Pea Weevils
Pest Common Name
- Pea Weevil (Bruchus pisorum)
- Peas
Adult pea weevils are not actually weevils, and thus lack the characteristic weevil “snout.” They are robust beetles, about 1/4 inch (6 mm) in length and 1/10 inch (2.5 mm) wide. Adults are brown in color, but with black, grey and white patterning on their backs. The wing coverings (elytra) do not entirely cover the abdomen, leaving the rearmost part exposed. Larvae can reach 1/5 inch (5 mm) in length, legless, with a white body and brown head. Eggs are bright orange and about 1/16 inch (1.5 mm) long (Figure 1).
Biology
Pea weevils have only one generation per year, with development taking 7-12 weeks. Adults overwinter in seeds and plant debris in the soil, or in stored seed. When adults emerge, they can fly as far as 4 miles (6.4 km) to find pea plants where they need to feed on pea pollen before mating. Females deposit eggs on the outside of pea pods. Larvae hatch one to two weeks later and burrow directly through egg and pod, and into the maturing seed. Larvae develop within the pea seed, slowly hollowing it out through their feeding. Larvae feed for about four to eight weeks before pupating. Adults may remain in the seed to overwinter or emerge and overwinter elsewhere.
Damage
Pea weevil larvae and adults feed within pea seeds, reducing the weight of the seed by up to 30%. While adult exiting the seed leaves a noticeable .08-.12 inches (2-3 mm) hole, damage from larvae feeding is not easily observed and may only be noted once seeds reach storage facilities. Pea weevil feeding significantly reduces market value of seed, making dry peas unfit for human consumption, and decreasing germination rates and stunting plant growth if seeds are sown. Pea weevils can cause 42-82% seed damage in the Pacific Northwest and even higher rates of infestation are documented worldwide.
Monitoring
Monitoring for pea weevils should run from flowering through crop maturity. Every two weeks sample eight to 10 sites around field edges, where infestation is most likely. At each sample site make 25, 180 degree sweeps and count the pea weevils collected. Average the number of pea weevils per 25 sweep sample across all eight to 10 samples. The economic threshold in the Great Plains is two adults per 25 sweeps.
Management
Primary Management Tactics
Remove volunteer plants and plant debris after harvest. Store pea straw well away from pea fields (at least 4 miles) or eliminate it before spring by burning it or feeding it to cattle. These practices reduce overwintering populations and eliminate potential sources of infestation.
Physical
- Disc soil after harvest and destroy crop residue that could harbor overwintering pea weevils
Cultural
- Control volunteer peas
- Pea cultivars with high concentrations of tannins in the pea or pea pod have shown resistance to pea weevil
- Early planting and harvest can reduce infestation and damage
Biological
- Parasitoid wasps such as Eupteromalus leguminis and Uscana senex have shown promise as biological control agents for pea weevil
Chemical
- Insecticides are most effective if applied before bloom, when pea weevils are present but before females have laid eggs
- Additional applications may be necessary if pea weevils are still present 10-14 days after first spray
- If pea weevils are not high in number, border spraying may be sufficient to achieve the desired level of control
- Fumigate if pea weevils are found in peas directly after harvest, in storage or in purchased seed
- Recommendations for pesticides to use in the management of pea weevil can be found on the PNW Pest Management Handbook website
Pesticide Warning
Always read and follow the instructions printed on the pesticide label. The pesticide recommendations in this University of Idaho webpage do not substitute for instructions on the label. Pesticide laws and labels change frequently and may have changed since this publication was written. Some pesticides may have been withdrawn or had certain uses prohibited. Use pesticides with care. Do not use a pesticide unless the specific plant, animal or other application site is specifically listed on the label. Store pesticides in their original containers and keep them out of the reach of children, pets and livestock.
Trade Names — To simplify information, trade names have been used. No endorsement of named products is intended nor is criticism implied of similar products not mentioned.
Groundwater — To protect groundwater, when there is a choice of pesticides, the applicator should use the product least likely to leach.
- Figure 1. Dennis Schotzko, University of Idaho
Desiree Wickwar, Entomologist, IPM Program Manager
Sanford Eigenbrode, Distinguished Professor of Entomology
2023