Weevils wobble but they won’t go away

Alfalfa weevils resistant to the most commonly used insecticides have been confirmed in the Rosemary area of southern Alberta, where many alfalfa seed crops are grown.

The wily weevils have been found to survive even after being treated with 50 times the recommended rate of Matador, a synthetic pyrethroid in the Group 3 chemical family.

Brad Alexander, research and extension manager with Alfalfa Seed Commission Alberta, said only 20 percent of the weevils from one field died when tested at the high rates of insecticide. Weevils from a second field had a 60 percent kill at the high rate, as determined by provincial entomologist Scott Meers.

“That definitely confirmed that there is resistance to Matador because the trial specifically used Matador, but the theory is, because of the nature of synthetic pyrethroids and of chemicals in general, I suppose, is that it’s a general resistance to all synthetic pyrethroids, not just Matador.”

Six insecticides are registered for use against alfalfa weevil. Matador and Decis are both Group 3, Malathion, Cygon and Imidan are in Group 1B and Coragen is in Group 28.

The latter four are not particularly effective against weevils, Alexander said.

“That’s a little bit alarming because it kind of sticks you to only two mode of action groups to use in rotation,” he said.

“If they’re not getting killed by Group 3, then there’s only the 1B category, which a lot of people don’t like anyway.”

Alfalfa seed grower Larry Wiens, who farms in the Rosemary region, said weevil control has been a major challenge for about two years.

“We’ve had to learn to live with higher levels,” he said.

“We’re just not getting the nice start we used to get in July.”

He said no other chemicals work as well as Matador and Decis, at least until recently.

“We really don’t have any other registered options so that puts us in a difficult position. It’s pretty frustrating right now.”

However, he said he is optimistic that a solution will be found, whether or not it is cheap or easy.

Alexander said farmers likely use Matador and Decis so much because they cost about $8 per acre compared to about $32 for Coragen.

He said growers are talking with the Prairie Pesticide Minor Use Consortium to see if other chemicals can be registered to fight the resistant weevils.

In the meantime, an extended period of -20 C weather, like that of mid-December, might be enough to kill them. The hardy insects manufacture their own type of antifreeze, but it is only effective to about -18 C, said Alexander.

“This cold snap that we had, with no snow, which is also very important because they weren’t insulated, hopefully that had some effect on the population.”

Some alfalfa seed growers have used tine harrows on their alfalfa fields to better expose the insects to cold. Alexander said some might also consider burning their fields, but that requires moving all the leafcutter bee huts, which is time consuming.

Alexander said the resistant weevils first came to light as a result of a grower survey, which seems to indicate the problem is limited to the Rosemary area.

“Our hope is to get it under control, anyway, to get some form of control happening in the Rosemary area before 2018 because by the end of summer 2018, just judging on a forecast, (the weevils) can migrate up to 13 kilometres in one year,” he said. “That would put them into a circle that encompasses Patricia and Brooks.”

Olds College researchers have collected and are raising resistant weevils in a controlled setting, said Alexander, and are testing the efficacy of various insecticides against them.

They’ve already tried a chemical called Spinosad, a natural bacterial treatment, with limited success.

“We did have good control … but by good, I mean better than water,” said Alexander.

“It wasn’t very good at all ,but all of the other ones that we tested, some of them weren’t even as good as water.

“(Olds College researchers) are kind of trying everything that they can on them, and next year we’re ramping up for more testing not only of different chemicals and the chemicals that we have … but we’re also looking at expanding the area.”

Alfalfa seed grower Fred Preston said he suspects weevil resistance has been building for years but became more obvious after several mild winters, which allowed higher weevil populations to survive.

He said the situation is troubling, and he is in favour of using safer products against insects if they are available and effective.

“I’m OK with using safer products. Coragen is very safe for people and bees. It’s expensive, but that’s kind of how it goes. Safe is better,” he said.

“I’m not going to change my cropping plans. I’m kind of in a committed relationship with alfalfa seed and the beekeeping, bee racket.”

Alexander said the weevil problem is likely here to stay.

“To be very, very honest, I have not found a single report online or ever heard of any resistance issue going away,” he said.

“Once it’s here, it’s basically a new pest. And that’s how we’re looking at this weevil because really we effectively don’t have anything to control it, so it’s almost like a new pest entirely to us.”

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