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Potato beetle resistance brewing in Manitoba fields

Farmers should be scouting early in the season and switching to alternative products if they suspect resistance

Andrew Ronald has spent a lot of time in recent months talking to Manitoba potato growers about a familiar pest problem that could soon be taking on a new prominence.

The Keystone Potato Producers Association agronomist says there’s growing evidence that Colorado potato beetle — the most significant insect pest of potato crops in Manitoba — is developing resistance to the neonicotinoid family of chemicals.

He says growers have been sporadically reporting fields where these products appear to be less effective than they were in the past. Laboratory testing of CPB samples sent to London, Ontario last season confirm that the naturally occurring genetic resistance is beginning to overtake more susceptible populations in Manitoba fields.

“It’s a straight-ahead issue of selection pressure being applied season after season,” Ronald said in a recent telephone interview. “The neonicotinoids are very effective, they come in at an attractive price point, and they’re either seed applied or in-furrow applications at time of planting, making them very simple to work with. That’s led to a lot of applications since their introduction, and I don’t think anyone is surprised to find we now have this situation developing. It’s not anything we haven’t seen in other production areas.”

Selection pressure is what happens when the natural genetic variability in any population of weeds, insects or crop disease meets a chemical control method. There is generally a small subpopulation within these populations that is naturally resistant to the mode of action. When the product is applied, they survive and breed with other survivors, producing more resistant organisms. Over time, the resistant population replaces the susceptible population and the insecticide, weed killer or fungicide in question is no longer effective.

While he takes the problem seriously, Ronald also stresses it’s not the immediate end of the line for this very important crop protection product in Manitoba’s potato industry.

“I would say we are at the point where a serious problem could develop — but we’re not there yet,” Ronald said. “We’re certainly not advising growers to stop using the product in 2014. It still provides a lot of protection against other insects, and it also continues to be very effective against Colorado potato beetle on many farms.”

Instead Ronald and other potato industry specialists are recommending heightened awareness of what’s going on in potato fields after planting time, when the Group 4 neonics are applied, as well as very careful management of the product when it is used. For example, Ronald says any growers who are using reduced rates should stop immediately, because lower rates appear to contribute to the development of resistance.

John Gavloski, Manitoba’s provincial entomologist, echoes this call for careful management and more awareness of the nature of Colorado potato beetle populations in Manitoba fields. It’s always important, but perhaps even more than usual in this case, because Colorado potato beetles appear to be more effective than most insect predators at developing chemical resistance, he said.

“They’re specialized feeders that feed on a group of plants that develop chemical responses to insects,” Gavloski said. “Potatoes respond to feeding by developing chemicals that either make them unpalatable or kill the pests that are feeding on them. Colorado potato beetle then responds to this pressure by developing resistance — so there’s been this evolutionary battle going on, and it makes Colorado potato beetle better at developing chemical resistance.”

One of the best ways to spot the problem is through early-season scouting, looking for Colorade potato beetles that have survived. They can be gathered for genetic testing, which will determine the scope of the problem.

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“If you’re out there early in the season, and you’re seeing a lot of them, you should probably be trying to find out why,” Gavloski said. “That’s when you’d expect good control from the seed treatments or in-furrow treatments.”

Ronald said if growers do find populations that have survived in numbers high enough to justify a followup foliar application, it’s then critical that it not be hit with another Group 4 product. That’s just going to exacerbate any resistance problem by applying yet another round of selection pressure.

“If they have to go back in and make a foliar application, they should be rotating to a different product group,” Ronald said. “If they don’t, they’re just applying another round of selection pressure.”

A different chemical group would knock back the resistant populations, prevent economic damage to crops, and potentially push back the appearance of full-blown resistance, keeping a valuable and economically viable control tool in the tool box for a few more seasons.

For some in the industry, it’s beginning to seem alarmingly like the situation back in the 1990s, prior to the registration of neonicotinoid products, when the previous generation of crop protection chemistry had fallen to resistant Colorado potato beetles. Potato growers in the province saw significant economic losses prior to the registration of the first new products, and some worry it could happen again. Entomologist Gavloski says it’s a natural reaction to the situation, but cautions against drawing too many conclusions based on past history.

“I don’t think it will ever get that bad,” he said. “To start with, we have alternative control products that are already registered. They might be a bit less convenient, or a bit more expensive, but they do exist.”

It’s those alternative control products that Ronald is urging growers to acquaint themselves with, citing the need to be prepared when the inevitable will happen. They include active ingredients like cyantraniliprole (Verimark) or the spinosyn family (Success, Entrust and Delegate).

“They should begin evaluating some of these different control strategies and products now, on some of their acres, so that they’re prepared for what the future might bring,” Ronald said.

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Gord Gilmour is Editor of the Manitoba Co-operator.

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