Use Trap Crops For Pea Weevil

The pea leaf weevil is not a widespread pest and it’s not a chronic problem such as the flea beetle in canola, for example. However in some fields in southern Alberta and in southwestern Saskatchewan it is a concern for growers,” says Dr. Hector Carcamo, an insect pest management researcher with Agriculture and Agri-Food Canada (AAFC) at Lethbridge, Alta. He is leading a comprehensive, multi-agency project to assess integrated pest management options for control of the pea leaf weevil.

This project, which started in 2007, is investigating the weevil’s current and potential distribution, its life cycle, and the effectiveness of insecticides, soil fertility, trap cropping and natural enemies for controlling the weevil.

A native of Europe and North Africa, the pea leaf weevil is fairly new to the Canadian prairies. It was first identified in 1997 in southern Alberta, and crop damage started to be noticed a few years later. Carcamo says, “The insect seems to be well established only south of Highway 1 in Alberta — south of Calgary and Brooks, into the Medicine Hat area — and that extends also into southwestern Saskatchewan. It seems to be moving east more than north.”

For 2010, the parts of the prairies most likely at risk of pea leaf weevil infestations are: the counties of Willow Creek, Vulcan, Lethbridge, Taber, Warner, Forty Mile and Cypress in Alberta, and the area southwest of Swift Current in Saskatchewan.

THE PEA LEAF WEEVIL LIFE CYCLE

As you might expect from the insect’s name, field pea is the crop most at risk of economic damage from the weevil. The larvae cause the most serious damage to the crop; they feed exclusively on the Rhizobium root nodules, disrupting the plant’s ability to fix nitrogen. The larvae can infest pea and faba bean crops, however faba bean is rarely grown in southern Alberta or southwestern Saskatchewan so it is not at risk at present.

In the spring, the adult weevils emerge from their overwintering sites, which can be perennial legume crops or tree shelters. The adults feed on the leaves of many types of legumes, such as alfalfa, beans, clover, lentil, lupin and vetch, producing a distinctive notching of the leaf margins. Plants can usually recover from this damage.

The adults begin laying their eggs in the spring, within a week of feeding on peas, and continue to lay eggs till mid summer. The larvae emerge from the eggs starting in late spring, feed on nodules, then pupate in the soil in late summer, and transform into adults during August and September. Adults will feed on any legume plant before overwintering.

Weather conditions that promote higher pea leaf weevil populations are a warm spring, which causes the adults to leave their wintering sites early, and a moist August. Carcamo explains, “During the pupa stage, they surround themselves in soil, and if the soil becomes too dry they become trapped in the soil and are unable to emerge into adults.”

Carcamo says the amount and type damage caused by the pea leaf weevil varies, but the research results so far show that, under the right conditions, larval feeding can lower pea yields and/or reduce pea protein content. This year, the researchers will be assessing the weevil’s impact on residual nitrogen from the pea crop.

“If the peas have an effect on residual nitrogen and the weevil is affecting that nitrogen, then there is also the potential to have that carry over as a detrimental effect on the next year’s crop. It could also affect crop input costs because nitrogen represents a significant input cost,” he says.

FOUR STEPS TO CONTROL PEA LEAF WEEVIL

Based on the project’s results, Carcamo has several recommendations for controlling pea leaf weevil.

1. Use a seed treatment. If pea growers already know they’re in a high-risk area, he recommends they consider a seed treatment to prevent damage by the weevil. Cruiser is the only registered seed treatment for pea leaf weevil in peas.

2. Use a trap crop. If growers in high-risk areas prefer not to use a seed treatment, then Carcamo recommends a winter pea trap crop along the field’s edges. When the adult weevils leave their winter shelters, they tend to move into fields starting from the field margins. So Carcamo did a two-year study with Ken Coles, an agronomist with the Southern Applied Research Association, to evaluate winter peas as a trap strip planted around the edges of a spring-seeded pea crop.

“In both years, the winter peas were able to survive the winter with enough density that they attracted the pea leaf weevil early on. The weevil numbers were much higher in the trap strip, and the farmer had the option to control them there with insecticides,” Carcamo notes.

He adds, “I think winter pea has a lot of potential either as a trap crop or [as the actual crop] in the future if we actually have winter pea cultivars that can replace spring peas. In that case I think the weevil will be less of an issue because the winter pea plants are too advanced for the weevil to have an impact. Also winter peas have greater yield potential than spring peas, so that would be a win-win situation.”

Winter peas are less susceptible to the weevil because, as the project’s results show, when weevil eggs are laid after the plant’s fifth node stage, the plant has enough nodules that have fixed enough nitrogen to withstand larval feeding. As well, the larvae are still fairly small so they don’t cause as much damage in the critical seedling stage.

3. Seed early. The ability of more mature plants to withstand the weevil also means that early seeding of spring-seeded peas could help reduce impacts, especially in a cool spring. In cool conditions, the peas will continue to grow but the adult weevils will be slower to leave their overwintering shelters and lay their eggs.

Inoculate or fertilize the crop. Another important finding from the project is that increasing the nitrogen available to the pea crop — whether that is by inoculating the peas or by applying nitrogen fertilizer — helps reduce the weevil’s impact on crop yield. Carcamo adds, “If a field has been manured within the last two or three years, I suspect the levels of nitrogen in the soil should be high enough that the effect of the weevil on the plant should not be an issue.”

ECONOMIC THRESHOLDS AND SCOUTING

The project also confirmed scouting methods and the economic threshold for spraying an insecticide. Carcamo says, “Using our data, we now can recommend to growers that if they have 30% of seedlings with damage on the clam leaf, then they have reached the threshold and they should consider an action.”

To scout for the insect, he advises that growers check five sites along the field edge and another five sites within the field when the pea crop is at the second or third node stage. At each site, look at the clam leaf of 10 plants for the notches caused by the adults. Check only the clam leaf; if the notches occur on the lower leaves but not on the clam leaf, then the weevil has likely already laid its eggs and it’s too late to spray. Also, you don’t need to count the number of notches on the clam leaf, just note whether or not there are any notches.

Matador is currently the only registered foliar spray for controlling pea leaf weevil in peas. Carcamo emphasizes, “It’s important to only spray insecticide if you absolutely have to because you have surpassed the economic threshold. There are beneficial insects, ground beetle predators, which can reduce the weevil population by feeding on their eggs. The foliar sprays will kill the beneficial insects as much as they kill the pest.”

Another issue with foliar applications is that proper timing can be difficult. He explains, “If growers have to spray, then they should continue to monitor the field after spraying. Sometimes if they spray too early there is the potential for the weevils to continue migrating into the field from other areas. But if they wait too long then the weevils could already have laid their eggs and the insecticide will not kill the eggs or the larvae, only the adults.”

He adds, “If you have a cool spring and the plants are at the seventh or eighth node stage when the weevils arrive, you don’t need to be concerned. Our research shows that if the weevils arrive late, then the crop is not going to suffer by the weevils so you don’t need to consider spraying.”

The pea leaf weevil research project is funded by the Alberta and Saskatchewan pulse grower associations, AAFC and its Matching Investment Initiative, the Alberta Crop Industry Development Fund, and Syngenta Crop Protection Canada Inc.

Carolyn King is a freelance agriculture writer based at Kingston, Ont.

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