Watch For Notches On Pea Leaves

The pea weevil is a tiny little thing that gnaws away at pea leaves, causing seemingly negligible damage. It’s what you can’t see that could end up costing you your pea crop this year and the residual nitrogen you were counting on for next year. The adult weevil, after feasting on leaves, lays her eggs in the soil where the larvae hatch and begin feeding on nitrogen-fixing nodules, rendering them useless.

“The adult weevil really doesn’t affect a vigorously growing plant. It’s the larvae that cause all the damage,” says Héctor Crcamo, research scientist, insect pest management with Agriculture and Agri-Food Canada. What’s particularly tricky about the pesky weevil is that, if conditions are favour-able, the adults can breed and lay eggs from April to August. “The eggs take two or three weeks to hatch and then begin feeding,” Crcamo says. “The pea plant is most susceptible at the five-to six-node stage, however heavy feeding or later hatches can affect the nodules and yield.”

It’s only been just over a decade since the pea leaf weevil was first reported in Alberta. Since then, there have been good years and bad years, but in all years the weevil has spread. It’s now been reported as far east as southwestern Saskatchewan — a big concern for pea growers there. “The weevil is native to North Africa, all the way up to Scandinavia, where it has destroyed crops,” Crcamo says, so there is potential for significant damage if numbers get high enough. In 2006 and 2007, up to 10,000 acres were considered infested enough to warrant a foliar insecticide application.

Weather is a significant factor for the timing and severity of pea leaf weevil infestation. The tiny insects need heat to get moving out of their overwinter grounds in perennial crops, such as alfalfa. A cool spring slows down early feeding and breeding activity and may mean pea plants are more advanced (the seven-to eight-node stage) once the weevil larvae begin hatching. But in a warm spring with days of 15C or higher, the adults could be out as early as April and may continue to lay eggs into August.

Foliar insecticides, such as Matador, are registered for foliar application on field pea and faba beans, though Crcamo cautions

that the current economic thresholds used were developed for use in Idaho and Washington state, not Alberta and Saskatchewan. “We’re working on validating these thresholds for our conditions,” he says. As it stands, spraying is recommended if you see the tell-tale notching of the clam (terminal) leaf on 30 per cent of plants at the two to three node stage. Because pea leaf weevil is a flying insect that moves in from surrounding crops, always scout the field margins first. If you see notching, keep moving in towards the centre as you scout to estimate the severity of the infestation. Crcamo adds that the weevil doesn’t move uniformly, so scout field margins thoroughly as you may find concentrated areas of feeding.

ACTION

Seed treatments are available through emergency use registration, but Crcamo cautions that not a lot of independent work has been done to measure the economic advantage of the treatment. His advice is to use a test strip or small-scale trial on farm to gauge the effectiveness on yield protection.

As for cultural control, pea fields with a history of weevil and seeded adjacent to alfalfa fields are at highest risk, but even neighbours’ fields a good distance away may still harbour the little devils. “Weevils can fly several kilometers. They are very strong,” Crcamo says. He adds that those establishing alfalfa fields should consider weevil infestation levels as well, as the early feeding of newly active adults could significantly damage young alfalfa plants.

Pea leaf weevil breeds on peas and faba beans only, but it overwinters in perennial crops and will feed on several types of crops before laying eggs in a pea crop. Crcamo and fellow researcher Ken Coles have used this piece of information to test if a trap crop of winter peas is effective in saving field peas. So far, the work looks promising. “The idea is that if you have winter peas ready for the weevils to eat, they’ll congregate there in the trap crop, then you only have to spray the trap strips to control the adults,” he says.

Lyndsey Smith is a field editor with Farm Business Communications. She is based in Regina, Sask. Email her at [email protected]

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