Ohio State University Extension Fact sheet

Ohio State University Fact Sheet

Entomology

1991 Kenny Road, Columbus, OH 43210-1000


Alfalfa Weevil

FC-ENT-0032-00

H. R. Willson
J. B. Eisley

Pest Biology and Host Injury

The alfalfa weevil, Hypera postica Gyll., is a small, brown, snout-nosed beetle approximately 3/16 inch in length with a wide dark stripe down its back. The alfalfa weevil larvae is green with a black head and a white stripe down its back. The larvae pass through four stages or instars, and the 4th instar stage is about 3/8 inch in length. Both the adult and larvae stages of the alfalfa weevil feed on alfalfa foliage. Foliar feeding injury by the adult is not significant. Foliar injury by early larvae in the 1st and 2nd instar stages is primarily confined to the growing tips. Late instar larvae (3rd and 4th instars) may extensively defoliate alfalfa when abundant. In general, foliar injury by alfalfa weevil occurs on the first cutting of alfalfa. During periods of heavy weevil activity, early growth of the second cutting may be impacted.

In Ohio, the life cycle of alfalfa weevil begins with the adult, which is the predominant overwintering stage. In regions south of Ohio and possibly some southern Ohio counties, the egg stage of the weevil may survive the winter. In the spring, when temperatures begin to exceed 48°F, the weevil becomes active and clusters averaging 9 to 10 eggs will be deposited into fresh alfalfa stems.

As heat units above 48°F accumulate, the eggs hatch and larvae development proceeds through the four instar stages. The peak activity of larvae in the 3rd instar stage from spring deposited eggs occurs at 575 heat units. In an area having overwintering eggs, an early peak of 3rd instar larvae will appear at 325 heat units. When the 4th instar larvae reach maturity, they spin a fibrous net cocoon and transform into a pupae, from which the adult stage emerges. The pupae becomes the predominant stage as accumulated heat units reach 800.

Figure 1. Adult alfalfa weevil.
Figure 1. Adult alfalfa weevil.

In Ohio, the life cycle of the alfalfa weevil normally is limited to a single generation per year, but in an abnormal year-especially when high temperatures prevail in June-a second generation may develop. Populations of alfalfa weevil seldom reach economic levels of abundance due to biological control by a complex of parasitic wasps and a fungal pathogen. Three beneficial wasps that attack the weevil larvae include Bathyplectes curculionis, Bathyplectes anurus, and Microctonus colesi. When the Bathyplectes sp. wasps are active, a dark pupal case with a white ring will be found within the weevil cocoon instead of the green pupae of the weevil. Another wasp called Microctonus aethiopoides attacks the adult weevil and renders it sterile. When an epizootic of the fungal pathogen, Erynia sp., is present, late larvae of the weevil will be found attached to foliage that are brown in color. The combined effects of these biological parasites and fungal pathogens tend to regulate the abundance of weevil populations to a point that the economic injury by alfalfa weevil on Ohio alfalfa is uncommon. However, when biotic agents do not effectively control weevil activity, significant defoliation of alfalfa may occur and the application of a rescue treatment of insecticide may be warranted.

Figure 2. Larvae of alfalfa weevil.   Figure 3. Shaking alfalfa stems in a bucket to determine 
        number of larvae per stem.
Figure 2. Larvae of alfalfa weevil.   Figure 3. Shaking alfalfa stems in a bucket to determine number of larvae per stem.

Prevention of Yield Reduction Due to Outbreaks of Alfalfa Weevil

Application of an insecticide to prevent excessive defoliation of alfalfa by weevil is justified when one or more late instar larvae are found feeding per stem and the stand cannot be harvested early. Since the alfalfa weevil is controlled most of the time by beneficial wasps, which are susceptible to chemical treatments, it is important that treatments not be applied unless absolutely necessary.

The feeding of one larvae per stem on 12-inch-high alfalfa will result in a loss of 0.4 tons of dry hay per acre, but one larvae per stem on 16-inch-high alfalfa will cause a loss of only 0.1 tons per acre. Thus, the yield impact of weevil feeding on alfalfa declines as alfalfa stand height increases, and decisions to treat alfalfa for weevil should be focused on an alfalfa stand when larvae can be readily found on alfalfa that is 12 inches or less in height. Once alfalfa is 16 inches or more in height, early cutting is a preferred option for reduction of weevil impact.

The proportion of stems exhibiting tip feeding is an indicator of weevil abundance. Detection of 25% tip feeding on 6-inch alfalfa, 50% tip feeding on 9-inch alfalfa, or 75% tip feeding on 12-inch alfalfa indicates a potential problem. Since evaluation of tip feeding is rather variable among field personnel, the final decision to apply a foliar treatment should be based on an actual count of weevil larvae per stem.

Table 1. Action thresholds relevant to stand height, tip feeding, and density of larvae per stem.

Stand Height Inches Indiction of Problem % Tip Feeding Problem Confirmation Larvae per Stem Recommended Action
6 25 1 Recheck in 7 days
9 50 > 1 Spray
12 75 > 2 Spray or harvest
16 100 > 4 Harvest early
When harvested early due to weevil, check within one week for regrowth.

A larvae count should be based on a series of 10-stem samples randomly collected from various locations in a field. Each stem should be carefully picked off at the base and placed top down in a bucket. When 10 stems have been collected, the stems should be vigorously shaken in the bucket and the number of larvae collected in the bucket counted. The shaking will dislodge the late 3rd and 4th instar larvae, which cause most of the foliar injury. Close inspection of the stem tips may be needed to detect the early 1st and 2nd instar larvae. The detection of one or more larvae per stem on alfalfa that is 12 inches or less in height indicates a need for rescue treatment. Where alfalfa is between 12 and 16 inches in height, the action threshold should be increased to 2 to 4 larvae per stem depending on the vigor of alfalfa growth.

Figure 4. Cocoon on foliage containing pupae of weevil.
Figure 4. Cocoon on foliage containing pupae of weevil.

Where alfalfa is 16 inches or more in height and multiple larvae are found per stem, early harvest should be considered. Following early harvest of infested stands, regrowth should be examined 4 to 6 days after harvest. If regrowth has been prevented by 2 or more larvae per crown, a stubble spray is warranted.

Whenever weevil activity on an alfalfa stand is being evaluated, it is important that indicators of declining weevil activity be noted, such as the pupation of mature larvae or the decimation of a weevil population by an epizootic of the fungal pathogen.

If few early weevil larvae are present in a sample and pupal cocoons are readily found on the foliage or on the ground, it may be assumed that the peak activity of larval feeding has passed.

If diseased larvae are readily observed on the foliage, it may be assumed that additional larvae are infected and that a decline in feeding activity is occurring. Economic prevention of defoliation by a weevil population depends on timely treatment of an infestation prior to peak feeding activity. Treatment of a declining population will not likely achieve an economic return on the cost of an insecticide application.

Authors

Harold R. Willson, Associate Professor
Department of Entomology, The Ohio State University
1991 Kenny Road, Columbus, OH 43210
Phone: (614) 292-1554
E-mail: willson.1@osu.edu

J. B. Eisley, Research Associate
Department of Entomology, The Ohio State University
1991 Kenny Road, Columbus, OH 43210
Phone: (614) 292-3851
E-mail: eisley.1@osu.edu

Acknowledgments

R. Mark Sulc, Associate Professor
Department of Horticulture and Crop Science, The Ohio State University
2021 Coffey Rd., Columbus, OH 43210

R. B. Hammond, Associate Professor
Department of Entomology, Ohio Agricultural Research and Development Center, The Ohio State University
1680 Madison, Wooster, OH 44691


All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.

Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.

TDD No. 800-589-8292 (Ohio only) or 614-292-6181



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