1. Insect
  2. Legume, Grass, and Field Seed Crops
  3. Pests of Alfalfa Grown for Seed

Alfalfa seed-Alfalfa weevil

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October 2015

Hypera postica

Pest description and crop damage Larvae are about 0.375 inch long, yellow to green, with a white stripe down the back. They feed in and on the buds of alfalfa.

Sampling and thresholds Treat when 20 to 30% of plant terminals show feeding damage - or - when larval populations reach 20 or more per half-circle (180° sweep). These thresholds are simply guidelines to help growers determine when to treat. Weather, plant vigor, irrigation schedules, history of weevils in the area, and a complex of factors may determine whether treatment is justified. In specific geographic areas we have been observing resistance to pyrethroid insecticides, specifically lambda-cyhalothrin. We have also observed increase tolerance to chloropyrifos and pyrethroids, specifically lambda-cyhalothrin in alfalfa weevil populations in select geographic areas, but we have no populations that can survive exposure to labeled rates of chlorpyrifos. Resistance management strategies detailed in insecticide labels should be followed to help reduce the risk of further insecticide resistance development.

Management-chemical control

  • bifenthrin (Brigade 2EC or Discipline 2EC) at 0.06 to 0.1 lb ai/A. REI 12 hr. Do not apply more than three times per season or at intervals less than 21 days. May not be used for hay, forage, or human consumption. Do not apply at ground level within 25 ft of aquatic habitat, 150 ft if applied by air. 24c SLN ID-070009 and 040009; OR-070011 and 040039; WA-070015 and 040027.
  • dimethoate 400EC at 0.25 to 0.5 lb ai/A. PHI 10 days. REI 48 hr. For suppression only. Do not apply if crop or weeds are in bloom. Effective only on cutting to which chemical is applied. Do not feed or graze livestock.
  • gamma-cyhalothrin (Proaxis, Declare) at 0.01 to 0.015 lb ai/A. PHI 1 day for forage, 7 days for hay. REI 24 hr. Do not exceed 0.06 lb ai/A per season or 0.015 lb ai/A per cutting. Do not apply on ground within 25 ft of aquatic habitat, 150 ft if applied by air, or 450 ft if applied from ULV.
  • indoxacarb (Steward EC) at 0.065 to 0.11 lb ai/A. PHI 7 days. REI 12 hr. Make no more than one application per season. Do not apply when bees are in the area. Chemigation applications through center pivot is permitted.
  • lambda-cyhalothrin (Warrior) at 0.02 to 0.03 lb ai/A. PHI 1 day for forage, 7 days for hay. REI 24 hr. Do not exceed 0.03 lb ai/A per cutting or 0.12 lb ai/A per season. Do not apply on ground within 25 ft of aquatic habitat, 150 ft if applied by air, or 450 ft if applied from ULV. Do not apply while bees are active. Advisable to move bee shelters for 2 to 3 days after application.
  • permethrin at 0.1 to 0.2 lb ai/A. PHI 0 days at rates equal to or less than 0.1 lb ai/A, 14 days for rates greater than 0.1 lb ai/A or more. REI 12 hr. Do not exceed 0.2 lb ai/A per cutting. If used during bloom, remove bees from field for 3 days.
  • zeta-cypermethrin (Mustang Max) at 0.014 to 0.025 lb ai/A. PHI 3 days for cutting or grazing, 7 days for harvest. REI 12 hr. Allow at least 7 days between applications. Do not exceed 0.1 lb ai/A per cutting or 0.3 lb ai/A per season. Do not apply on ground within 25 ft of aquatic habitat, 150 ft if applied by air, or 450 ft if applied from ULV.