Pea leaf weevils have completed their eastward trek across the Prairies, and while they’re generally not a huge issue, pea and faba growers need to learn the habits of these hard-to-spot pests.
Adult pea leaf weevils are tiny and were first found in Canada near Lethbridge in 2000. By 2019, they had crossed the Manitoba border and have since been found in much of that province, although it takes a sharp eye to find them.
Like so many pest species, they aren’t from North America. They’re an old-world species that can be found in northern Europe and ranges all the way to the northern tip of Africa. They’re a member of genus Sitona and related to a number of weevils with similar dietary habits. They like pulses and legumes.
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“They all have a very peculiar life cycle,” said Hector Carcamo, an entomologist at Agriculture and Agri-Food Canada’s Lethbridge research centre. “The larvae are associated with the roots of the crop. Sometimes they feed directly on the roots, but most commonly they’ll attack the bacteria that form the nodules that fix nitrogen.”
While the tiny adults feed on green leaves, leaving a characteristic notching pattern, it’s the larvae that do the real damage. By eating the root nodule bacteria, they reduce the amount of fixed nitrogen available to the plant, leaving it unable to make protein and set seeds.
“If the field already has nitrogen, no problem,” said Manitoba Agriculture entomologist John Gavloski. “But people who are seeding peas and faba beans won’t be putting on as much nitrogen because the plants can fix their own.”
“In faba beans we have documented yield reductions that are quite substantial,” Carcamo said. “Sometimes half a tonne per hectare is possible, so that’s quite a lot of money.”
Scouting is a challenge. Tell-tale leaf damage may be evident but the weevils themselves are so small that they are not easily seen.
That is why it helps to know their habits. For a start, they overwinter as adults in field vegetation.
“They’re emerging in the spring from our perennial forages as well as our ditches and shelterbelts,” said Laura Schmidt, western production specialist with Manitoba Pulse and Soybean Growers. “In the early spring they’ll fly to peas and faba beans, their host crops.”
The weevils are specific to faba beans and field peas. Although the adults will feed on other plants during the fall, the larvae are tied to these two crops. Carcamo said a grad student tried forcing the weevils to breed on other legumes to see if the larvae would attack them too.
“We tried them on alfalfa and we got nothing out of alfalfa. We tried a vetch also, I can’t recall which one, but we couldn’t get anything out of the vetch,” he said.
“We tried soybean and we got only one larva out of that, so soybean might be an occasional host but not really an important one. They’re very specific.”
Once the weevils start feeding, they leave a distinctive, almost scallop-like notch pattern on the leaves. This feeding pattern indicates they’re around and may be a problem, though control options are limited.
“There are foliar insecticides that are registered for pea leaf weevil control,” Schmidt said. “They’re ultimately ineffective, partially because those eggs have already been laid, so the damage is already done. It’s a revenge spray.”
Besides, there may be several waves of emerging adults and spraying may only kill one wave. There will soon be others.
“There are insecticide seed treatments that will provide some control, and I say ‘some’ because it’s not complete control,” Schmidt said. “The thiamethoxam seed treatment is only killing a third of the weevils in the field.”
Since spraying is not the answer and seed treatments are only partially effective, managing the pea leaf weevil can be a chess match based on observations by farmers or agronomists.
It starts with the history of the field and previous problems with pea leaf weevils. They like to overwinter in perennial forages. If there was a problem last year and there’s an alfalfa field nearby, growers may want to consider a seed treatment for field peas and faba beans.
The weather is another consideration. Weevils like it moist.
“In Saskatchewan, with some of their surveys, they saw their numbers go down quite a bit in some of the drier years that we had over the last few years,” Gavloski said. “Manitoba is predicted to be an area where they should do quite well because we do have the climate conditions that would favour them. We’ll just have to wait and see how the population establishes here.”
When it comes to weevils, there’s an endless variety
If you open an entomology textbook, find the index and look up ‘weevil,’ you’ll find a stack of them, most named for a plant grown in agriculture or forestry.
There’s cabbage seed pod, carrot, clover seed, coffee bean, cranberry… it covers most of the alphabet from alfalfa to white pine.
It’s obvious that people have a storied history with weevils. In the early 1900s. the cotton boll weevil decimated the cotton crop in the southern U.S., inspiring a number of blues singers right up to the Boll Weevil Song made famous by Brook Benton in 1961.
The University of Arkansas-Monticello even uses the infamous boll weevil as the team name and their mascot, Weezie the Weevil, leads the cheers as they take the field in Boll Weevil Stadium.
For as long as humanity has been farming, it has been plagued by weevils in one form or another.
Weevils are beetles and beetles make up close to 40 per cent of all insect species. We don’t know how many different types there are but it could be as many as two million.
The number of different types of weevils is unknown because people are still arguing about what a weevil is.
There’s agreement that true weevils belong to the family Curculionidae and there are about 83,000 different known species. When talking insects, you’re talking huge numbers.
The true weevils are also called ‘snout beetles’ because of their distinctive long snout. Their antennae also have odd, almost elbow-like joints. Beyond that, there are huge variations in size and shape.
Most are plant eaters and some can be serious pests in agriculture and forestry. They can also be used for biological control of pest plants such as purple loosestrife, which is now kept in check with the loosestrife flower weevil and the loosestrife root weevil.
