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A potato tuber showing extensive damage from wireworms.

The Battle Rages On

Potato growers battling wireworms have been given a three-year reprieve by Canada’s Pest Management Regulatory Agency, which had intended to deregulate the widely used insecticide, Thimet LNL 15G, this year.

With few effective alternative controls available for wireworms in potatoes, the PMRA has decided to extend the deregulation deadline to 2015.

Thimet is a systemic insecticide and is the only product currently registered in Canada (except for British Columbia) that is effective in killing wireworms and can be used by all growers.

Wireworm control is tricky, partly because of the multiple species—around 30—that exist across Canada, which are specific to different areas and environments, making it hard to develop control products that are effective in all provinces against all species.

Click beetles (the adult stage) exist in undisturbed areas, such as pasture, and constantly seek new areas to colonize. Wireworm populations typically take four to five years to complete a life cycle, all of which is spent underground, so it’s often hard to detect their presence until there is crop damage.

Few options for wireworm control, and increasing wireworm populations, are causing concern for many potato growers. Researchers believe a number of factors are contributing to population increases. Initial insecticides used for wireworm control—organochlorines—were banned around 40 years ago because of their residual effect in soils. “We think that those long-lived chemicals have now left most of the fields, so wireworm populations can build up once again,” says Robert Vernon, a research scientist at Agriculture and Agri-Food Canada in Agassiz, B.C.

As well, cereal crops, grown in rotation with potatoes, are one of the preferred habitats (along with grasses) for egg laying by adult click beetles, and there are no control products registered that effectively kill wireworms in cereals.

Up until its ban in 2004, an organochlorine chemical called Lindane was used as a seed treatment on cereal crops every three to four years to control wireworms, which killed 60 to 70 per cent of resident wireworms, the neonate (baby) wireworms that hatch later in the season, virtually eliminating the problem in subsequent crops.

Products recently registered for use in cereal crops, Imidacloprid and Thiamethoxan, provide damage suppression but don’t eradicate wireworms in the same way. These products knock out the resident wireworms just long enough to allow the crop to establish, after which they recover, and they don’t kill neonates.

“You have a good cereal crop stand but no kill of the larvae,” says Vernon. “If you plant potatoes the following year you have no reduction in wireworm population. That’s why farmers growing potatoes in rotation with cereals are compelled to use Thimet to control wireworms. We would like something registered on cereal crops that can wipe out wireworm populations, so growers don’t have to be reliant on Thimet.”

It’s an important consideration, because Thimet can be variable in its efficacy, especially when wireworm populations are heavy or when a field contains other green material. Green matter ploughed under before planting produces large amounts of carbon dioxide, which attracts wireworms and holds them underground. The Thimet-treated potato crop developing above won’t attract the wireworms, which have plenty of food and carbon dioxide where they are. Later in the season the wireworms will move up to feed on the daughter tubers, but by then the weakened insecticide does not provide effective control.

“If growers plant potatoes with Thimet, they must have no other carbon dioxide-producing sources in the field besides the mother tubers,” says Vernon. He suggests top killing and rotivating (not ploughing) green material during the fall before planting, or leaving as stubble and rotivating the killed residue in the spring.

Alternative Controls

Two products recently registered in the United States for wireworm control are showing promise, but neither is currently registered for use in Canada. Bifenthrin is an in-furrow spray, and studies on wheat have shown that it controls crop damage through repulsion rather than mortality.

Fipronil is registered on potatoes in the United States and Vernon rates it as the best insecticide to be developed for wireworm control in many years. “Tests on Fipronil as a wheat seed treatment have shown that it can control wireworms at very low dosages,” he says.

Whether Fipronil will be registered in Canada is uncertain, but Canadian growers are at a disadvantage without it. “If we could get it registered in Canada as a cereal crop treatment, the wireworm problem would disappear,” says Vernon.

Another alternative control method may be crop rotation. A three-year study conducted in Prince Edward Island has shown that growing brown mustard or buckwheat for two consecutive years as part of a three-year rotation significantly reduces wireworm damage in the potatoes grown in year three.

AAFC research scientist Christine Noronha and her team at Charlottetown planted brown mustard, buckwheat, barley and alfalfa in three different fields in three different areas of the province.

The number of holes on potatoes following two consecutive years of brown mustard and buckwheat ranged from three to five holes per tuber compared with 14 holes per tuber on potatoes following two years of barley. The amount of tubers with no damage averaged at 16 tonnes/hectare following brown mustard and 12 tonnes/ha following buckwheat, compared with 2.3 tonnes/ha following barley. In terms of marketability, 17 tonnes of potatoes per hectare were lost due to wireworm damage in the crop following barley, compared with half a tonne per hectare following brown mustard and three tonnes per hectare following buckwheat.

Noronha is confident that similar rotations could reduce wireworm damage across Canada. “I don’t think that this method is as species-specific as insecticides, which tend to change with the environment,” she says, adding that local trials will still be necessary. “It seems to be working in other places where they have different species to the ones in Prince Edward Island.”

But the clock continues to tick for existing wireworm control products like Thimet, which is one of the reasons that AAFC has recently formed a cluster project with the Canadian Horticultural Council, with the aim of developing new methods of controlling wireworms in locations across Canada.

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