Alfalfa Weevil

The alfalfa weevil is an important pest of established alfalfa stands in New York.

New seedings are not typically affected. The alfalfa weevil overwinters primarily as an adult in New York.

These adults return to alfalfa fields in the spring when temperatures permit and begin to feed and lay eggs in the stems of developing alfalfa plants. In an “early” spring, weevils emerge sooner, more eggs are laid, and damage to the alfalfa plant occurs in the plant’s development. In general, both the weevil and the alfalfa plant respond to favorable spring temperatures, but at lower temperatures alfalfa develops more quickly than the weevil. Normally larvae hatch from eggs in early to mid-May, and their development is completed by late May to mid-June. During this developmental process the larvae go through four distinct stages: first-stage, second-stage, third-stage, and fourth-stage larvae. The larvae in each stage are progressively larger and consume increasingly more alfalfa. Actually, the larvae in the last stage consume about 80 percent of the total foliage eaten in all the stages. Therefore, the last instar causes the most damage and is the target of the management program. Fourth-stage larvae are about 3/8 inch long, green with a white stripe down the back and sides, and have a black head.

After completing its larval development, the alfalfa weevil spins a netlike cocoon on the plant, dead leaves, and stems or litter on the ground below the plant on which it fed. In about 1-1/2 weeks the adults emerge from the cocoon and feed on alfalfa regrowth for a brief period. In early July the adults migrate from the alfalfa field to adjacent protected areas such as the edge of woods, fencerows, and old fields. They then enter a resting period. In late September the adults return to the alfalfa field, feed for a brief time, mate, and lay a few eggs. Most of these eggs never make it through the winter. In early to mid-November the adults again move to protected sites to overwinter. The alfalfa weevil goes through only one generation per year.

Management

Alfalfa weevil monitoring should begin in early May and continue through first harvest and early regrowth. While walking alfalfa fields, pick at least 50 alfalfa stems at random throughout the field. Pick stems close to the ground. After collecting stems, check terminal buds and upper leaves for the “shot-hole” signs of weevil feeding. Severely damaged stems may have minimal amounts of green leaf tissue remaining, giving the leaf and stem a grayish-white frosted appearance. An alfalfa stem is considered “positive” for weevil feeding if at least one leaf shows the shot-hole damage. Count the number of weevil injured stems as a percentage of all stems collected.

Note: Alfalfa weevil infestations may vary as much as 100-fold between adjacent fields. There is no substitute for close and frequent observation in each alfalfa field. Even in the most heavily infested regions, many fields will be well below the treatment threshold and not require treatment.

Before first cutting an action threshold has been reached if 40 percent of alfalfa stems are positive for signs of weevil feeding (20 of 50 stems). Note: Count the percentage of stems showing alfalfa weevil tip-feeding injury. Do not count the percentage of leaves with weevil feeding.

If weevil populations are approaching threshold, several management alternatives should be considered: early harvest, use of a recommended insecticide (Table 4.10.1), or biological control.

Whenever possible, harvest the first cut early for hay or silage. If the field is within 10 days of a normal harvest, cut the crop for alfalfa weevil control. Harvest the alfalfa field before the fourth-stage larvae can cause damage.

This management option is practical in a “normal” or “late” spring, but difficult in an “early” spring when fourth-stage larvae peak early and alfalfa development has barely reached peak protein yield. Figure 4.10.1 shows how these peak larval periods can vary from year to year.

Remember, if harvest or other action is not taken or is badly timed, yield losses and crop quality losses can occur. After harvest, check stubble and regrowth for signs of weevil feeding. If 50 percent of regrowth shows signs of weevil feeding, larvae are <3/8 inch long, and there are few or no weevil cocoons, the field may need to be treated with an insecticide.

Insecticidal control by a wide variety of chemicals (Table 4.10.1) is another option that provides temporary reduction of the larval population.

Cost, date until harvest, residue constraints, and weather factors can limit practical implementation of this option. Like harvesting, accurate timing of the insecticide application is necessary for optimal control.

Apply an insecticide when 40 percent of the alfalfa tips show signs of injury from larval feeding. Precise timing is critical because damage builds up very rapidly. If spraying is neglected or badly timed, yield losses and crop quality losses can occur. Remember that most insecticides require a waiting period between application and harvest.

With frequent close observation of each alfalfa field and with proper timing and careful application of insecticides, good alfalfa weevil control can be achieved with the presently labeled and recommended insecticides (see Table 4.10.1). For best results, follow these guidelines:

• Do not treat any alfalfa field unless absolutely necessary as shown by an assessment of feeding damage and weevil populations during the current year. Parasites and predators controlling the alfalfa weevil are building up in these fields, and insecticide application may interfere with future weevil control.
• Do not treat fields with an insecticide with less than 50% of the stand alfalfa. In these fields, the economic return from the insecticide is marginal and these fields serve as an important refuge for the alfalfa weevil parasites which usually do an excellent job in controlling alfalfa weevil below economic levels.
• Treat only if necessary and only at the proper dosage and time intervals. Sometimes, two applications may be necessary.
• Do not spray alfalfa in bloom; this is a violation of state law. Protect our bees. Do not spray weedy alfalfa fields when weeds are in bloom. Poor stands of alfalfa do not justify chemical treatment. They serve as desirable sources of weevil parasite buildup.
• Biological control of the weevil by a number of parasites is another control option.

Thirteen species of parasites of both alfalfa weevil larvae and adults have been identified in New York alfalfa fields. As these beneficial parasites multiply and become more widespread, the alfalfa weevil will become less troublesome.

One of these parasites, Bathyplectes curculionis, a small parasitic wasp, is particularly effective. Bathyplectes wasps lay their eggs in the small alfalfa weevil larvae. As the parasite develops, it consumes the weevil larva and forms a cocoon inside that of the weevil. The parasite’s small, brown, football-shaped cocoon, generally with a white band around the middle, can be often found inside the alfalfa weevil cocoon cases.

Harvesting alfalfa early, when practical, is an effective method of minimizing risk of weevil damage. This method is also more “friendly” to populations of beneficial organisms present in the field.