Pea leaf weevils are starting to spring up in pea and faba bean crops in southern Alberta, but the cool, damp start to spring could keep infestation levels low this summer.
“The years when we have the highest risk are actually years with early, warm springs,” said Hector Carcamo, research scientist with Agriculture and Agri-Food Canada.
“This kind of spring is actually conducive to reduced risk of pea leaf weevil, except for those farmers who seeded very, very early before the snow fell. I suspect that in most of the fields planted in middle to late May, weevils are not going to accumulate in very high numbers.”
At this point in the growing season, adult pea leaf weevils will be causing the most damage to crops, eating the foliage and creating distinct notches along the edges of the leaves.
“They’re quite easy to spot,” said Carcamo. “It looks almost like somebody came with a hole punch and made them. They’re quite uniform.”
But that damage is “of very little consequence to the plant.”
“It looks quite bad… but plants can compensate for the foliage that is removed by the weevil.”
The “real damage” is going to happen later in the growing season, when the larva hatch and begin selectively eating the nitrogen-fixing nodules on the plant roots.
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“It’s the interference with the nitrogen fixation that can produce the yield losses,” said Carcamo. Once the plants are past the fifth or sixth node, pea leaf weevils present serious risk to the plant in terms of reducing yield.
“A pea field that was planted in mid-April would still be between the second- and the fifth-node stage, and at that stage, it’s vulnerable to the insect because the larva will then be synchronized with the nodule production of the plant.”
Damage to the nodules won’t cause any symptoms on the plant itself.
“The only way to detect them is by pulling the roots and looking at the nodules closely. It’s more subtle damage,” he said.
Producers can protect their crops from pea leaf weevils by using a seed treatment prior to seeding and insecticide at the two-to three-node stage, after establishing whether yield loss is likely to occur.
“You want to establish an economic threshold when your crop is before the sixth-node stage,” said Carcamo. “From emergence to the sixth node, that’s the crop stage of interest.”
To determine if spraying is necessary, producers will need to scout at least 10 areas of their fields — five from the border and five from the middle — and look at the damage to the clam leaf from 10 different seedlings in each area.
“If you have an average of 30 per cent of the seedlings with damage on the clam leaf, you know that you are near the threshold and there might be a risk of yield impacts from the weevil,” he said.
And the time to start scouting for pea leaf weevil damage is now, especially in the Lethbridge area.
“If growers have peas that were planted in mid-April, they should be looking at their crops now to see if they will need to make a decision about controlling them or not.”