Alfalfa Weevil Seasonal Update and Control Recommendations

Crop and Soil Environmental News, April 1998

Rod R. Youngman, Associate Professor and Extension Specialist
Curt A. Laub, Research Associate
Tom P. Kuhar, Graduate Research Assistant
Department of Entomology (0319), Virginia Tech, Blacksburg, VA 24061

The recent onset of warm weather in late March can only mean one thing for Virginia's alfalfa growers: alfalfa weevil season is upon us once again. Since November, we have surveyed alfalfa weevil egg laying every two to three weeks in over a dozen fields in the piedmont and ridge and valley areas of the state (zones A and B, respectively, 1998 Pest Management Guide for Field Crops [PMG], VCE publication 456-016). The results of this survey have revealed dramatically higher alfalfa weevil egg densities in the piedmont fields compared to those fields west of the Blue Ridge. We noted also that egg hatch had begun the first week of March; however, below freezing temperatures during the second week of March (i.e., March 10-13) caused many newly-hatched weevil larvae to die. Although temperatures are now ideal for alfalfa weevil adults to resume egg-laying, weevil development has been set back in many areas.

Given the present situation, alfalfa growers should begin scouting their fields to determine if and when they need to take action against weevils. By taking just a few minutes to scout for weevils, most growers will save money either by avoiding unnecessary spraying or from identifying those fields where a foliar insecticide application is warranted. The bucket-shake method, as described on page 123 of the PMG, is the proper method for scouting alfalfa weevil.

Alfalfa Weevil Scouting
Scouting for alfalfa weevils is very simple. The only equipment needed is a 3-5 gallon white bucket (a white bucket makes it easy to see the green alfalfa weevil larvae), a clipboard with pencil and paper, and a tape measure. To begin scouting, mentally divide the field into 6 equal sections and walk to the approximate center of the first section (take a few extra samples in fields greater than 20 acres to improve sampling accuracy). Randomly pull 10 stems and place them tip-end first into the bucket. Be careful not to lose any larvae that may fall off as you pull each stem. While holding the clipboard over the top of the bucket and the 10 stems by their base, vigorously shake the stems against the sides of the bucket for at least 5 seconds. Then carefully remove the plant material from the bucket and count the number of weevil larvae in the bottom of the bucket. Record this number on your clipboard. Next, measure the length of two of the stems you selected and record the stem lengths on your clipboard. Repeat this sampling procedure for each of the remaining 5 sections of the field. When you have finished sampling the field, determine the average number of larvae per stem and the average height of your alfalfa plants. Plot these numbers on the decision making chart (refer to Fig. 1 in the PMG) in order to determine whether an insecticide needs to be applied to control alfalfa weevil.

Insecticide Control Options for Alfalfa Weevil
Currently several insecticides are available for controlling alfalfa weevil. Our 1997 alfalfa weevil efficacy trial in Floyd Co., Virginia demonstrated that most labeled products performed well under heavy weevil pressure for nearly a month after insecticide application. The untreated check plots averaged 2.4 weevil larvae per stem at 24 days after application. Labeled insecticides that gave excellent control of alfalfa weevil larvae (i.e., ¾ 0.2 larvae/stem) up to 24 days after application included: Baythroid 2E (0.025 lb AI/ac); Furadan 4F (all three rates: 0.5 lb AI/ac, 0.75 lb AI/ac, and 1.0 lb AI/ac); Karate 1E (0.025 lb AI/ac); and Lorsban 4E (both rates: 0.5 lb AI/ac and 1.0 lb AI/ac). Good weevil control also was obtained from Lannate 2.4LV at rates of 0.45 lb AI/ac (0.7 larvae/stem) and 0.90 lb AI/ac (0.6 larvae/stem).

Remember, alfalfa weevil is an early-season pest of alfalfa that typically affects the first cutting and sometimes the early regrowth of the second cutting. Most alfalfa weevils that survive the first cutting begin leaving the field and become inactive over the summer months.

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