LETHBRIDGE – Farmers who plant peas in southern Alberta this season will likely require an additional insecticide treatment or two.
In the future, producers prairie-wide also will be looking to add pea leaf weevil control to their list of spring practices.
Scott Meers, a pest risk-management specialist for Alberta Agriculture, said the pea leaf weevil has become a major problem for pea and fababean growers in southern Alberta.
Pea leaf weevil damage begins just weeks after plants sprout. Adults migrate into fields and feed on the pea seedlings, leaving telltale scalloped edges on the leaves.
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Adult weevils are grey, about five millimetres long and have three light-brownish-grey stripes down the thorax and sometimes the abdomen and wing covers, which distinguishes them from the sweet clover weevil.
Rising commodity prices have complicated the calculation of thresholds at which it becomes economically beneficial for farmers to apply pesticides. But typically if one or more feeding notches are found on three pairs of clamshell leaves, farmer should apply foliar treatments.
In 2007, the federal Pest Management Regulatory Agency approved the minor use registration of Matador, or cyhalothrin-lambda, as a foliar control of adult weevils.
Producers must apply the treatment early enough to prevent significant egg laying. That means spraying in mid to late May, said Meers.
“It’s a challenge because scouting and spraying conflicts with seeding that is still going on for many growers,” he said.
Producers with evidence of weevils in their fields or neighbouring fields in 2007 should count on treating for the insect, said entomologists.
For producers who saw significant damage last year or those that are seeing greater evidence of the insects, emergency registration has been granted for Cruiser 5FS seed treatment, or thiamethoxam, also known as Cruiser Maxx Pulses, from Syngenta.
The seed treatment is highly effective, said Meers.
The higher cost of the seed borne control compared to foliar applications should be weighed against the challenge and cost of field scouting and spraying, said Meers.
“Each producer’s situation is different,” he said.
Leaf damage generally causes few problems for the plants. However, the larval assault on the rhizobium inhabited root nodules, which the plants need to convert atmospheric nitrogen into plant available nitrogen, can cause yield losses of up to 30 percent.
Meers said farmers must scout fields two weeks after planting, when seedlings are just out of the ground. Spraying should take place before the five or six node stage of growth. If new damage is present after foliar control, a second application may be necessary.
“If there is damage you need to get on the spraying immediately because you know egg laying is going on,” said Meers.
No biological control is available yet, but Hector Carcamo of the Agriculture Canada Research Centre in Lethbridge is seeking parasites that will attack the pea leaf weevil. Researchers are also looking at the role ground beetles may play in control of the pest.
“Chemicals are the only control at this point,” said Meers.
Ken Coles of the Southern Applied Research Association said his organization is in its second year of working with trap crops. They planted fall-seeded winter peas that could be sprayed before the spring crop were advanced enough to attract the insects.
In 2007, however, Coles said the success of trap crops was so good, the weevils overran the trap crop’s borders and the whole field had to be sprayed.
“We are early in this research. We have some ground to cover to catch up with the weevils.”