Researchers find possible alfalfa weevil control

Alfalfa growers have new control options for alfalfa weevils, but the application window is tight to avoid damage to leafcutter bees.  |  File photo

Alfalfa seed growers want more weapons in their arsenal against alfalfa weevil and research now shows promising possibilities.

The weevils can drastically reduce seed production, and populations in the Rosemary, Alta., region have become resistant to available pesticides.

Paul Tiege, a research scientist with Olds College, has undertaken studies on other chemical options on behalf of the Alfalfa Seed Commission.

“The growers have sort of noticed in the last few years, especially that the Group 3As that they’re using, like Matador and Cylence, have become less and less effective. The control that they’re getting isn’t really control. It’s more like suppression,” said Tiege.

His research indicates two Group 5 chemicals, a spinetoram and a spinosad, showed effective control when used on alfalfa weevil larva. Last year, he tested Entrust, a pesticide already registered in Canada, and this year also tested Delegate.

Both showed effective control of the weevil larva.

“We’ve found a couple of chemistries that were definitely able to control the weevil very effectively in the lab. We’re hoping that next year we can move some of that to the field,” said Tiege.

He focused his research on chemicals already registered in Canada in hopes of quicker approval.

“We think that if we can find one that’s already registered, even if it’s for a different crop and pest, the pathway to get a label extension will be a lot quicker with the (Pest Management Regulatory Agency.) If we’re starting from a product that’s already well known and the registrant is willing to work with us, it just makes it that much quicker.”

Alfalfa weevil is a major pest for alfalfa seed growers because their crops must mature to the seed stage. In forage crops, the insects are controlled through periodic cutting of the crop.

The other wrinkle for seed growers is the use of alfalfa leafcutter bees to pollinate the crop and facilitate seed production. Pesticides that kill alfalfa weevil also kill bees, which are themselves an income stream for growers.

That means alfalfa weevil has to be controlled at the larval stage, when they are hatching and moving from the stems to the leaves for feeding.

It can be a narrow window for chemical application, and several hatches can occur over about a six-week period. That might require multiple chemical applications, which increases the risk that the weevils will develop resistance.

Leafcutter bees are generally put into fields around July 1, so any spraying has to take place before then.

“Because there’s been a limited number of control options, the producers use the ones that are available to them, but if you’re not hitting the weevil with an upper cut, a body blow and a kick to the shins all at the same time, they become resistant to the one thing that you keep hitting them with,” said Tiege.

“So you really need two or three modes of action at a minimum and mix it up all the time.”

Brad Alexander, research manager with the commission, said weevil numbers were high this spring.

“However, a lot of producers were having much better control than they’ve had in previous years. One possible reason for that is the heat because a lot of the chemicals actually need higher heat” to be most effective.

Alexander said Aug. 1 that seed yield potential is looking very good, and leafcutter bees are also thriving.

“The bees love the heat and the crop loves the heat. The bees are just filling up blocks like crazy because of the high temperatures, so both yields of bees and crops look like they’re going to be really good.”

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