What is alfalfa, and what are alfalfa weevils?
Alfalfa (Medicago sativa) is a valuable forage crop in Wisconsin, that supports dairy and livestock production. However, alfalfa production can be threatened by the alfalfa weevil (Hypera postica), a beetle pest that can significantly damage alfalfa foliage if left unmanaged.
What do alfalfa weevils look like?
Larvae: The larvae are legless, green caterpillar-look-alikes with a white stripe along the center of their backs and a black head capsule. They are approximately ¼ to 3⁄8 inches long when fully grown
Adults: Alfalfa Weevil adults are typically small, grayish-brown beetles with elongated snouts. They measure about ¼ inches long and have distinctive longitudinal stripes on their backs.
Photo of alfalfa weevil larva (left) and alfalfa weevil adult (right). Photo credit: Bryan Jensen.
What are the life cycle and biology of alfalfa weevils?
Alfalfa Weevils overwinter as adults in leaf litter and debris near alfalfa fields.
In spring, adults emerge and 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, sometimes attached to the leaves.
The next generation of adults emerges later in the season and does not produce another larval generation before overwintering. These adults usually migrate out of the field for the summer before re-entering to the field to overwinter.
Source: Alfalfa weevil life cycle chart was 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 feeding. Whitney Cranshaw, Colorado State University, Bugwood.org
How do you scout for alfalfa weevils?
Scouting and Growing Degree Days: Scouting timing is critical. Start spot checking your alfalfa fields at 300 degree days (sine base 48°F) which will give you an advanced warning of significant feeding. Concentrate scouting on areas with past significant feeding or south facing slopes or sandy knolls. Find more information about degree days here.
Inspect alfalfa fields for signs of larval alfalfa weevil infestation, including skeletonized leaves and the presence of larvae. Begin monitoring early in the growing season (at 300 degree days) as alfalfa weevil damage can occur rapidly.
Use a sweep net (standard 15-inch net), visual scouting for skeletonized leaves and tip feeding, or stem dislodging to monitor weevil populations.
Photo of alfalfa weevil larvae in sweep net. Photo credit: Krista Hamilton, DATCP.
Stem Dislodging: For more accurate scouting of alfalfa weevils, hand-collect alfalfa stems, and dislodge alfalfa weevils into a bucket. This can help you determine the number of alfalfa weevils per stem and what the size distribution of alfalfa weevils is.
What is the threshold to reach for alfalfa weevil treatment?
40% of alfalfa stems with positive signs of alfalfa weevil larval feeding is the economic threshold in Wisconsin. Thresholds are NOT based on the number of larvae per stem.
More specifically, the economic threshold is to treat when 40% of the alfalfa stems have alfalfa weevil damage AND you are more than 7 days from your scheduled harvest date. For those fields with significant damage, scout regrowth thoroughly to ensure weevils are not feeding on new stems or crown buds.
As a general guideline, consider implementing control measures if larvae exceed an average of 1 to 2 per stem or if significant foliage damage is observed.
Integrated pest management (IPM) strategies for alfalfa weevils
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 the larvae. Feeding damage may continue to occur to the windrow, and some larvae may drop to feed on the next cutting regrowth.
Other best practices include:
- Encourage healthy alfalfa stands through proper fertilization, irrigation, and cutting schedules.
- Rotate alfalfa fields with other crops to disrupt the alfalfa weevil life cycle.
- Avoid planting alfalfa adjacent to overwintering sites such as wooded areas or tall grass.
Biological Control: Beneficial insects such as parasitic wasps and predatory beetles may help suppress alfalfa weevil populations.
Chemical Control: Insecticide applications may be warranted if Alfalfa Weevil populations exceed threshold levels. Choose products labeled for alfalfa and follow label instructions carefully. See Table 1 for common insecticide options for alfalfa weevil.
Resistance to insecticides has been seen commonly in alfalfa weevil in areas where chemical control is common. Be sure to rotate modes of action when using chemical control methods.
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 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
Alfalfa weevil response and management options
Monitor weather conditions and alfalfa growth stage to determine optimal timing for harvesting or insecticide applications, as needed.
Apply insecticides according to label instructions and consider factors such as application method, rate, and pre-harvest intervals.
Consider alternative forage crops or management practices in severely infested fields.
Environmental and ecological considerations for alfalfa weevil treatment
Minimize pesticide use whenever possible to reduce potential impacts on non-target organisms and environmental health.
Implement IPM practices that promote biological diversity and ecosystem resilience.
Conclusion
In specific locations, alfalfa weevil infestations can pose a significant threat to alfalfa production in Wisconsin. By implementing proactive monitoring and integrated pest management strategies, farmers can effectively protect their alfalfa crop.
References
Additional resources
For assistance with alfalfa weevil management and other agricultural pest issues, contact your local agricultural extension office or entomology expert.
Managing Soybean Aphids in Wisconsin Soybean Fields
Managing European Corn Borers in Wisconsin Corn Fields
Managing Corn Earworms in Wisconsin Corn Fields
Managing Western Bean Cutworms in Wisconsin Corn Fields
