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The Path of Least Resistance

While neonicotinoids are currently controlling most insect populations affecting potato production in Canada, experts say pest resistance is only a matter of time.

Twenty years ago, all was going well on Gary Linkletter’s potato operation in Prince Edward Island. Linkletter, who currently serves as chairman of the P.E.I. Potato Board and was also co-chair of the Potato Council of the Canadian Horticultural Council, was doing everything right—not only did he use regular crop rotations, but he also had a hard and fast policy of rotating chemistries. Then the unthinkable happened.

“All of a sudden, one year Colorado potato beetles came in and were resistant to everything—we lost 200 or 300 acres,” he says. “They were so bad there was just a solid mass of beetles on the drills. We lost 20 to 25 per cent of our crop that year. That was the same year, though, that new chemistry in the form of Admire became available. If it hadn’t become available, I don’t know what we would have done.”

Colorado potato beetle larva. All photos courtesy of Christine Noronha, AAFC.

Colorado potato beetle larva.
All photos courtesy of Christine Noronha, AAFC.

Linkletter’s point is a sobering one—combating insecticide resistance is a complex community effort that relies on producers making the right choices season after season. Rotating chemistries to combat resistance is important not just for individual growers, but for everyone across the industry.

“It’s not just what a grower does—it’s what his neighbours do [too],” says Linkletter, adding that regardless of an individual grower’s sound management practices, everyone feels the impact of insecticide resistance. “We use a two-in-five crop rotation, and longer rotations certainly are beneficial. But looking back at that incident 20 years ago, I don’t know what else we could have done,” he says, “[The beetles] showed up on our doorstep and just took over the field. Rotation’s important, but rotation as an industry as well as a farm,” he emphasizes.

The reasons for insecticide resistance are difficult to quantify, but according to Christine Noronha, a research scientist and resistance specialist with Agriculture and Agri-Food Canada in Charlottetown, P.E.I., the chief cause was initially the long-term overuse of foliar, broad-spectrum insecticides. In response, insects began to develop cross-resistance. She says that even though narrow-spectrum insecticides are mainly used in Canada currently, resistance always builds up with an overdependence on any insecticides in lieu of a variety of management techniques.

Colorado potato beetle adults and larvae feeding on a potato crop.

Colorado potato beetle adults and larvae feeding on a potato crop.

“Insecticide resistance is quite widespread. If you take just the Colorado potato beetles themselves, there are beetles in every province, and they are resistant to at least one or two of the insecticides,” says Noronha. “It’s not as bad in Western Canada because they haven’t been growing potatoes as long as in Eastern Canada. But you see it everywhere.”

Linkletter says they no longer uses a broad-spectrum, imidacloprid insecticide on his farm, as they have seen some resistance starting to develop to that class of chemistry. Thiamethoxam, another neonicotinoid insecticide, is being utilized instead, which Linkletter says has been so successful it’s now used on 95 per cent of his fields. However, he believes it’s only a matter of time until resistance to this chemistry builds up in insect populations.

“We don’t have resistance right now—it’s working so well—but if you do the math you know that your days or your years are numbered that you’re going to have that success,” he says. “We just don’t have the availability of other products to rotate [it] with.”

Always Follow the Label

Brian Beaton is Prince Edward Island Department of Agriculture and Forestry’s potato industry coordinator. He says the neonicotinoid class of chemistry is providing excellent control of Colorado potato beetles and, so far, the Island has not documented a great deal of insecticide resistance to neonicotinoids among beetle populations.

“The current selection of pesticides, in-furrow or seed-piece applied, are working right now in P.E.I. Almost everybody [uses pesticides] in-furrow or treats the seed with a neonicotinoid insecticide,” says Beaton. “In most cases, it’s giving season-long control. There’s lots of insect pressure, but we can still control them.”

Even though Colorado potato beetles are always a force to be reckoned with, it’s wireworm that is currently causing the Island’s potato industry some concern because of the pest’s long life cycle. Other insects such as aphids and the European corn borer also need to be controlled depending on the population pressure.

The female European corn borer moth (pictured here, right, beside a male moth) lays its eggs on the stems and undersides of potato leaves.

The female European corn borer moth (pictured here, right, beside a male moth) lays its eggs on the stems and undersides of potato leaves.

However, despite the current efficacy of products like neonicotinoids, resistance is a risk that exists wherever the same modes of action are used against pests long term. “Just like any population, there are always resistant ones in there,” says Beaton. “If you go with a lower rate or if you use the same product over and over and don’t rotate between modes of action or families of products, those one or two [insects] that are resistant escape and continue to build the population. Then the population shifts from being sensitive to being resistant.”

According to Beaton, following the label is key to proper insect management. Anyone using pesticides should take time to compare the product name and active ingredient so they know if they are using products from different chemical families if possible.   Growers should also be mindful of when patents expire on familiar chemistries and when generic formulations of the same products appear on shelves under new labels. “It’s important to follow the label—a lot of work goes into the label by the companies [that make the products],” says Beaton.

“Rotating products is important,” he adds. “If you can prolong the life of crop protectant products, history has shown the prices do come down, and that benefits the growers and the companies too because they have the product on their portfolio for a longer time.”

A mixed bag of management techniques is most effective in controlling insect populations, however, including proper calibration and use of application equipment, ensuring the correct rates of application, and even the use of mechanical control methods. “It’s important to have a multi-pronged approach,” Beaton sums up.

Researching Solutions

Noronha and her team have been studying the use of biological control organisms, including the parasite Trichogramma brasicae, which infects the eggs of European corn borers when released into a field. This biocontrol option is already commercially available, and Noronha says she has begun talking to growers about it.

“It’s up to researchers to find ways to make [integrated pest management] easier for growers. And it’s up to [growers] to try these techniques out,” she says. “So it works both ways for both the researcher and the grower.”

However, biocontrol techniques like the use of T. brasicae may not be the best solution for everyone. “You have to be quite precise in timing of release,” says Noronha. “With the way [biocontrol methods] work, a farmer has so many other things going on, and it would be more difficult for larger operations. It would be more beneficial for smaller farms to use.”

Plant damage caused by the European corn borer.

Plant damage caused by the European corn borer.

According to Noronha, this is also true for a number of insecticide alternatives, including some mechanical methods.

“The mechanical techniques can be harder to use. For instance, for Colorado potato beetle, you can put a trench around the field, but for a large grower this would be hard to do. Some [methods] may be time consuming if growers have to go into their fields over and over again,” says Noronha.

Another example of a time-consuming management technique is ensuring fields are clean by removing debris which can harbour pests and diseases. This can be difficult for a grower rushing against the weather to harvest his crop. “There are certain methods that only work on small farms,” she says.

There are methods any grower can use to control insect pest populations, including scouting, monitoring and using thresholds. “The other thing growers can do is rotate classes of chemicals,” says Noronha, echoing Beaton and Linkletter.

It’s up to researchers to find ways to make [integrated pest management] easier for growers. And it’s up to [growers] to try these techniques out. So it works both ways for both the researcher and the grower.

– Christine Noronha

Supporting New Development

While Noronha maintains there are new classes of chemistries in development, Linkletter feels that the existing choices for growers fall short of what is required, resulting in a “ticking time bomb” for the potato industry. He argues that chemical companies need the industry’s support in developing new products and easing the pressure on existing chemistries. Linkletter also believes the regulatory system in Canada has an important role to play.

“It is critical that we have a regulatory system that is timely, efficient and cost-effective to get these chemistries registered and into the hands of growers,” stresses Linkletter. “So often in Canada, we have seen new chemistries in the hands of growers in other countries, but blocked from Canadian use either because the Pest Management Regulatory Agency was very slow in processing applications, or that the chemical companies did not even submit applications in Canada because the time and cost was not justified in our smaller market. PMRA has made a lot of progress but there still is a ways to go”

In addition, new products often have to pass a demonstration of efficacy in Canada even when such tests are not required in other markets, which also slows the process. “I really have concerns over year after year of the same chemistries being used,” Linkletter says.

With limited options for insect control, the path of least resistance might be a difficult one to achieve in Canadian potato production, but growers are doing their best. “Growers really are doing a good job overall,” says Noronha. “In Canada, it’s really hard because you have such a short growing season, but they are trying their best [to combat] resistance and diseases.”

But as insect pressure continues unabated, more growers may be forced to realize that pest control is a community affair, and every decision they make ultimately affects their neighbours.

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