The pea leaf weevil, which has become fairly common in southern Alberta and southwest Saskatchewan over the past 15 years, doesn’t necessarily have a direct impact on pulse crop yields, but it can affect the ability of the crop to fix nitrogen.
While the adults feed on the leaves of the plant and cause distinctive notching of leaf edges, the larvae feed on the nitrogen-fixing root nodules, says Hector Carcamo, (left) an Agriculture Canada research scientist at the Lethbridge Research Centre. Peas and faba beans are pulse crops most susceptible to the pest.
Carcamo speaking at a recent southern Alberta Farming Smarter “crop walk” of research plots, says the damage caused by the larvae can affect nitrogen production and crop yield if the soil is nitrogen deficient. Lack of nitrogen can certainly affect the current standing crop (and yield), and the nutrient won’t be in the soil for subsequent crops in rotation. Carcamo says if the pulse crop is being grown on soils that have been heavily manured (high in nitrogen) don’t worry about the pest.
So what is the best way to manage the pest? Research assistant Amanda St. Onge is leading a project to develop a reliable trapping system in hopes of being able to better monitor pea leaf weevil numbers. With a better handle on the pest, farmers can be alerted and take control measures as needed.
Seed treated with thiamethoxam has produced more consistent yield responses than application of foliar insecticides but
additional farm-level trials are required. As pest numbers increase, spraying is an option. The economic threshold during the second to fifth node stages in field peas is 30 per cent of seedlings with damage to the terminal (clam) leaf. If the pest appears after the fifth or sixth node stage, Carcamo says don’t worry about any control measures, the crop is no longer susceptible to the weevil.
Other control options include establishing a trap crop by planting a border in the fall with winter peas or adjacent, earlier planting of a spring cultivar. Close monitoring of the trap crop is needed to control the weevils in the trap crop area before they move to the rest of the field. Fields under no tillage regimes suffer less damage than those under conventional tillage. Biological control is a research area that should be pursued for sustainable management of this pest.
And an interesting fact, pea leaf weevils are prolific little pests. Research shows on average the female will lay up to 3,000 eggs (compared to the lygus bug that lays about 300 eggs). The important question as Carcamo pointed out – that’s an interesting fact, but who has the time or patience to count those microscopic eggs?
Lee Hart is a field editor for Grainews in Calgary, Contact him at 403-592-1964 or by email at [email protected]