Search IPCM Newsletter Archives

Problems with alfalfa weevil larvae began this past week with a few alfalfa fields requiring insecticide applications in southern and central Missouri. Alfalfa weevil larvae grow through 4 instars or larval stages (worm stages) on their journey from egg to adult weevil. Eggs are laid inside alfalfa plant stems beginning in late fall, on winter days when temperatures go above 50 degrees F, and in early spring. At present, numerous larvae (1st and 2nd instars) have hatched and can be found in the upper terminal buds of alfalfa plants in fields throughout southern and central regions of the state. These larvae are likely the result of fall or early winter laid eggs and are often seen initially as problems on the south facing slopes of fields. Small larvae in the 1st and 2nd instar stages of growth typically feed inside plant terminals where they are difficult to find and are protected from harsh spring weather conditions. Damage from these early instars is often minor and seen as "pin hole" damage on expanding alfalfa leaflets.

As temperatures warm, additional eggs will hatch, plus existing small larvae will rapidly grow into 3rd and 4th 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. One unknown, but important factor is whether the recent sustained rainfall over most of Missouri will result in the development of a fungal pathogen (Zoophthora phytonomi) in the alfalfa weevil larval population. This fungal pathogen is present in most years, but infection rates of the 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. If this pathogen develops early in the season, it can decimate larval alfalfa weevil populations. Whether is does so this year in Missouri is yet to be determined.

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.

Recommended methods for control of the alfalfa weevil larvae include: insecticide application, early cutting, grazing and biological control.

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 (See table below).

Early Cutting

Early cutting 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. 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

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

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.

Wayne Bailey, 573-864-9905