Pea leaf weevils were first observed in Manitoba over 5 years ago in 2019, arriving from the western prairies into the Swan River Valley and they have moved further south and east every year since.
Adults are slender, greyish-brown, and approximately 5 mm long. They can be distinguished by three light-coloured stripes on their thorax. They have a short snout (unlike most weevils), and they tend to “play dead” when disturbed.
Pea Leaf weevil, Shortdale, MB May 12, 2025, NOTE: Temperature 27°C photo credit: Wendy McDonald
Adult weevils hide under the snow to survive the winter snuggled in amongst alfalfa and other perennial legumes, in roadside ditches and shelterbelts. They emerge in the spring, when temperatures are warmer than 15 °C and those adults fly to pea or faba bean fields and then walk around in those fields with the aim to reproduce. It seems like the adults keep coming in waves from the ditch through out the spring. Applying foliar insecticide would only kill the adults present in the field at the time of application and more pea leaf weevils would be back the next day. We can tell pea leaf weevils are in a field when we see what they have eaten. We count the number of notches on the plants to get an idea of how large the pea leaf weevil population is. Pea leaf weevils uniquely nibble only on the leaf margins of peas and faba beans which leaves a crescent-shaped notch missing. It looks like the leaf was “hole punched” or has a “scalloped edge”.
No bites on the pea clam shell leaf, Dauphin, MB June 9, 2025 photo credit: Wendy McDonald
Pea Leaf Weevil notches on faba bean Minitonas, MB June 4, 2025 , photo credit: Wendy McDonald
The pea plants can typically withstand 50 percent defoliation without the damage impacting yields. By the time we see notching, that means that the adults have already ben there, mated, and laid eggs.
Females lay up to 1,500 single eggs on the soil surface or on developing plants as they feed and move through out the field. The majority of eggs are laid in May and June and require 18 to 20 days to hatch, depending on temperature and humidity. Eggs are small, smooth and oblong. They are white when laid, but turn nearly black at hatching.
In July, eggs hatch and larvae develop under the soil over a period of 30 to 60 days. These larvae grow through five instar stages. They are “C” shaped, legless, with a dark brown head capsule. The rest of the body is a milky-white color. First instar larvae bury into the soil after hatching, and search out root nodules of field pea and faba bean plants.
Larva of pea leaf weevil. Source: Michael Dolinski
Larvae enter and consume the microbial contents of the root nodules. We think nitrogen-fixing bacteria are just about the only thing these larvae eat. Larvae crawl to, crawl in, hollow out and eat the inside of the nitrogen producing nodules. Actually, the pea leaf weevil larvae consume the bacteria that live inside root nodules.
We suspected that high pea leaf weevil populations could destroy a large percent of the nitrogen-fixing root nodules (Rhiozobium). The larvae feeding on nodules are hard to see because it occurs underground but may cause significant yield losses and weaken plants which makes them less drought tolerant and then they produce less seed.
When pea leaf weevil larvae have grown to their 5th instar, they are 3.5-5.5 mm long and they burrow into the soil and pupate. Pupation takes place in the soil and requires 15 to 20 days. The new generation of adults emerge from late July to August. We have heard reports of large numbers of weevils on the cutter bar during pea harvest this year. Adult pea leaf weevils come out and are looking for anything green to eat including volunteer peas post harvest. When peas and faba beans are not available, adult pea leaf weevils will consume foliage of all available legumes (wild and cultivated), including alfalfa, clover, and vetch. Adult weevils fly and migrate to their overwintering sites.
October and still nibbles on volunteer peas, Grandview, MB October 3, 2025 photo credit: Wendy McDonald
So how do we control pea leaf weevils in Manitoba?
Other Canadian prairie research has not seen a significant protection of yield by applying a foliar insecticide for pea leaf weevils.
No insecticide exists to target those larvae snacking on the bacteria inside nodules, so it’s critical to control adult weevils before they mate and lay eggs.
In theory, a farmer could spray insecticides on a trap crop to control the pests before they emerge or go to over-winter, but that seems very tricky to time appropriately in our crop cycle.
Insecticide coating on pea seeds can limit weevil activity and populations early in the growing season, as the insecticide moves systemically through the plant as it grows. (but only is effective for approximately 2-3 weeks following seeding)
When the adult pea leaf weevil feeds on the new leaves, it ingests the active ingredient. The insect will stop feeding and delay egg laying. This has been shown to reduce the number of larvae feeding on nodules.
It was important to research what might be effective management techniques against pea leaf weevils in Manitoba. Since no 2 growing seasons are ever alike, MPSG undertook a 3 year study on pea leaf weevils, the effectiveness of seed treatment in management, and how yield is affected. Insecticide seed treatment is not cheap and MPSG wanted to examine if it was economically effective in protecting yield of peas. We decided to conduct the research in Minitonas, Roblin, and Melita over a 3 year study. Anand Aneja, agronomist in residence at Assiniboine College oversaw the 2025 research, and will be presenting the final results, considering all sites and all 3 years of the study, in early 2026.
July 23, 2025 Pea research trials at Minitonas, MB conducted by New Era Ag Technolgies photo credit: Wendy McDonald
In the research locations, we wanted to mimic field management techniques. The plots had a pre-plant-incorporated herbicide like Authority or Edge on the whole trial site, a pre-seed burnoff with glyphosate on the whole trial, coupled with a post emergent herbicide at the 3rd to 6th true node stages (e.g., Viper ADV or Odyssey) to eliminate weed competition. They also used hand weeding to ensure the trial was weed free. Insects and diseases were controlled with registered products before they reach threshold levels.
We experimented with the following seed treatments:
Assessments were made for each rep, of each treatment for plant stand, pea leaf weevil notch counts (at the 6 true leaf stage), and yield.
Generalizing from all locations and results from 2023 and 2024, it showed there was a trend of less pea leaf weevil notching where Cruiser (thiamethoxam) and Stress Shield (imidacloprid) seed treatments were used, but this did not translate to any yield improvements. The thoughts were that pea leaf weevil populations may have be so high in these areas that any seed treatment effects were masked by excessive pest pressure.
Another theory was that pea leaf weevils will arrive in fields early if warm conditions (above 20°C) persist for more than a few days in late April or early May, resulting in higher yield loss. In years when cool weather persists, the arrival of pea leaf weevils can be much later, and the yield impact less serious.
Dry years make plants more susceptible to the pea leaf weevil. In wetter years there is more nitrogen available to the plant,
making it less susceptible to the pest.
MPSG definitely got to put that theory to the test in 2025.
Pea Leaf Weevil Seed Treatment trials Seed Minitonas, MB June 4, 2025 , photo credit: Wendy McDonald
In 2025, the Minitonas (Swan River Valley) site got off to a fantastic start and was seeded May 6, at 12°C soil temperature, with excellent soil moisture, on 10 inch spacing, and the peas emerged on May 16. With no rain, daily high temperatures reached above 25°C every day in the last week of May, and June 1 reached 34°C! There was no statistical difference when it came to plants counts between the treatments.
Intense Pea leaf Weevil Notching on leaf margins Minitonas, MB June 4, 2025 photo credit: Wendy McDonald
In this hot and dry growing season, the Minitonas site experienced the highest numbers of pea leaf weevil notching and defoliation the evaluators had ever seen. They didn’t even count notches from the bottom 3 leaves because the nibbling had been so intense that the majority of the bottom 3 leaf had been defoliated.
Severe Pea leaf Weevil Notching Minitonas, MB June 4, 2025 photo credit: Wendy McDonald
After analysis, Cruiser (thiamethoxam) and Stress Shield (imidacloprid) seed treatments showed lower numbers of notch counts than the other treatments. There was no difference in the number of notches counted between the untreated and the Lumivia CPL (chlorantraniliprole) plots. There was no difference between any of the treatments when it came to total nodule numbers per plant, pea leaf weevil larvae present on roots, number of pink nodules and damage done to nodules by the larvae.
healthy pink pea nodule photo credit: Wendy McDonald
After a season of intense pea leaf weevil populations, the trial was harvested on August 21. There was no statical yield difference between any of the treatments and overall yield was in the high 60’s/low 70’s bu/acre.
Remember, this is only a snap shot of only 1 site in only 1 year.
Stay tuned for the final outcome of the study, coming soon.
Conducting a leaf notch survey June 10, 2025 Strathclair, MB photo credit: Wendy McDonald
Can you tell how much yield loss to expect based on the spring notch counts on your peas?
We chatted with Manitoba Agriculture Entomologist John Gavloski and received an emailed answer from Meghan Vankosky, with AAFC in Saskatoon, and she has done a lot of research on pea leaf weevil.
“In my MSc work, we found a significant, but quite weak relationship between the % of plants with damage on the terminal leaves and yield loss. A few other authors also observed similar, weak relationships in field pea. I think that factors like soil quality and larval numbers confound our ability to predict yield loss based on feeding damage. For example, in the same study, I found that despite the number of adult weevils, we found about 12 larvae per plant. I argued that field peas have a carrying capacity for larvae, and that even one female can lay enough eggs to meet the larval carrying capacity of 32 plants (based on the number of plants in our cages). Other studies in labs had similar results, where they added 50 or 100 eggs to individual plants and only about 10-15 adults emerged later. So while feeding damage gives a rough estimate of population density, it unfortunately doesn’t really allow us to estimate subsequent larval populations, root nodule damage, or yield loss. “
What can we expect for pea leaf weevil numbers in 2026?
According to Meghan, unless something drastic happens, like a major change in precipitation, the chances are good that areas with high weevil densities last year could have high densities again the next year.
Research done around Swift Current, Sask., demonstrated that populations of pea leaf weevils completely crashed when there was a period of dry weather. Meghan’s research showed between spring of 2017 and spring of 2018, it was really, really dry. The next year (2018), they had basically no weevils.
It’s possible that the larvae cannot burrow through the soil crust, or they die on the soil surface when the weather is hot and dry.