Adult alfalfa weevils are 3/16-inch long and light brown with a dark mid-dorsal line extending down the middle of the back. Adults possess a distinctive snout and readily fall to the ground when disturbed. Eggs are laid inside alfalfa stems in fall or spring. Small, light green, black-headed, legless larvae have a distinct white stripe down the center of the back. Larvae feed on the upper leaves and growing terminals of the plant early in the spring, reaching a quarter-inch in length when full grown. Larvae spin loosely-woven, silken cocoons, generally found on the ground among the leaf litter. Most damage occurs before the first cutting, but damage by larvae and adults can suppress yields by delaying regrowth.
Alfalfa weevil larva. (Pat Porter)
Fall or winter grazing of alfalfa, or crushing the stems with a heavy roller, can destroy overwintering eggs and reduce spring larval populations, but the level of suppression provided by dormant-season habitat manipulations is not reliable.
Alfalfa weevil cocoon. (Ken Wise)
Parasitic wasps and fungal pathogens also may help limit damage, but do not usually provide sufficient levels of control in Kansas. Weevil-resistant alfalfa varieties are being developed, but insecticide sprays and timely cutting remain the most practical means of preventing economic damage.
Treatment and Decision-Making
Treatment decisions should be based on the stem-count method.
Alfalfa weevil larval defoliation.
Carefully cut 30 to 50 stems selected at random across the field and shake them individually into a deep-sided bucket. Given the propensity of larvae to fall easily, the method used to harvest infested stems can influence the accuracy of population estimates, potentially leading to the wrong management decision. The finger-shear, palm-tip, or knife methods are recommended for taking weevil larval samples:
In the finger-shear method, the stem is held horizontally between the thumb and forefinger while the middle finger is used to break the stem off.
In the palm-tip approach, one hand grasps and encloses the tip of the stem while the other is used to break the stem.
The knife method involves severing the stem with a small knife using the thumb to push the stem into the knife blade.
These techniques will minimize the loss of larvae from the stem before it reaches the bucket. Count the stems, noting the average stem height, and count the larvae, calculating the average number of larvae per stem. Refer to the Alfalfa Weevil Stem Count Decision Guide (below) to determine the action recommended. As the alfalfa value increases, fewer larvae are required to reach a treatment threshold. The first relationship (a) was developed for situations where alfalfa sells for $35 per ton, and indicates that nearly three larvae per stem are required to justify treatment, whereas in (b) with the crop valued at $70/ton, alfalfa should be treated at just over two larvae per stem.
Early Season Infestation. When alfalfa is 3 to 7 inches tall, teat when feeding is evident on the top inch of growth – this usually requires one to two larvae per stem, depending on expected value of the hay. Retreatment may be necessary before cutting.
Mid-Season Infestation. When alfalfa is 8 to 14 inches tall, treat when significant feeding damage to the top 1 to 2 inches of growth is evident on 30 to 50 percent of terminals. High larval populations (four or more per stem) can cause severe foliage loss in only three or four days. Stubble sprays may be necessary in addition to treatment at this time.
Late Season Infestation. Early cutting may be advisable if the alfalfa is 2 weeks or less from a normal harvest date, provided the hay is expected to dry quickly, and windrows will be rapidly removed from the field so that larvae are exposed to bright sunlight and drying winds. However, spraying before cutting is advisable if the top 2 to 3 inches of most plants are suffering injury and the field cannot be harvested immediately, if weather conditions favor larval survivorship, or if other stresses such as drought are likely to impede rapid regrowth after cutting.
Post-harvest. Stubble sprays may be necessary to protect regrowth if eight or more larvae per square foot of stubble are present. As few as four larvae per square foot may prevent regrowth under unfavorable growing conditions. Adult weevils may also 'debark' stalks and may require treatment, especially under conditions of drought that delay regrowth. Alfalfa is a good source of beneficial arthropods, and use of older, broad-spectrum materials to control weevils can eliminate predators and parasitoids and flare other pests such as aphids. Formulations that contain indoxacarb or spinosad as active ingredients will spare natural enemies, but tend to come at a higher cost.
Spray gallonage for ground equipment: 10 to 12 gallons per acre for 7-inch alfalfa; 15 to 20 gallons for 8- to 15-inch alfalfa; at least 20 gallons per acre on alfalfa more than 15 inches tall. Use 30 psi and hollow cone nozzles, and adjust spray pattern as suggested by the nozzle manufacturer to overlap near the top of the canopy.
Regrowth damaged by adult weevils.
Spray gallonage for aerial equipment: The use of less than 2 gallons of spray per acre can result in less than satisfactory control. Overall efficacy increases as more carrier is used.
Close-up of adult weevil 'debarking' damage.
Please refer to the most recent version of the Alfalfa Insect Management Guide for control options. More information can be found in KSRE publication MF2999: Alfalfa Weevil.
