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The Watch for Alfalfa Weevils Begins

March 22, 2002
Alfalfa weevils are the "earliest-rising" insect pests of field crops in Illinois, becoming active whenever the temperatures rise above 48°F. Alfalfa weevil adults lay eggs in the fall in southern Illinois, where they overwinter as both eggs and adults. When temperatures exceed 48°F, larvae begin to develop within the eggs and adults resume mating and egg laying.

Two distinct peaks of larval activity usually occur in southern Illinois, one from fall-deposited eggs and one from spring-deposited eggs. Larvae hatch from overwintering eggs when approximately 200 degree-days (above a base temperature of 48°F) accumulate beyond January 1. Scouting for alfalfa weevil larvae should commence 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.

Figure 3 and Figure 4 show actual and projected degree-day accumulations (base 48°F) in Illinois from January 1 through March 17 and March 30, respectively. According to current degree-day accumulations, alfalfa weevil larval activity is still in our future. However, accumulation of degree-days varies within an area, depending on topography and other factors. For example, south-facing slopes usually warm up faster in the spring, so alfalfa weevil activity could begin in such fields earlier than projected. For several counties in southern Illinois, alfalfa weevil larvae should be evident before the end of March.

I recently visited Oklahoma State University and got some information about alfalfa weevils from Phil Mulder, Extension entomologist. At the time of my visit (first week in March), Phil and his students were processing samples of alfalfa weevil eggs from several areas in their state. He indicated that samples taken during January indicated an unprecedented density of eggs per square foot, the highest densities he had ever observed. The range in densities was about 57 (Payne County, central Oklahoma) to 1,487 alfalfa weevil (Stephens County, southern Oklahoma) eggs per square foot. The average was 348 alfalfa weevil eggs per square foot.

However, the freeze that occurred in February took its toll on both alfalfa weevil eggs and larvae that had hatched in some areas (as much as an 85% reduction in larval numbers). Nevertheless, the viability of eggs ranged from 73% to 81%. (Data taken from the article "Alfalfa Weevil Egg Population Up in 2002," in Oklahoma State University's Plant Disease and Insect Advisory, Vol. 1, No. 5, March 12, 2002; http://plants.okstate.edu/Pddl/PDIA1-5.pdf).

You may wonder what the situation in Oklahoma has to do with our situation in Illinois. Well, we can keep track of occurrences of insects like the alfalfa weevil in states to the south (e.g., Kentucky, southern Missouri, Oklahoma) to get potential insight into our own situation. If alfalfa weevil eggs survived the mild winter in Oklahoma, they probably survived our mild winter, too. Another forewarning of the possibilities.

We'll keep you apprised of degree-day accumulations, population densities, effects of natural enemies, and options for management in future issues of the Bulletin. Please feel free to let me know when you first observe alfalfa weevil activity this year.--Kevin Steffey

Author: Kevin Steffey


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

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