Pea Leaf Weevil Emergence (05/27/21)
As we move into late May, pea leaf weevil (PLW) will likely begin emerging in western areas of North Dakota. PLW adults are greyish brown, around 3/16th of an inch long, and recognizable by their broad snout. Adult weevils are similar in appearance to another local weevil, such as the sweet clover weevil, but can be separated by the presence of three light stripes on the thorax that extend onto the wing covers of young PLW adults. PLW is an economic pest of field peas and faba bean. Secondary hosts include clover, dry bean and alfalfa. Adult weevils will feed on these secondary hosts, but the feeding injury by PLW is not economic and they cannot fully develop on these secondary hosts.
PLW was first observed in Golden Valley County, ND during the 2016 growing season, and has since increased its distribution across the field pea growing regions of the state. As of 2019, PLW had been confirmed in 13 western North Dakota counties.
Scouting should begin in tandem with the emergence of field pea or faba bean in the spring, especially as temperatures warm above 63°F. When scouting, it is easier to scout for their feeding injury over the adult weevil. The adult feeding injury causes half-moon leaf notches on the lowest leaves along the field edges. Notches are small and are symmetrical in appearance. This type of injury does not typically result in yield loss as the crop can recover. However, larvae feeding below the soil surface on nitrogen-fixing bacteria within the root nodules can lead to significant damage. Overall, this can result in poor plant growth and lower crop yields.
Utilizing cultural and chemical integrated pest management (IPM) strategies can help lower the incidence of injury by the PLW. For cultural control, late seeding of field peas by around 10 days can reduce foliar leaf feeding injury as pea emergence is delayed until after peak emergence of PLW. The use of insecticidal seed treatments can also help protect crops. Seed treatments can reduce defoliation from adult weevils, lower egg laying potential, and lower larval feeding on the nitrogen-fixing nodules. Two active ingredients are currently registered in field peas: thiamethoxam (Cruiser Max, Cruiser 5F) and imidacloprid (Dyna-Shield imidacloprid 5, Gaucho 600, and others).
If economic threshold for crop injury is met (30% of plants with half-moon shaped notches between the seedling and 6th node stage), a foliar insecticide treatment may be warranted. However, control timing can be difficult as foliar insecticides should be applied before the female PLW has the opportunity to lay eggs. More information on the PLW can be found by searching for the Integrated Pest Management of Pea Leaf Weevil in North Dakota E1879 NDSU Extension publication or the North Dakota Field Crop Insect Management Guide E1143 (revised).
Extension Specialist/Crop Protection
NDSU North Central Research Extension Center