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 and burrow belowground to feed on the root nodules, resulting in poor plant growth and reduced nitrogen fixation, causing yield loss. 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 V5 (fifth node stage).
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, it’s presence was confirmed in northwest Manitoba near Swan River.
Since then it has been confirmed in most of western Manitoba. The furthest east it has been found was Gladstone and Holland in 2021.
Only field peas and faba beans are at risk of damage from pea leaf weevil. Other crops in the legume family (soybeans, dry beans) may be fed on by adults but do not suffer significant damage from larvae.
In 2022, we are trying to determine the distribution and abundance of pea leaf weevil within Manitoba by participating in the Prairie Pest Monitoring Network’s PLW survey. Adult weevils chew the leaf margins, resulting in a characteristic notching along the leaf margins. For this survey, we count the number of leaf notches per pea plant and use it to create a risk map for the province.
Pea farmers or those scouting pea crops who think they may have pea leaf weevil in their field in central and eastern Manitoba are advised to contact John Gavloski, entomologist with Manitoba Agriculture, or a production specialist with Manitoba Pulse & Soybean Growers. That will assist us in tracking the distribution of pea leaf weevil in Manitoba.
In field peas, economic thresholds for foliar insecticide applications are once 30% of plants have notches on the uppermost closed stipule or clam leaf at V2 – V3 (two to three true nodes). In faba beans, a nominal threshold when 15% of plants have notches on the uppermost closed bifoliate leaf at V2-V3 has been proposed. However, insecticide applications do not prevent yield loss since healthy adult weevils will continue to travel into the field from elsewhere and, if there are notches on the expanded leaves below the clam leaves, female weevils have likely already laid eggs in the field. Foliar insecticide applications are considered ‘revenge sprays’.
Preventative insecticidal seed treatments are a more effective option. Seed treatments containing imidacloprid and thiamethoxam 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.
Ground beetles and rove beetles are predators of pea leaf weevil, eating 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.
- 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.