Alfalfa Weevil

The larvae and adults of alfalfa weevil (Hypera postica) are the primary defoliators in alfalfa. Heavy infestations can reduce tonnage and forage quality. Use this article to learn how to identify alfalfa weevil and properly scout for and manage this pest.

Identification

Adult: Adult alfalfa weevils are approximately ¼ inch long beetles with a blunt “snout,” thick golden hairs covering the body, and a distinct dark, narrow stripe down the back (Photo 1).

Egg: Eggs are yellow and oval and can be found inside alfalfa stems (Photo 2).

Larva: Alfalfa weevil larvae are legless with a prominent black head (Photo 3). Larvae are wrinkly and yellow-green in color with a white stripe along the middle of the back. Mature larvae reach 3/8 inches in length.

Pupa: The pupal stage is a pea-sized, loosely woven cocoon, which may have leaves attached to it.

Be aware of look-alikes! The alfalfa weevil is commonly confused with the clover leaf weevil, which rarely causes economic injury to alfalfa in Iowa. The clover leaf weevil has a few key differences in the larval and adult stages: larvae have a brown head, and the stripe is bordered by pink or red; adults are 5/16 inches long (approximately 2 times the size of alfalfa weevils!).

Biology

Alfalfa weevil has one generation per year and overwinters as an adult. Adults typically become active in April or May, and females will deposit clusters of up to 25 eggs in alfalfa stems. Females can lay 800-4,000 eggs in a lifetime. Eggs hatch within two weeks, and larvae feed for 3-4 weeks, depending on temperatures and alfalfa quality, and go through four instars before pupating. Pupation occurs in small cocoons near the base of the plant, sometimes attached to leaves. The time it takes to reach the adult stage is dependent on temperature but is usually around eight weeks. Adults emerge and feed for 2-3 weeks before seeking sheltered areas outside of alfalfa fields to enter summer dormancy. Most adults will overwinter in these sheltered areas, but some return to the field to lay eggs in the fall when temperatures are warm enough.

Eggs laid in the fall or winter rarely survive normal winters in the Midwest, but if they do, alfalfa may be injured earlier in the spring, leading to a prolonged feeding period for alfalfa weevil. In 2021, entomologists in several states noticed multiple peaks of larval activity, likely from successful overwintering by both adults and eggs, that led to damage to regrowth after the first cutting (see these articles from Iowa State University and the University of Nebraska-Lincoln).

Plant Injury

Newly hatched larvae can be found feeding on terminal leaves. Feeding by first-instar larvae appears as pinholes in the leaves. Maturing larvae move down the plant and begin feeding between leaf veins, which may result in skeletonization. The third and fourth instar larvae cause most of the feeding injury. Peak larval activity occurs around 575 GDD. A heavily infested field will look frosted or silver due to the dying plant material (Photo 4). Adults cause less plant injury than larvae. They feed along the leaf margin, leaving irregular notches. Usually, alfalfa plants suffer the greatest injury prior to the first cutting, although injury to regrowth (crowns) can occur, especially under windrows where larvae may take shelter after the first cutting. Feeding reduces yield, quality, and stand health.

Scouting

After reaching benchmark degree days (200 in southern Iowa and 250 in northern Iowa), use a sweep net to sample for adults and larvae (Photo 5). South-facing slopes warm up faster and may be a place to start sampling.

Once the first larvae are collected in your sweep net, sample the field using the sampling plan below to determine number of larvae and plant height: these are two of the four pieces of information needed to determine if the economic threshold has been reached.

1. Walk the field in an “M” pattern, stopping at 5 random locations throughout the field. Larger fields may require more sites (and total stems). Make sure the sample is representative of the entire field.
2. At each location, collect six alfalfa stems by breaking them off at the base, making sure to be gentle so as not to lose larvae during the process.
3. Measure the height of the plants collected.
4. Shake the plants into a bucket. Most larvae can be dislodged by vigorously shaking the plants into the bucket, but small larvae can be difficult to separate, so carefully inspect the plants after shaking.
5. Take the average plant height for all 30 plants and record the total number of larvae found.
6. Use either Table 1 or Table 2 to determine if management is warranted. Table 2 provides thresholds for shorter alfalfa.

Scouting helps determine plant height and total number of larvae in the field, but you also need to know the market value of the hay ($/ton) and control costs ($/acre).

Table 1. Economic thresholds for alfalfa weevil, based on the total number of larvae in a 30-stem sample (Originally published by John Tooker, Penn State Extension).

For an example of how to interpret this table: if the estimated hay value is $180/ton, insecticide cost is $14/acre, and your plants are 19 inches tall, the economic threshold would be 58 alfalfa weevil larvae/30 plants. If you found at least 58 larvae, it would be worth it to spray an insecticide if cutting is not an option.

Table 2. Economic thresholds for alfalfa weevil, based on the average number of larvae in a 30-stem sample (Originally published by Marlin Rice at Iowa State University).

Thresholds in Table 2 are ranges - use the smaller threshold if alfalfa is stressed (drought, other pests, etc.) or control costs are relatively low. Use the larger threshold if plants are healthy, diseased larvae are present, or control costs are relatively high. An alternative ET table from North Dakota State University uses the average larvae/30 stems and includes lower crop values and treatments costs. 

Continue to scout through the first cutting. After harvest (whether planned or used as a management tactic), be sure to scout for larvae seeking shelter in the windrows; these larvae can feed on the regrowth. If eight or more larvae per square foot are found OR regrowth is delayed due to feeding, consider an insecticide application.

Management

Tracking development: The degree-day model in Table 3 is derived from a model developed for southern Ontario and does not account for egg overwintering, which may result in earlier larval feeding. Alfalfa weevil has a lower developmental threshold of 48°F and an upper threshold of 90°F. Eggs begin hatching around 300 GDD, but we suggest scouting should begin at 200 (south of I-80) or 250 (north of I-80) GDD in Iowa. You can track alfalfa weevil development on the Pest Maps and Forecasting page.

Table 3. Alfalfa weevil development and associated activity.

*GDD accumulated from January 1, base 48°F.
**See Biology section. Adults feed for a few weeks and then enter dormancy but may return to alfalfa in the fall to lay eggs.

Cultural: Cutting alfalfa (early harvest) is an effective management tool for alfalfa weevil, and an insecticide application may be avoided if harvesting occurs within a few days of reaching the economic threshold. Harvest removes food and shelter and exposes larvae to harmful UV rays. Windrows also create a favorable environment for entomopathogenic fungi. Harvesting is preferred to chemical treatments once plants are 16 inches tall.

Biological: A few parasitic wasps have been introduced to control alfalfa weevil, but the parasitism rate in Iowa is unknown. A fungal pathogen (Zoophthora phytonomi) also provides good control of alfalfa weevil in warm, humid conditions. When scouting, take note of any discolored larvae; these larvae have likely been infected. Natural enemies can prevent alfalfa weevil from reaching economic levels, so insecticides should be used only as necessary.

Chemical: In alfalfa, foliar insecticide applications are typically a last resort if early harvest is not an option. Many products are available, primarily in the pyrethroid class of insecticides, and are highly effective. Pyrethroid resistance has not been reported in Iowa but be aware that western states have reported pyrethroid resistance in alfalfa weevil. Some important considerations when choosing insecticides:

• Pre-harvest or pre-grazing intervals. Which product you apply will depend on when you intend to cut or graze the crop. As plants mature, it will be necessary to choose products with shorter pre-harvest intervals than when plants are young.
• Pollinator safety (aka the Bee Rule). Most insecticide labels now include language to protect pollinators. Do not spray plants in bloom and only apply insecticides early in the morning or late in the evening when bees are not foraging.