LETHBRIDGE – A new insect turning up in southern Alberta pea fields may not be causing economic levels of damage, but insect specialists and scientists are keeping a wary eye on the pea leaf weevil.
“The first records of it in Western Canada are 1997 or 1998, when Bob Byers picked it up in some of his collection work,” said Scott Meers, pest risk management specialist with Alberta Agriculture in Brooks.
“In 2001 we noticed pretty good numbers at the diagnostic field school in Lethbridge. We did a survey in southern Alberta, so those were the first well documented records in 2001.”
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As far as Meers knows, it was only found in the immediate Lethbridge area that year. A new survey in 2005 found it in all five counties surrounding Lethbridge, along with the County of 40 Mile. It appears to be radiating out from the Lethbridge area, but hasn’t made it to Calgary yet.
“Pea leaf weevil is a common insect almost everywhere peas are grown. It’s not unexpected we would find it. It’s been in the (U.S.) Pacific Northwest for many years, so it probably made its way along on native legumes and finally made it here,” said Meers.
American pea growers have established economic thresholds for it. They are most concerned about it attacking peas in the seedling stage, in a similar situation to flea beetles in canola, where there’s early seedling damage before the plants get established.
But Meers said the damage is happening later in southern Alberta. The weevils aren’t moving in from their overwintering site until the peas are in the three to four node stage, so the plants are more able to withstand the damage.
“We’re not certain the level of damage is economic yet. In field situations there’s been virtually no spraying and we don’t think it’s economic at this point,” he said.
Life cycle
The weevil goes into the winter as a sexually immature adult. Meers said it will move into headlands and alfalfa fields, then snuggle down against the roots of the alfalfa plants and under leaf litter to wait out the winter.
He said the new generation of adults appears in August and starts feeding on the pea plant to fatten for winter. As the pea field dries, the weevils move into their overwintering sites.
“In the spring, the adults move out of the alfalfa. We’re not sure on the timing in southern Alberta, but we’re noticing damage at the three to four node stage, starting sometime in late April.”
When they first move into the fields, these adults have to feed on pea leaves to reach sexual maturity. Once they’re done feeding, they mate and lay eggs at the base of the pea plant.
That egg hatches into a grub, which makes its way down onto the roots of the pea plant and hollows out the nitrogen-fixing nodules. Meers said that could be a second worry for the crop economics.
“They seem to prefer the nodules. I’ve seen feeding damage on other parts of the root, but they seem to prefer to feed in the nodule.”
The grub goes through a number of stages before a short pupa stage in the field. It emerges as an adult in August to complete the cycle, fatten up for winter and then migrate to where it overwinters.
Damage symptoms
Meers said as far as he knows, adult damage only occurs on the pea leaves. He hasn’t seen any documentation on pod or seed damage.
“It’s very distinctive. If anyone is familiar with the feeding damage of the sweet clover weevil, it looks exactly the same. In fact they’re in the same genus. The feeding damage is like a crescent shape chewed out of the side of the leaf. It almost looks like someone took a hole punch and punched out the edges of the leaves,” said Meers.
“(The larvae) are actually quite visible. If you slowly work your way through the roots of the plants, you’ll notice the little grubs will crawl out as you break the dirt away from the nodule. The nodules that were fed on a while back will appear hollowed out and rotten. Others, you’ll see where they’ve been fed on and sometimes you’ll find the little grub right inside the nodule.”
He said the field damage he’s seen has not involved a significant amount of a plant’s nodules.
“At our diagnostic field school in 2001, we saw virtually every nodule was eaten or hollowed out, but that was a case of concentration of the insect on a small plot. In the field, we haven’t seen every nodule fed on. We just see the odd one.”
The adult weevil is a nondescript brownish grey with some lighter brown stripes. Meers said it looks like a little dirt lump.
“The best way to see it is to squat down in a pea field and watch for the little beetles scurrying around on the ground. If you go into the field carefully, you can actually pick them up right off the leaves. But if you stomp into the field and shake the plants around at all, or throw your shadow across them, they drop off the plant and play dead,” he said.
“They don’t seem to be getting into the top leaves as much. They seem to like to feed down in the canopy more. We haven’t seen them so much on the top leaves.”
Meers said his take on the weevil is that it might be a problem and should be monitored.
“We haven’t seen early enough damage to be really concerned about it, or heavy enough feeding to get extremely excited. What we’re concerned about is that it is a pest in other parts of the pea growing world and we have seen a dramatic increase in acres and level of damage between the 2005 survey and the one we did in 2001.”
He said most plants can withstand substantial feeding by leaf-eating insects before crops start losing yield.
“We want people to be aware of it, but we don’t want them to overreact and think as soon as they start seeing crescent-shaped feeding that they have to spray. The truth of the matter is that the true pest status hasn’t been worked out. We haven’t seen economic damage yet.
“It’s one of these things that as scientists, we need to start working on the thresholds under Alberta conditions. We need to learn how it acts under Alberta conditions and keep an eye on it.”
For more information, contact Meers at 403-362-1366.
