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Keep a watchful eye for pea leaf weevil

Be ready to scout your fields. This pest is headed north

There’s a new pest heading north in Alberta and it’s eersoving fast. Although a regular in southern Alberta and southern Saskatchewan, the pea leaf weevil has now been sighted as far north as Athabasca, Alberta. “There’s been a real range expansion,” says Scott Meers, insect management specialist with Alberta Agriculture. “It’s a concerning issue for the Highway 2 corridor in Alberta. We’ve seen some very significant numbers, especially in the Red Deer and Lacombe area.”

As damage from the pea leaf weevil can be up to 20 percent of crop yield, it’s worth taking note of this insect. According to Meers, the best chemical control practice is insecticide seed treatment so farmers would be wise to inform themselves now of the risk to their fields.

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The life of the pea weevil

The adult pea leaf weevil emerges from its winter habitat of ditches, shelterbelts or perennial legume fields in late April/early May. It begins feeding on available leguminous greens and then moves into pea or fava bean fields to reproduce. The pea leaf weevil is a five millimetre, slender, greyish-brown insect with a short snout. Three light-coloured stripes run along its abdomen. It’s not an easy insect to spot, as it drops to the ground on approach, where it’s hard to see. Evidence of its presence comes from crescent-shaped notches on pea leaves.

It’s not the feeding of the adult pea leaf weevils that causes enough damage for economic concern. The problem is the larvae that develop and feed on the nitrogen-fixing nodules of the pulse plant, the Rhizobium. The soft C-shaped, white-coloured larvae with a brown head will feed on the nodules for up to six weeks, sometimes completely destroying them. Without these nitrogen-producing nodules, the plant is weakened, is less drought tolerant and produces less seed.

“It’s not so critical in high organic soils, if the soil produces lots of nitrogen,” Meers says. Should farmers then place more nitrogen with their pea seed? Meers thinks that would be contradictory, as peas are usually planted for improved soil fertility. “The implications of the pea leaf weevil are for this crop and the next one too — it’s more subtle,” Meers says. Farmers should consider that when thinking about pea leaf weevil control. The economic benefits of a healthy pulse crop extend past the current year.

The pea leaf weevil is one of the longest living insects. A female pea leaf weevil lays up to 1,500 eggs over a period of three months. Although there is only one generation per year in Alberta, that generation often survives the emergence of the next generation, one year later. The insect is an extremely good flyer, easily covering several kilometres. “The pea leaf weevil has a strong ability to increase in numbers,” Meers says.

Economic damage

Thankfully, economic damage doesn’t go beyond the five- to six-leaf stage of the plant, although nodules continue to be damaged. So it’s important to control the pea leaf weevil in that time. To date, treating the seed with an insecticide such as Stress Shield by Bayer Crop Science seems to be the most successful. “In Southern Alberta, with the lower organic soils, seed treatment is generally recommended,” Meers says. Central Albertans should take a good look. If farmers sprayed for the pea leaf weevil the year before, they should seriously consider treating their seed this spring. “We’ve had poor luck for most part in foliar applications,” Meers says. “The insects lay so many eggs so quickly and in waves, they’re very difficult to control with spray… it’s better to spend money on seed treatment.”

Farmers should begin scouting fields as soon as the plants emerge, and continue up to the six-leaf stage. Eggs laid after this period don’t affect yield. If three out of 10 seedlings have at least one crescent-shaped notch, it could make sense to proceed with a foliar application. Dry years make the plant more susceptible to the pea leaf weevil. In wetter years there is more nitrogen available to the plant. The pea leaf weevil flies at temperatures above 17 C. Warm weather at emergence in spring provides the insect with optimal conditions to spread from the winter habitat to pea fields. Prolonged cool weather, in contrast, can lay the pest low until after main economic thresholds have been passed. While early seeding is cited as one way to manage the pea leaf weevil, that isn’t part of Meers’ recommendations.

Lacombe Research Station will be doing some field trials for the first time this year on control measures for the pea leaf weevil. Research has been done regarding biological control methods which show some promise, utilizing the native ground or carabid beetle, which eats the pea leaf weevil eggs. In northern Germany studies using entomophagous nematodes, (heterorhabditis bateriophora), have been quite successful.

Life is never dull around insects, Meers concludes.

Managing pea leaf weevil

Here are six ways to manage pea leaf weevil, as listed on the Alberta Agriculture and Forestry website.

  1. Plant early to maximize yields and potentially escape the weevil in cold springs.
  2. No-till cropping systems, integrated pest management systems and good crop rotations are recommended.
  3. Use inoculants and adequate levels of fertilizer to maximize crop yields.
  4. Registered seed treatments are recommended if high populations of pea leaf weevil were in the area the year before.
  5. Trap crops can be planted along field borders in the fall or early spring. If warranted, spray trap crops with a registered insecticide to control pea leaf weevils before they move into the main crop.
  6. Scout for pea leaf weevils as soon as peas emerge and continue up to the sixth node growth stage.

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