Life Cycle of the Pea Leaf Weevil
In spring, pea leaf weevil adults fly in from overwintering sites (perennial legume crops) to their host crops, peas and faba beans. Once they start feeding on these host crops, they begin laying eggs.
Larvae hatch from eggs and burrow belowground to feed on root nodules, reducing nitrogen fixation, causing yield loss and poor plant growth. Larvae feeding also creates wounds for root rots to infect, further damaging roots.
Adult weevils feed above ground on leaves, but rarely cause economic damage. The characteristic leaf notching on leaf margins can be used to monitor adult weevil presence (Figure 1). The best time to look for potential feeding damage is at the end of May to early June when peas and faba beans are emerging up to V6 (sixth true node stage).
Only field peas and faba beans are at risk of damage from pea leaf weevil. Other crops in the legume family (soybeans, dry beans, alfalfa) may be fed on by adults but do not suffer significant damage from larvae feeding on root nodules.
Surveillance and Distribution
The first record of pea leaf weevil in Alberta was in 1997, and it was first found in Saskatchewan in 2007. In the fall of 2019, their presence was confirmed in northwest Manitoba near Swan River.
Since then their distribution has been confirmed in most of western Manitoba. The furthest east pea leaf weevils have been found were in fields near Gladstone and Holland.
Starting in 2022, surveyors in Manitoba began to participate in the Prairie Pest Monitoring Network’s pea leaf weevil survey to determine the distribution and abundance of pea leaf weevil within Manitoba. Adult weevils chew the leaf margins, resulting in a characteristic notching along the leaf margins. The number of leaf notches per plant directly correlates to the abundance of adult weevils in the field. For this survey, the number of leaf notches per pea plant are counted and distribution maps are created from the results.
In 2022, pea leaf weevil populations were greatest in northwestern Manitoba and occurred at low population numbers in southwestern Manitoba. You can contribute to this pea leaf weevil survey! If you’re growing peas and are willing to have surveyors enter your field to count leaf notches, contact MPSG’s production specialist to sign up. Biosecurity protocols are followed and you will receive a report for your field.
Economic thresholds are available for foliar insecticide applications, however, foliar applications do not prevent yield loss and are considered revenge sprays.
- Field peas economic threshold: when 30% of plants have notches on the uppermost closed stipule (clam leaf) at V2 to V3
- Faba beans proposed nominal threshold: when 15% of plants have notches on the uppermost closed bifoliate leaf at V2 to V3
Foliar insecticide applications do not prevent yield loss since healthy adult weevils will continue to travel into the field from overwintering sites and, if there are notches on the expanded leaves below the clam leaves, female weevils have likely already laid eggs in the field. The damage has already been done.
Preventative insecticide seed treatments are a more effective option. Seed treatments containing imidacloprid, thiamethoxam or chlorantraniliprole offer protection against the pea leaf weevil. However, while adult defoliation, egg laying and larval feeding are reduced with a seed treatment, they are not eliminated entirely. There are no forecasting models available to determine when a seed treatment is likely to be economical, so seed treatment decisions are based on regional history of pea leaf weevil populations and damage levels. Current populations of pea leaf weevil are low in Manitoba and seed treatments are unlikely to provide a return on investment at this time. To better understand when farmers are likely to see a benefit to an insecticide seed treatment, MPSG has initiated pea seed treatment trials in Roblin and Swan River starting in 2023. MPSG’s On-Farm Network has also begun hosting pea seed treatment trials with interested farmers. To learn more about participating in these trials, visit here.
Natural enemies of pea leaf weevils include ground beetles and rove beetles, which eat weevil eggs on the soil surface.
Cultural control methods are being investigated. Reduced tillage systems, trap cropping, and intercropping are options to reduce the impact of pea leaf weevil. Factors that promote rapid crop establishment also produces more resilient plants that can better tolerate feeding damage. Adding nitrogen to offset larval feeding damage has not shown consistent results in research conducted in Western Canada.
- For more information regarding pea leaf weevil scouting methods, thresholds and management, see SPG’s Pea Leaf Weevil factsheet or NDSU’s Integrated Pest Management of Pea Leaf Weevil factsheet.
- For a video explaining the action thresholds of pea leaf weevil and how to scout for feeding damage, view Pulse School: Scouting the Elusive Pea Leaf Weevil.