Pea leaf weevil advance slowed by cold, dry prairie climate

Pea leaf weevils are a widespread invasive pest but populations are currently low. | Agriculture Canada photo

The pest typically spreads into new areas by walking, which is another reason its expansion across the Prairies has been slow

It’s been tough slogging for the pea leaf weevil to expand its range in prairie pea and fababean crops.

“One of the factors that we think is contributing to diminishing populations, at least here in Saskatchewan, are these hard cold snaps without a whole lot of snow. So these animals are a little bit more cold-sensitive than some other insect pests and we suspect that might be having an effect on them,” said James Tansey, provincial specialist of insect/vertebrate pest management for Saskatchewan Agriculture.

“There’s also some indication from some of the modelling that dry conditions can have a negative effect on them as well.”

The pea leaf weevil is an invasive species in Canada so pathologists are keeping a close eye on it because it does pose a threat to prairie pea and fababean crops.

It’s a common pest in its native Europe, and researchers are building models to help understand if a warming prairie climate will make the area more hospitable for the pest.

In-season scouting for leaf notching is the most common way to determine if the pest is present in your fields because both larvae and adults are difficult to find.

Notching is caused by the adult pea leaf weevil, which leaves a u-shaped indentation along the edge of a leaf.

The main cause of yield loss comes from pea leaf weevil larval feeding on root nodules that causes reduced nitrogen fixation by the plant, which is why boosting nitrogen fertility can help prevent yield loss in affected crops.

The pea leaf weevil typically spreads into new areas through walking, which is another reason its expansion across the Prairies has been relatively slow.

Localized forecasts for the pea leaf weevil are based on leaf notching observations from the previous year.

Pandemic-related travel restrictions placed on researchers hampered efforts to monitor the pest last summer, but the pea leaf weevil does not appear to be a major concern for most growing areas on the Prairies.

“The population is up a tiny bit. So right in that south-central region (in Saskatchewan), and a little bit in the southwest. But overall, the population is really low,” Tansey said

Over the last few years, Manitoba Agriculture and Resource Development trapped pea leaf weevils in the northwestern region of Manitoba, with weevils found near Swan River, Kenville, Minitonas, Gilbert Plains and Dauphin, according to a Manitoba Pulse Bean report.

Pea leaf weevils leave a distinctive notching pattern on leaves. | B. Olson photo

The pea leaf weevil started to become a problem in southern Alberta around 1996, then it jumped the Trans-Canada Highway and got into central Alberta and it has been found as far north as the Peace region in northwestern Alberta, said Dr. Maya Evenden of the University of Alberta.

“The notching survey that was conducted during the season of 2020, most of the heavier notching activity was in farms around Edmonton, and there still was in the far southern parts of the province around Lethbridge. All of the other fields that were surveyed didn’t show appreciable damage, so probably wouldn’t have high levels this season,” Evenden said.

To get a better sense of the pea leaf weevil population in Alberta, Evenden adopted a pheromone for the pest that was first identified in England.

“It’s the male weevil that produces the aggregation pheromone, and both the females and males respond to the signal. That can be useful for pest management because you get a better picture of the whole population,” Evenden said.

“What we did was we looked at different lures. So the same compound but different rates of release, so high release versus low release. And we also combined the pheromone signal itself with chemical cues that are released by pea plants.”

She said the research found the pheromone alone is just as effective as traps that use the pheromone and chemical cues from peas.

“There’s two times in the season where there’s adults and so we trap in the early spring, like from April through mid-June when they’re entering into the field, and then we trap from August to mid-September when they’re leaving the field. So those are the two times we can catch adults, but they’re actually adults from different generations,” Evenden said.

She said the notching surveys that monitor adult feeding damage are useful for growers to know whether there is likely to be high levels of damage the following season.

However, these surveys are labour intensive and they’re not as sensitive in areas where there’s not a high population density.

“If you want to know where the pea leaf weevil is going next, it’s better to have a more sensitive tool for attracting the insects and that’s sort of where the pheromone traps come in,” Evenden said.

“They really are good at detecting the leading edge of invasion. That’s how we detected their presence in the Peace River area. It wasn’t by seeing notching damage at all, it was just by catching them in our pheromone-baited traps.”

This weevil was feeding on fababeans. | Saskatchewan Pulse Growers photo

She said when the pheromone-baited traps are used in the fall they can provide a good indicator of the population going into the winter, which can help inform growers if they should use a seed treatment the following spring.

“The best way to control for pea leaf weevil is to use a treated seed in the spring and you won’t know whether you need to treat or not based on the spring catch because that happens after you’ve already seeded,” Evenden said.

Another reason to trap in the fall is to confirm that pea leaf weevils are causing notching damage, because there are several species of weevil that make similar feeding damage, Evenden said.

She said pitfall traps are the best kind of traps to use with the baited lure to catch the pea leaf weevil.

“A pitfall trap is essentially just a cup that’s dug into the ground, and then there’s a little lid that’s put over it to prevent water from getting in. The lure is hung from the lid and then in the bottom of the cup we put antifreeze to kill and preserve the insects that fall in,” Evenden said.

Funding for the pheromone trap research has not been renewed in Alberta, so the pea leaf weevil monitoring and forecasts rely solely on the notching survey.

These pheromone-baited traps collect a lot of bugs and it can be difficult to correctly identify the pea leaf weevil, so it may be best to leave them to the researchers.

However, the lures are available through ChemTica, the pitfall traps are easy to make and there is nothing stopping producers from brushing up their bug identification skills to see if the pest is present in their fields.

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