Pulse Growers preach prudent patience pertaining to pea leaf weevils

Local Journalism Initiative Reporter

While it may be tempting to “revenge spray” when it looks like someone’s taken grandma’s pinking shears to their plants, Alberta Pulse Growers reminds producers that the use of non-selective insecticides in response to pea leaf weevils does more harm than good.

2021 saw a lower level of damage from the pea leaf weevil than previous years, according to surveys done by the province of Alberta. This is suspected to be due to dry growing conditions over the course of the year.

The pea leaf weevil is a small insect that feeds on pea and fava bean plants when they emerge in the spring, said Nevin Rosaasen, Sustainability and Government Relations Lead with Alberta Pulse Growers. 

“The adult bin lays a dig at the soil surface right at the base of the stem of the plant. And then when that egg hatches, the larva actually travels down the crown root in search of nodules. nodules are these nitrogen fixing powerhouses, they kind of look like cancerous growths on the root,” said Rosaasen. 

These nodules, a symbiotic relationship between rhizobium bacteria and the plant, fix nitrogen present in the atmosphere and turn it into a form usable by the pea or fava bean plant. 

“When you have pea leaf weevils that are eating the nodules, that impacts the potential yield of that plant. There are some years when pee leaf weevils are in higher numbers and actually represent perhaps an economic concern to farmers,” said Rosaasen. Years where there are warm conditions in April or May pose a great risk to the yield of the plants, as the weevils may emerge earlier.

Thankfully, there are options to help combat this, said Rosaasen, though it may not be necessary every year.

“There are seed treatments available so you can treat your seed with a type of insecticide that actually paralyzes those weevils and allows the plant to grow through its seedling stage to adult plant, or mid season plant that isn’t isn’t impacted by the nodule feeding,” said Rosaasen. “We’ve been doing lots of research on pea leaf weevils with Alberta Pulse Growers. We have a plot to field program where we’re actually doing field scale research. And we looked at the benefits of seed treatment versus not treating your seed, and to be honest, it was only maybe one in 20 years where producers would see a benefit from the additional costs of seed treatment versus the yield that they would lose.”

Alberta Pulse Growers helps combat this cost by conducting surveys, looking for signs of the insect and publishing maps in late January to early February to allow for producers to make the decision whether to opt for a seed treatment that year.

While the weevil is an insect of concern, they rarely cause enough damage to present a serious economic impact, said Rosaasen, due to the presence of “field heroes” who serve as predators for the insect.

This is one of the reasons that Alberta Pulse Growers doesn’t advocate for the use of non-selective insecticides, as well as due to the fact that in many cases spraying will not change the outcome.

“We do not actually recommend spraying an insecticide that would be a full or non selective insecticide. Because by the time you find out you have pea leaf weevil notching, they’ve already laid those eggs. So we call it a revenge spraying event where it doesn’t actually give you any benefit. And you’re actually doing more harm than good by killing the beneficial pollinators, the parasitoids, the predator insects in your fields,” said Rosaasen. “[Spraying] definitely hurts the ecology that is your bread and butter. So producers are there, they’re not no longer spraying for pea leaf weevil. So that’s a good thing.”

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