Alfalfa weevil problems, which have occurred in southern Missouri during the past two weeks, are now moving into central and north Missouri alfalfa fields. Larvae from spring laid eggs have moved from feeding sites in plant terminals to more exposed plant surfaces to feed. In several fields surveyed, numbers of small larvae were three to four times over the economic threshold level of an average of one or more larvae per stem. Alfalfa weevil larvae grow through four instars or larval stages with most damage to alfalfa plants caused by the last two instars. These larger instars readily move about the plant and feed on alfalfa foliage. They may consume significant amounts of leaf tissue, which typically results in substantial economic loss of alfalfa yield and forage quality. In addition, heavy defoliation also reduces alfalfa competition with weeds and may result in increased weed populations.
Alfalfa producers should scout alfalfa fields throughout the state as problems can quickly develop and result in substantial loss of forage yield and quality. Scouting for alfalfa weevil is best accomplished using a 3-5 gallon bucket and a sharp knife. Producers are encouraged to sample 10 alfalfa stems at each of five random locations in a field for a total of 50 stems per field. At each of the five locations, the scout should carefully cup the terminal of each alfalfa stem and then cut the stem off near the soil surface. The stem is then carefully placed inside the bucket and vigorously tapped to dislodge any larvae present. It is necessary to cup the terminal with your hand during removal of the stem from of the plant to prevent the larvae from being flipped from the terminal during stem removal. If the alfalfa weevil population has reached the economic level of one or more larvae present per stem of alfalfa (50 or more larvae per 50 stems) and 30 percent or more of the alfalfa stems show feeding damage, then control is justified.
Although several management options are available, applications of foliar insecticides are commonly used. Other options include early harvest of the crop, grazing by livestock, and the presence of biological control agents. In surveys this past week in central Missouri, the fungal pathogen (Zoophthora phytonomi) was found in low numbers in alfalfa weevil larval populations. This fungal pathogen is present in most years, but infection rates of this pathogen are most successful in wet years. Infected alfalfa weevil larvae slow their feeding activities, turn from light green to pale yellow in color, and die within a few days of becoming infected by the fungal pathogen. If this pathogen develops early in the season it can decimate larval alfalfa weevil populations. Whether this pathogen builds to endemic levels and causes high larval mortality in the alfalfa weevil larval populations will be determined over the next few days.
See Integrated Pest and Crop Management newsletters for additional information about alfalfa weevil management options and recommended pesticides.
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REVISED: February 14, 2012