Agronomy Rising Resistance

Rising Resistance

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[deck]Neonicotinoids are becoming less effective in the fight against Colorado potato beetle – a resistance trend that appears to be moving westwards across Canada.[/deck]

It’s important to keep an eye out for Colorado potato beetles during the growing season because of their potentially devastating impact on both yield and quality. In recent years, farmers have relied heavily on neonicotinoid insecticides to help keep this resilient bug in check — but of growing concern to producers is the fact that what once worked, actually may not now.

In addition to growing CPB resistance to neonicotinoids, another disturbing trend is that this resistance appears to be moving from east to west across the country. According to leading ag specialists both in Canada and the United States, your best defence against the beetles truly depends on your farm’s chemical management practices.

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Adult Colorado potato beetles.

Vikram Bisht, pest management specialist with Manitoba Agriculture, Food and Rural Initiatives, says there are growing signs of CPBs developing resistance to neonicotinoids within the province. “We have seen some fields in Manitoba that have shown evidence of CPB resistance in 2013,” explains Bisht. “However, this is a fairly recent discovery in the province and is not as extreme as in other areas such as in Eastern Canada and potato growing regions on the U.S. Atlantic Seaboard, such as in the state of Maine.”

Bisht notes that if growers discover the presence of the beetles after using a neonicotinoid insectide, either as a seed treatment or in a foliar application, they should contact their local agriculture rep and send samples for testing. “If your field still has evidence of beetles, then the product you used has become less effective, and it is necessary to change and use a new group of chemistries,” he says.

Bisht and his colleagues have been sending CPB samples for laboratory testing to London, Ont., where an ongoing study led by Agriculture and Agri-Food Canada’s Southern Crop Protection and Food Research Centre scientists shows clear evidence that the beetles are becoming more resistant to certain classes of chemicals — particularly, neonicotinoids, which have been used extensively for potato growers in recent years due to their effectiveness and overall control.

Research scientist Ian Scott has been with the AAFC team monitoring CPB resistance since in 2007. During this time, he’s seen resistance in CPBs moving from east to west across Canada. “Resistance is more evident in eastern Canadian provinces as they have been using neonicotinoids much longer than in western regions,” explains Scott. “Growers used them year after year because they were reliable…. It’s this continued use of the same class of chemicals that now is becoming less effective as a result.”

Ian MacRae is an extension entomologist with the University of Minnesota’s Northwest Research and Outreach Center. He agrees that overreliance on one or a few chemicals spells trouble when it comes in insecticide resistance. “We have the tendency that when we find something that works we tend to keep using it, and this simply is not going to be an operable strategy in the future as tolerance builds,” he says.

Determining the levels of CPB resistance to insecticides within the neonicotinoid chemical family is increasingly important as growers in more regions of the country grapple with the problem. The AAFC study Insecticide resistance and cross-resistance development in Colorado potato beetle populations in Canada 2008–2011 details the level of neonicotinoid resistance in a sampling of more than 150 Canadian CPB populations.

The survey confirmed previous studies showing that many CPB populations have become less sensitive to imidacloprid, and that cross-resistance with the second-generation neonicotinoids — thiamethoxam and clothianidin — is a growing concern.

“Resistance and cross-resistance is a growing problem, so using several modes of action over the course of several years will help to slow the incidence of resistance in CPB,” explains Scott. “There are many other modes of action producers can use rather than a neonicotinoid class of chemistry; however they may be more costly and not have the same strength.”

Scott says continued funding in this area of CPB resistance research is critical to building the store of knowledge required to help producers fight against destructive pests across Canada.

Bisht agrees that continued study is vital to help the industry to maintain the efficacy of products in the marketplace. “Further research will help the potato-farming industry, which is a community,” states Bisht. “How neighbours are controlling problems in-field can ultimately affect the surrounding growing region, and producers are very aware of this.”

Poster Child for Resistance

CPB has been referred to as “the poster child” of insecticide resistance as research shows that the beetle has already shown various signs of resistance to some 51 insecticides. “So, this proves that many of the insecticides we already use have beetles that show resistance at some level,” states MacRae. “We need to ‘husband’ what we have and ‘husband’ the newer materials coming online so that we don’t lose the resistance in the new products as well.”

MacRae explains the insect’s resistance to certain chemicals in genetic terms. When CPB are exposed to one mode of action over a period of time and all of the insects susceptible to that chemical are killed, a few insects with a rare form of a gene that enables them to detoxify that particular insecticide may remain behind, he says.

“As the susceptible insects die, the only ones left to reproduce are the ones that have the trait to make it resistant,” says MacRae. “So, what you are left with is a population of CPB that will not respond to the insecticide.”

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Colorado potato beetle larvae.

Bisht notes that signs of resistance are evident when some of the CPB population dies, but not all. “It should then be clear that resistance is now in some of the population,” says Bisht. “It’s those that have survived that will multiply the resistant trait to their offspring. But there can be a mix of resistance, such as one mode of action [having] more of an impact than another. That’s when using a different class of chemistry can help to control a resistant population.”

Scott notes that the AAFC survey found that there is the need for growers to adopt effective integrated resistance management strategies for continued CPB control. “Neonicotinoids were once the silver bullet, but now as control is dropping for some populations, new management strategies are vital,” he says.

MacRae and Bisht agree that preventative measures and proper chemical management are critical in helping fight against CPB resistance to neonicotinoids. “It’s always desirable to use a different chemistry than what was used in the seed treatment,” says Bisht. “For instance, producers should use a different foliar insecticide group of chemistry to help prevent escape.”

According to the experts, scouting your crops early for signs of CPB infestation is also useful. Repeated field walks — including along field edges — are recommended, because there are now so many areas with CPB resistance, and because of the fact that two generations of the beetle can occur within a single crop year.