degree-days in southern Illinois, and we probably are past 200 degree-days in central counties. Therefore we encourage people throughout the southern half of the state to watch for quickly developing infestations of alfalfa weevils. We will provide actual and projected accumulated degree-days in issue no. 2 of the Bulletin, which will be published during the first week in April.

Alfalfa weevil larvae.

Skeletonization of alfalfa leaves by alfalfa weevil larvae.

As you begin to scout alfalfa fields for alfalfa weevils, look first in areas of the field that might warm up early, such as south-facing slopes and areas of the field with lighter soils. After 300 degree-days have accumulated, you should be able to find small, first-instar weevils in the folded terminal leaves. As these small, yellowish larvae with black heads feed on these leaves, you will observe some pinholes. This injury is not economic because the larvae are too small to cause significant defoliation. However, by the time alfalfa weevils grow into third instars, they begin to cause more economic damage by skeletonizing the leaves. At this stage of development, alfalfa weevil larvae are bright green with a distinct white stripe along the center of the back.

An insecticide may be warranted when 25% to 50% of the tips are being skeletonized and there are three or more larvae per stem. More dynamic decision-making guidelines will be provided in the next issue of the Bulletin. If you decide to apply an insecticide, please read all precautions carefully, and follow all directions on the label. Insecticides suggested for control of alfalfa weevil larvae are presented in Table 1.

This early start to the alfalfa weevil season occurred before we began publishing weekly issues of the Bulletin. Nevertheless, provide us with any information you have, and we will distribute it as soon as possible. The Update section of the Bulletin on the Web is an ideal place to look for new information before the next issue is published.--Kevin Steffey

Author: Kevin Steffey