Pests & Predators, Ep 8: Good vs pea leaf weEVIL

Pea leaf weevil is an invasive pest of Western Canada and attacks field peas and faba beans. The larval stage damages nodules on roots of both these crops, which can hinder biological nitrogen fixation happening in those nodules.

In this episode of the Pests & Predators podcast, host Shaun Haney is joined by Dr. Meghan Vankosky, research scientist in field crop entomology at Agriculture and Agri-Food Canada in Saskatoon, Sask., to talk all about this evil weevil.

“We often like to grow pulses in our crop rotations, in order to improve soil nitrogen naturally, and the pea leaf weevil cancels out that beneficial effect of growing pulses because they destroy those root nodules and then the plants cannot produce their own nitrogen,” says Vankosky.

Pea leaf weevils are brown-grey in colour, with stripes on their back, and are only three to four millimetres long, and have blunt, broad, long noses. There’s one generation of pea leaf weevil per year. Adults overwinter and emerge in spring, then move into host crops, then will mate and lay eggs. They’re quite long lived, says Vankosky, and can lay a lot of eggs into July. The new generation that hatches in July or August will feed on the crop later in the summer as well, which means that adult pea leaf weevil populations will overlap.

Damage from this pest is variable and depends on population density, says Vankosky. “If you have a low population density in the field, it’s probably not going have any substantial or noticeable impact on nitrogen fixation or yield or any economic effects. But, if the weevils occur in really high numbers, really early in the growing season especially, then they can be a more significant threat to crops.”

A 10 to 30 per cent yield loss can happen if the population is significant enough.

Listen in on the full conversation between Vankosky and host Shaun Haney:

Scouting is an area of improvement when it comes to being proactive in controlling pea leaf weevil. Scouting early in the spring when the crop is at two to six nodes, and counting feeding notches on the plants, but the annual survey method isn’t actually the greatest way to be proactive. As Vankosky explains, the most effective insecticide option to manage pea leaf weevil is a seed treatment, which doesn’t line up with the timeline for scouting when the crop has reached to the two to six node stage. There are foliar options, but they’re not nearly as effective as the seed treatment.

The Prairie Pest Monitoring Network publishes pea leaf weevil population density maps, based on that two to six node stage monitoring and damage reports. Vankosky notes that if you’re in an area with high populations, it might be worth using an insecticide seed treatment. If you’re in an area where the populations are low, it’s probably not worth using the seed treatment.

Another work-in-progress is information on what beneficial insects that might be affecting pea leaf weevil. Vankosky’s own research for example looked at whether different sized beetles predate on pea leaf weevil eggs, in a lab setting. Small beetles could eat the pea leaf weevil eggs, but larger beetles couldn’t, because their mandibles were too large.

Research out of the U.S. on other stages of pea leaf weevils — larvae, pupae, or adults — show that larger ground beetles can and will eat pea leaf weevils, a question that a member of Vankosky’s research team will work on in Canada using bioassays and gut content analysis with molecular techniques.

“Not every insect that you encounter in your field is a pest,” says Vankosky. “A lot of them are actually probably beneficial insects or neutral insects that just might be passing through. These species are really important to conserve, and if we can avoid foliar sprays especially, then that’s going to be a lot better for the agro-ecosystem as a whole.”

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