Hypera postica Gyllenhal
Appearance and Life History
The most important early season insect pest of alfalfa in the Midwest is the alfalfa weevil. Infestations of this insect have been so destructive in some areas that alfalfa acreage should be closely watched.
The adult alfalfa weevil is a light brown snout beetle distinguished by a darker brown strip down the center of its back. It is typically about 3/16 inch (4.7 mm) in length. The beetle is found in alfalfa during the spring, early summer, and fall. It is usually inactive during the hot summer months, entering a dormant state called aestivation.
The alfalfa weevil larva is a small, light green worm with a wide, white stripe down the center of its back, paralleled by a lighter strip down each side. When fully grown, it will reach 3/8 inch (9 mm) in length. It is very similar in appearance to the larva of the clover leaf weevil. To distinguish between larvae of these two species, carefully examine the head. The alfalfa weevil larva has a black head, while the larva of the clover leaf weevil has a brown head.
In early spring, alfalfa weevil larvae hatch from eggs deposited in the plant stems and begin feeding within the folded leaves at the growing tips. As these leaves unfold, the early larval instar damage appears as tiny "pinholes" in the leaves.
As the larvae grow, they chew larger holes, giving the plants a shredded or skeletonized appearance. A heavy infestation of larvae can consume enough foliage that an entire field may take on a grayish appearance.
Adult weevil feeding damage is usually minor, taking the form of small, circular cuts along leaf margins. Adults usually feed on lower leaves. Adults feeding on the stems shortly after the first cutting can produce damage known as "bark feeding."
Before First Cutting - Field scouting for alfalfa weevil damage should begin when approximately 250 heat units [base 48°F (8.9°C)] have accumulated from January 1. Sampling a field to determine the extent of alfalfa weevil damage and average stage of weevil development is best accomplished by walking the field in an M-shaped pattern. Five alfalfa stems should be examined in each of 5 areas of a field for a total of 25 stems for the entire field. Each stem should be examined for:
- Evidence of feeding by alfalfa weevil larvae
- Maturity of the stem, i.e., pre-bud, bud, and/or flowers
- Stem length
The average size (length) of weevil larvae should also be noted. Although large larvae are relatively easy to find, small larvae are difficult to see. Therefore, developing shoots showing "pin-hole" feeding may have to be torn apart to find the small larvae.
Warm, wet springs can encourage the development of a naturally occurring fungal disease, Zoophthora phytonomi. Alfalfa weevil larvae are very susceptible to this pathogen, and it can spread quickly throughout the population. Infected larvae move slowly and become discolored, progressing from yellow to brown and finally black. Diseased larvae are likely to be noted during scouting activities.
After First Cutting - Even though harvesting a weevil-infested field is usually an excellent means of control, alfalfa weevil may continue to cause severe damage by feeding on the stubble and new growth. Fields should be scouted 4 to 5 days after the first cutting has been removed to determine if the weevils are still present and feeding.
Closely examine at least 5 plants in each of 5 areas for clipping of developing shoots by weevil larvae or stem or leaf feeding by adult weevils. Determine the percentage of stems with feeding damage and/or note if weevils are feeding on the shoots.
Forage Insect Control Recommendations: E-series 220-W (PDF)
Use the following charts for southern and northern Indiana to determine if control is warranted.
If control is necessary, contact your state Cooperative Extension Service or click here for control materials and rates.