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Keep an Eye Out for Alfalfa Weevils

March 20, 2003

As spring commences, the first of the field crop insect pests in Illinois begin to make themselves known. Included in this group is the alfalfa weevil. Egg laying takes place in southern Illinois in the fall and continues through April when temperatures are above 40°F. Consequently, the alfalfa weevil overwinters as both eggs and adults. Alfalfa weevils generally become active when temperatures exceed 48°F, usually in late March and April in southern counties. Larvae begin to develop within the eggs, and adults resume egg laying. On hatching, larvae move to the terminal leaves and begin feeding.

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 250 to 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.

When 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. Small first instars should be present in terminal leaves after the accumulation of 300 degree-days. The small, yellowish larvae with black heads feed on these leaves, causing injury resembling pinholes. The injury is not economic because the larvae are too small to cause significant defoliation. It is not until alfalfa weevils grow into third instars that they begin to cause more economic damage by skeletonizing the leaves. Third instars are bright green with a distinct white stripe along the center of the back.

At the time this article was written, accumulated degree-days were not available from the Illinois Climate Network run by the Illinois State Water Survey. However, temperatures in January and February were lower in 2003 than they were during the same months in 2002. A rough estimate of degree-day accumulations thus far in 2003 indicates that 200 degree-days have not accumulated yet even in the most southern locations in Illinois. However, as temperatures increase, degree-day accumulations will occur rather quickly. People in areas near Dixon Springs, Belleville, and Carbondale should begin to spot-check for alfalfa weevils soon. Degree-day accumulations in those areas will approach 200 in late March or early April. We will provide actual and projected accumulated degree-days in future issues of the Bulletin.

We can often get insight on potential insect occurrences, like the alfalfa weevil, by keeping track of these insects in states to our south (e.g., Kentucky, Oklahoma, southern Missouri). The survival of alfalfa weevil eggs in Oklahoma was not as high this year as it was at this time last year. This past January, egg populations in 11 counties ranged from 49 to 390 alfalfa weevil eggs per square foot. The average was 125. In February, numbers of alfalfa weevil eggs decreased in six counties, from the January sampling date to the February sampling date. Eggs per square foot ranged from 4 to 435. However, the viability of the eggs remained at 80% on average for both sampling periods. (Data were taken from the article "Alfalfa Weevil Egg Populations in 2003," in Oklahoma State University's Plant Disease and Insect Advisory, vol. 2, no. 4, March 7, 2003; http://entoplp.okstate.edu/Pddl/advisory.htm).

Look for more information on the alfalfa weevil, population densities, management options, and degree-day accumulations in future issues of the Bulletin.--Kelly Cook

Author: Kelly Cook


The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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