As temperatures warmed up in the past weeks, producers in southeast, south-central and central Nebraska have noticed alfalfa weevil activities in alfalfa fields, with multiple sizes of larvae that may have come from overwintering adults and eggs laid in the previous fall that were able to survive the winter (Figure 1).

Producers growing high quality alfalfa hay should make time to scout alfalfa fields for weevils now and over the next few weeks. Severe alfalfa weevil feedings will make the crop look droughty. Producers are recommended to use the economic thresholds (Table 1) to make decisions on insecticide application.

Alfalfa weevil has become a significant issue for the past few years. Alfalfa weevil feeding on leaves can reduce hay yield and quality. The feeding of weevil larvae can skeletonize or defoliate the first cutting alfalfa, which causes the most significant economic loss. However, both adults and larvae can feed on the regrowth after the first cutting. If regrowth does not begin within four or five days, scout for weevil larvae and adults and treat the field when necessary.

Alfalfa Weevils

Life Cycle

Most alfalfa weevils overwinter as adults, become active as temperatures increase and lay eggs. After summer estivation, some adults may lay eggs in the stem during fall. If winter is not too severe, eggs will successfully overwinter. These eggs will hatch earlier than those laid in spring.

Overwintering eggs are common within alfalfa fields in Kansas and Oklahoma. While we do not have direct evidence to prove that weevil eggs overwinter in Nebraska, multiple sizes of larvae co-occurring with adults in the early spring when only adult emergence is expected strongly suggests the success of eggs surviving through winter in Nebraska.

The weevil larvae progress through four instars — the first two instars chew holes in the upper leaves, while the third and fourth instars cause the most damage to the crop by feeding between leaf veins, resulting in skeletonization or defoliation.

Scouting and Economic Thresholds

Alfalfa weevil damage consists of small holes and interveinal feeding on the newest leaflets near the stem tips. The larvae are small (1/16- to 3/8-inch long) and pale yellowish-green, becoming a darker green when larger. These legless worms have black heads and a white stripe the length of the back (Figure 1). The alfalfa weevil larvae spend nearly all their time on the plant. They curl into a C-shape when disturbed.

Once the alfalfa is high enough to use a sweep net, take a sample to establish whether weevils are present. If they are, randomly select at least five sampling sites from across the entire field. At each site, gently pick or cut at least 10 alfalfa stems at ground level. Shake the larvae off the stems by beating the stems into a deep-sided bucket. Count the larvae and determine the average number of larvae per stem. Make sure to check for small larvae that may be enclosed in new, folded leaflets at the tips of the stems. Measure stem lengths and determine the average stem height. Use these averages to find economic thresholds in Table 1 to determine the appropriate action.

Economic thresholds have been developed to aid decision making on alfalfa weevil control (Table 1). These thresholds were derived by North Dakota State University entomologists (Beauzay et al. 2013) from a two-year study conducted at the UNL Eastern Nebraska Research, Extension and Education Center near Mead in 1990 and 1991 (Peterson et al. 1993). These guidelines can fluctuate depending on growing conditions and variety.

Deciding whether to treat or re-sample depends on the average number of weevils per stem, the stem length, treatment costs and the value of the alfalfa. When alfalfa reaches 50% or more bud stage, it may be more profitable to cut the alfalfa early than treat it.

Insecticides

Because natural enemies of alfalfa weevil (e.g., lady beetles and parasitoid wasps) have the potential to keep weevils from reaching economic injury levels, use insecticides only when necessary.

Many insecticides are registered to control alfalfa weevil larvae. See the most recent edition of the Guide for Weed, Disease and Insect Management Management in Nebraska (EC130) for rates and restrictions of commonly used insecticides for alfalfa weevil larval control. They differ in their modes of action and pre-harvest intervals.

Previously, highly effective insecticides for alfalfa weevil control included pyrethroids (active ingredient ends in "thrin"), indoxacarb (e.g. Steward), and chlorpyrifos. However, alfalfa weevils have developed resistance to multiple pyrethroid insecticides across the western United States (including Arizona, California, Montana, Oregon, Washington and Wyoming). Although resistance has not been confirmed in Nebraska, some producers have noticed poor control with pyrethroid products, including lambda-cyhalothrin and beta-cyfluthrin.

It is important to note that because of complications due to overwintering of multiple life-stages of this insect (i.e., adults and eggs), control failures within a region could be the result of insecticide resistance or overwinter survival of eggs, which leads to a continuous early hatching of larvae. Therefore, scouting is necessary, and insecticides should only be applied when weevils exceed the economic thresholds. Pyrethroid insecticides also provide aphid control but can have detrimental effects on beneficial insects.

Besides pyrethroids, products containing indoxacarb (e.g., Steward) are effective and more selective. They do not affect most beneficial insects but will not provide aphid control.

Due to the recent reinstatement of food tolerances, existing stocks of products containing chlorpyrifos that are currently labeled for alfalfa in Nebraska can be used, although regulations may change in the near future and should be watched closely. However, chlorpyrifos will have negative impacts on the beneficial insects that keep aphids and other secondary pests under control.

Resources

• Integrated Pest Management of Alfalfa Weevil in North Dakota (E1676), by Patrick B. Beauzay, Janet J. Knodel, G.A.S.M. Ganehiarachchi, 2018. NSDU Extension Service, Fargo, North Dakota.
• Alfalfa Weevil, by Ashley Dean and Erin Hodgson. Iowa State University Extension and Outreach, Ames, Iowa.
• Yield Responses of Alfalfa to Simulated Alfalfa Weevil Injury and Development of Economic Injury Levels by R.K.D. Peterson, S.D. Danielson and L.G. Higley, 1993. Agronomy Journal 85: 595-601; view abstract.
• Alfalfa weevils (Coleoptera: Curculionidae) in the western United States are resistant to multiple type II pyrethroid insecticides, by Erika A Rodbell, Christopher G Caron, Silvia I Rondon, M Umer Masood, Kevin W Wanner, 2024. Journal of Economic Entomology 117(1): 280-292.