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The plots with the most green are plots with wheat growing amongst the potatoes. (Photo: Bob Vernon, AAFC)

Companion planting offers hope for wireworm control

“Attract and kill” method could hit pest populations hard while costing growers less.

Wireworm is persona non grata in many Canadian potato-growing areas, particularly in P.E.I., Alberta and B.C., where the pest causes millions of dollars of damage each year.

“P.E.I. is a good example of where a wireworm problem is out of hand,” says Bob Vernon, a research scientist with Agriculture and Agri-Food Canada (AAFC). “There’s a new species that’s come in from Europe that is now causing most of the damage: Agriotes sputator. The populations in P.E.I. are almost unheard of.”

Wireworms are the larval stage of the click beetle; there are about 30 species of the pest across Canada. Although all species of the pest prefer cereal crops, wireworms also feed on potatoes, canola and pulse crops.

According to Vernon, wireworms lie dormant in a field over the winter and become hungry in the spring. In their natural habitat, they’d prefer to munch on grass, but they’ll feed on “anything in the ground,” he says.

The wireworm stage lasts for roughly four years. If producers grow grasses or cereals in an infected field and convert it to potatoes at any point in those four years, the wireworms will make their way to the potatoes, following the tubers’ respiring trail of carbon dioxide, and cause significant damage.

Where potatoes are typically grown in short rotations of two to three years, as in P.E.I., this presents a major problem for growers.

But help is on the way.

Vernon has been working on what he calls an “attract and kill” or “companion planting” method for a couple of decades, and his research has borne fruit.

Companion planting concept, showing treated wheat co-planted with potatoes. (Photo: Bob Vernon, AAFC)

Here’s how it works: in the spring, when waking wireworms are hungry, treated wheat seed is planted along with potato seed in rows one metre apart. All germinating crops respire, but wheat seed does so more quickly than potatoes in the spring. Wireworms are attracted to the wheat seed’s carbon dioxide “trail,” following it from up to half a metre away, and consume the toxic seed. The pests are wiped out before they have a chance to impact the potato crop.

“If you plant the wheat seed in a very dense strip so that the seeds are almost touching in each row, you’ll collect most of the wireworms in that field,” says Vernon.

 “The technique works and offers advantages that all of the other approaches do not,” he adds. “It opens up opportunities to reduce the amount of insecticides to negligible levels, because you’re attracting the wireworms to the poison versus hoping the wireworms will encounter the poison by chance.”

Registration Issues

So far, Vernon and his team have tested the technique in Ontario, B.C. and Alberta on about four different wireworm species. They’ve seen positive results each time.

In the first trials, about 15 years ago, Vernon used Lindane to treat wheat seed. But Lindane has been banned in Canada since 2004, and was never registered for use against wireworm.

In subsequent studies, Vernon found that Fipronil, a broad-use insecticide, can kill in excess of 85 per cent of wireworms when used in the companion planting technique. “We found we could get the same level of control with wheat seed treated with Fipronil put in with potatoes as if farmers were using Thimet 15G,” at a significantly lower cost, both economically and environmentally.

Thimet 15G (which is no longer registered in Canada), along with Thimet 20G, its successor, are highly toxic. Vernon says Thimet is generally used at the rate of 3250 grams of active ingredient per hectare. “Fipronil is way less toxic,” he says. “Instead of 3250 grams of a highly toxic insecticide, you’re using one gram of an insecticide that’s 100 times less toxic.”

Fipronil is currently registered in the U.S. for use as an in-furrow spray on potatoes, but it isn’t used in companion planting south of the border because the product is not registered on wheat. The insecticide is not registered in Canada either, and there are few alternatives: none of the other insecticides currently registered on wheat in Canada are effective against wireworms.

But Vernon is hopeful new insecticides will come along that can be used in his companion planting approach. “We’re working on new chemistries all the time,” he says.

Management Techniques

In the meantime, Vernon says there are alternatives to his novel approach. In P.E.I., growers are well aware rotation is key to managing wireworms. But rotating potatoes with cereals is inadvisable in areas hard-hit by the pest.

Christine Noronha, a research scientist with AAFC based in Charlottetown, has been conducting rotation studies for many years. Her research has found that brown mustard and buckwheat can significantly reduce wireworm populations if they are grown prior to potato.

Last year, 25,000 acres were planted to brown mustard in the province, she says. By sacrificing a year to two years of potato production, growers are achieving good control of a host of pests and diseases, from wireworms to nematodes, verticillium and scab, and seeing higher yields and improved quality. There is a market for brown mustard, but growers prefer to disc it into the field in the fall so that the crop’s natural biofumigant properties can wreak vengeance on pests.

Noronha has begun a new study interplanting mustard with potatoes in what’s called a “nurse crop,” to evaluate whether the mustard can help reduce damage to potatoes. The project is in its preliminary stages, but Noronha says the technique has been used successfully in barley.

Vernon says planting brown mustard works. His studies have shown that if potatoes are planted with Thimet 20G the first year, there will be a reduction in wireworm egg-laying. If brown mustard is planted the following year, populations are reduced again; another year of brown mustard will significantly lower wireworm populations in the field. The only problem with this approach is that producers lose income in brown mustard years.

No solution is a silver bullet, but researchers are working year-in and year-out on possible controls for the insatiable pest, and companion planting might soon become a viable option for growers when the right chemistry hits the market.

“The companion planting approach is something we test every year with Fipronil as well as new candidates that come our way,” says Vernon.

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