Over the past decade, pea leaf weevils have made a relentless march across Alberta pea fields. The larvae feast on the nodules of field peas and faba beans, preventing them from fixing nitrogen, which limits growth. To manage pea leaf weevils, growers need to think ahead and make management decisions before their crop goes in the ground.
“There has been a significant range change for this insect, especially in the past three years, and we don’t have a clear reason why,” says Scott Meers, insect management specialist with Alberta Agriculture and Forestry in Brooks, AB.
Since 1997, when the pea leaf weevil was first reported in southern Alberta, it remained for many years south of the TransCanada highway. Pea leaf weevils are now well established throughout southern Alberta, west central areas of the province and as far north as Sturgeon County, north of Edmonton.
“We suspect the milder winters in recent years have facilitated the movement. Regardless of how they are moving, they are reducing yields. In the most affected fields in 2016 we saw damage in the range of 15 to 20 per cent yield loss,” says Meers.
Based on the wet weather in August 2016, researchers are predicting the potential for widespread damage by the pea leaf weevil in 2017. Predictions are based on how many notches per plant were damaged in the previous season. Experience has shown that areas with greater than nine notches suggests a very high potential for damage the following year.
“Seed treatments are the best way to reduce losses from pea leaf weevils,” says Meers. “There is no real economic threshold developed in-season, as once the insect arrives it’s too late to apply a seed treatment,” he explains.
Meers cautions that current studies show minimal economic benefit from foliar insecticides in controlling pea leaf weevils. In addition, foliar insecticides should be applied with extreme caution so as not to injure beneficial ground beetles.
Part of the reason people grow peas is for the crops’ nitrogen fixing properties and unfortunately pea leaf weevil larvae consume nitrogen fixing nodules. “So while there may be some yield benefit to adding nitrogen, it won’t help growers use peas or faba beans to fix nitrogen in the soil.”
Pea leaf weevils don’t fly when temperatures are below about 18 C so during a cool spring their arrival in pea fields may be delayed. If the weevils do not arrive until after the six-node stage, there will be very little yield loss from feeding activity. However, every spring season usually has a stretch of warmer weather that will enable the migration of pea leaf weevils into fields.
“This really is a new pest for many growers in Alberta and there is always a transition period,” says Meers. “It takes a few years of yield loss for growers to adopt new strategies. But given the spread of these insects and the impact they have on yield, managing pea leaf weevils should be part of every pea growers’ annual pest management plan.”