High Numbers of Alfalfa Weevil Larvae

Published: May 9, 2008

Alfalfa weevil larval numbers have significantly increased during the past week in many fields in central and southern Missouri. In several fields surveyed this past week, numbers of small larvae were three to four times over the economic threshold level of an average of one or more larvae per stem. All sizes of larvae were present with most in the 2nd or early 3rd instar stage of development. As they mature as 3rd and 4th instars the amount of foliage they consume substantially increases. At this time, damage to most fields was confined to the upper whorl of leaflets at the plant terminal. This will quickly change as warm temperatures allow for rapid larval growth and increased consumption of leaf tissues. 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. Producers are encouraged to scout alfalfa fields to determine weevil numbers. If the economic threshold of 1 or more larvae per alfalfa stem is reached or exceeded, then treatment is justified.

Proper scouting is the key to obtaining good estimates of weevil numbers. 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.

Several management options are available, although application of a foliar rescue insecticide is the most common management strategy used in most years. In addition to insecticides, early harvest, grazing, and biological control are other viable options depending on larval numbers, plant growth stage, and field conditions.

Insecticides

If an insecticide application is required in order to control alfalfa weevil larvae, select from the list of insecticides labeled and recommended for alfalfa weevil control on alfalfa. Rates are given as amount of product applied per acre.

Early Mechanical Harvest of alfalfa is an alternative to insecticide applications if the alfalfa is within 7-10 days of the normal harvest stage of 1/10 bloom. This season early harvest may be a viable option as alfalfa plants have grown rapidly with the cool, wet conditions experienced this spring. Early cutting will cause the death of most alfalfa weevil larvae through mechanical crushing by hay conditioners or dehydration from the sun following the removal of the alfalfa canopy. After forage removal, the field should be monitored to detect a possible resurgence in larval numbers.

Grazing is being used by some Missouri producers to reduce the numbers of alfalfa weevil eggs and larvae. Grazing is initiated when weevil numbers reach or are approaching the economic threshold and the alfalfa plants are more than 6-8 inches in height. Grazing is generally accomplished using a management intensive grazing method in which a large number of cattle are placed on a small amount of acres and quickly remove the alfalfa growth. As the alfalfa is grazed to normal harvest level, eggs and larvae that are present are destroyed. Data from Missouri indicate that alfalfa weevil larval numbers are reduced by about 98 percent with mechanical harvest and about 90 percent by cattle grazing in a management intensive grazing system. These reductions in larval numbers can effectively eliminate the risk from alfalfa weevil as long as most spring laid eggs have hatched. This method of alfalfa weevil control is not without risks. Fields should not be grazed when wet and susceptible to damage from cattle hooves. Bloat also must be a concern as producers must take precautions to prevent bloat from occurring to cattle. Your local extension office can provide additional information concerning grazing precautions. Producers should continue to scout alfalfa after grazing to determine whether larval or adult alfalfa weevil numbers again reach economic levels and require further control.

Biological Control is a long-term control strategy that can help keep alfalfa weevil numbers below damaging levels. Five species of biotic agents are now commonly found associated with the alfalfa weevil in this state: four parasites and a fungal disease. The parasites are all introduced species from Europe as is their host the alfalfa weevil. Bathyplectes curculionis, a larval parasite, moved into the state with the alfalfa weevil in the 1960s. Similarly, the fungal disease, Zoophthora phytonomi, was first detected in Missouri in the early 1970s. Both of these biotic agents occur throughout the state and cause some mortality of alfalfa weevil larvae. The three other parasites have a limited range in the state, but are increasing in distribution. The two larval parasite, Bathyplectes anurus and Oomyzus incertus, and an adult parasite, Microctonus aethiopoides, have been established in Missouri as a result of parasite release programs conducted during the 1970s and 1980s. These parasites have a limited distribution, but should increase in importance as they move to other Missouri counties.

Producers can help conserve and increase the number of parasites on their farms by using pesticides only when needed and leaving a small area of alfalfa standing when the first cutting is removed. The alfalfa that has not been treated with an insecticide and is not harvested during first cutting will serve as a reservoir for many parasites and predators that attack alfalfa weevil. This alfalfa can be harvested at second and later cuttings because most of these parasites will mature shortly after removal of first alfalfa harvest. The fungal pathogen (Zoophthora phytonomi) is most effective at causing larval mortality in wet years. However, the wet conditions experienced this spring seem to have little effect on weevil numbers to this point in the season. 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 it does so this year Missouri is yet to be determined.