Extension agents have been finding pea leaf weevil in the pulse growing areas in Montana and western fields in North Dakota.
“Pea leaf weevil emerges over a wide window,” said Janet Knodel, NDSU Extension entomologist. “There have been some found this spring, but not as many as other years, but there are some hot spots – Beach in Golden Valley is one hot spot."
Beach is the area in North Dakota where the pea leaf weevil was first discovered.
The pea leaf weevil has been found in 13 counties in the western region, increasing in the number of counties over the last few years. However, it is possible, the pea leaf weevil was always in the area more than five years ago, just not scouted for and at very low densities.
The weevil is 3/16th of an inch in length, greyish-brown in color, with stripes and has three distinct lines on its thorax.
When scouting for adult pea leaf weevil, producers should look for the “half-moon notches” on the leaves.
After overwintering, the adult pea leaf weevil feeds on the early clam leaves (leaves that are folded up like a clam), causing a symmetrical pattern of half-moon notches.
“The notches on the leaves look similar to the damage done by cutworms. However, the difference is the pea leaf weevil feeds all the way through the folded up leaf, and when it is unfolded, it leaves a symmetrical pattern. The cutworm damage is not symmetrical,” Knodel explained.
Female weevils must find a pea or faba bean field to lay their eggs, and the emerging larvae cause economic damage due to feeding on the nitrogen-fixing root nodules.
“You have to open up the nodule to find the larvae inside feeding,” she said. “The feeding on those root nodules impacts the ability of the plant to fix nitrogen for itself and for the soil around it.”
To scout for the pea leaf weevil, begin on the edges of the field and 100 feet into field, and look for leaf notches in the canopy. Count the number of leaves with notches on the lowest leaves of the plant.
“Check 10 plants in each spot for the notches and calculate the percentage plants infested with notches,” Knodel said.
The pea leaf weevil can fly, but it won’t leave the area if food is there. They will stay in shelterbelts, pulse crops or forage crops and overwinter there.
Foliar insecticides are applied when 30 percent of the leaves have notches. However, research has found that it does not work as well as insecticide seed treatments because of the long window of weevil emergence in the spring and multiple movements into fields.
“Insecticide seed treatments work better, because they reduce adult defoliation, egg laying and larval feeding on the root nodules,” she said.