Managing Alfalfa Weevil in Wisconsin Alfalfa Fields

What is the alfalfa weevil?

Alfalfa is a valuable forage crop in Wisconsin that supports dairy and livestock production. Alfalfa can be threatened by the alfalfa weevil (Hypera postica), an early-season pest that feeds on alfalfa leaves and may severely damage first-crop and second-crop alfalfa fields. By implementing proactive scouting and integrated pest management strategies, Wisconsin farmers can protect and extend the productivity of their alfalfa stands.

What do alfalfa weevils look like?

Alfalfa weevil larvae. Photo credit: Krista Hamilton, DATCP.

Larvae: The larvae are legless, green grubs with a white stripe along the center of their back and a shiny black head capsule. Larvae grow to a length of approximately 3⁄8 inch.

Adults: Alfalfa weevil adults are small (about ¼ inch) gray-brown snout beetles with a distinct V-shaped strip down the center of the thorax and abdomen. The dark V-shaped marking makes the alfalfa weevil easy to identify.

Photo of alfalfa weevil larva (left) and alfalfa weevil adult (right). Photo credit: Bryan Jensen.

Introduction
Identification
Life Cycle and Biology
Damage Symptoms
Monitoring and Detection
Threshold Levels
IPM Strategies
Response and Management Options
Environmental and Ecological Considerations
Additional Resources

What are the life cycle and biology of alfalfa weevils?

Alfalfa weevils overwinter as adults in leaf litter and debris near alfalfa fields.

In early spring, the adults emerge from overwintering sites and migrate to alfalfa to lay eggs in alfalfa stems. The eggs hatch into larvae, which feed on alfalfa foliage.

Larvae develop through four instars before pupating in a woven cocoon on the alfalfa plant, often attached to the leaves.

The next generation of adults emerges in early summer but does not produce more larvae until the next spring. Instead, these adults usually migrate out of the field for the summer months then re-enter to the field to overwinter.

Source: Alfalfa weevil life cycle chart created by the Crop Protection Network.

What are the symptoms of alfalfa weevil damage?

Alfalfa weevil larvae feed on alfalfa foliage, causing skeletonization of leaves and reduced plant vigor.

Severe infestations can lead to yield loss and reduced forage quality.

Image Source: Alfalfa weevil leaf feeding injury. Whitney Cranshaw, Colorado State University, Bugwood.org

How do you scout for alfalfa weevils?

Scouting and Growing Degree Days: Start spot checking your alfalfa fields at 300 degree days (sine base 48°F) for newly hatched larvae and early signs of feeding. Concentrate scouting on warmer south facing slopes or sandy knolls where leaf pinhole feeding is likely to appear first. Find more information about degree days and alfalfa weevil development in the Alfalfa Weevil Thermal Model.

Sweep Net: Using a standard 15-inch net, walk a M-shaped pattern in the field and take two sets of 50 sweeps for a total of 100 sweeps. The size and number of larvae collected in the net indicate the development stages (instars) present and overall weevil abundance. For example, a sweep with many early first and second-instar larvae signals the population is young and feeding damage is likely to increase as the larvae grow larger. By contrast, a net containing larger third and fourth-instar larvae may represent the peak damage period, although the larvae are also closer to reaching the non-feeding pupal stage. Peak weevil damage often occurs as the first alfalfa crop is ready for harvest. Note that control decisions should not be made based on sweep net counts.

Photo of third and fourth-instar alfalfa weevil larvae in sweep net. Photo credit: Krista Hamilton, DATCP.

Check out this video for instructions on how to use a sweep net, Emily Bick, UW-Madison Extension & Crop Protection Network

Assess Tip Feeding: For more accurate scouting, walk an M-shaped pattern and hand-collect alfalfa stems or examine the tops of 50 alfalfa stems at random. Closely inspect each stem for signs of weevil feeding. Count all the stems with feeding and divide by 50 to determine the percentage of tip feeding.

What is the threshold for alfalfa weevil treatment?

The treatment threshold for alfalfa weevil in Wisconsin is when 40% of alfalfa stems in a field show signs of alfalfa weevil larval feeding. Thresholds are not based on the number of larvae per sweep.

More specifically, the threshold is to treat when 40% of the alfalfa stems have alfalfa weevil damage and the field is more than seven days from the scheduled harvest date. For first-crop fields with severe damage, scout second-crop regrowth thoroughly to ensure weevils are not feeding on new stems or crown buds.

As a general guideline, check regrowth 4-5 days after cutting, take a sweep net sample to confirm larvae are present, and collect or examine a total of 50 stems to assess feeding injury. The treatment threshold for regrowth is when 50% of stems show feeding and larvae are still active in the field.

Integrated pest management (IPM) strategies for alfalfa weevil

Cultural Control: Timely harvest is often the best management strategy for alfalfa weevil. Keep in mind cutting without the use of mower conditioners or rollers will not necessarily kill all the larvae. Feeding damage may continue beneath windrows, and some larvae can survive and feed on the regrowth.

Biological Control: Beneficial insects such as parasitic wasps and predatory beetles are very effective in controlling alfalfa weevil populations, which is why insecticides should only be used as a last resort.

Chemical Control: Insecticide applications may be warranted if alfalfa weevil tip feeding damage exceeds the 40% threshold for first crop alfalfa and 50% threshold for second-crop regrowth. Choose products labeled for alfalfa and follow label instructions carefully. See Table 1 for common insecticide options for alfalfa weevil.

Alfalfa weevil resistance to insecticides has developed in areas where chemicals are commonly used for control. Be sure to rotate modes of action when using chemical control methods.

Alfalfa Weevil Management in the Upper Midwest, Bryan Jensen, Crop Protection Network

Table 1: Insecticide options for alfalfa weevil

This is not an exhaustive list and does not endorse specific products or trade names. See UW-Madison Extension publication Pest Management in Wisconsin Field Crops (A3646) for an extensive list.

Group	Class	Active ingredient	Trade Names
1A	Carbamate	Methomyl Carbaryl*	Lanate Sevin
1B	Organophosphate	Malathion† Phosmet†	Malathion Imidan
3A	Pyrethroid‡	Alpha-cypermethrin‡ Beta-cyfluthrin‡ Cyfluthrin‡ Gamma-cyhalothrin‡ Lambda-cyhalothrin‡ Permethrin‡ Zeta-cypermethrin‡	Fastac Mustang-Maxx Baythroid Tombstone Declare, Proaxis Warrior and generics Arctic, Permethrin, others Mustang Maxx
22	Oxadiazine	Indoxacarb	Steward

*Carbaryl can burn regrowth if applied after cutting.
†Single group 1B products tend to only have ~50% control but are closer to 100% efficacy if in a mixture (e.g., malathion + phosmet).
‡Resistance to pyrethroids appears likely, especially in western and central MN, but is not yet officially confirmed. If you have what appears to be an insecticide failure in a field, do not use the same group twice in a row.

Table 1 Source: https://blog-crop-news.extension.umn.edu/2023/05/updated-alfalfa-weevil-management.html