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Colorado potato beetle has developed resistance to 52 different compounds belonging to all major insecticide classes, and resistance is growing to neonicotinoid chemistries. (Photo: Tracy Shinners-Carnelley)

Colorado Potato Beetle Marches On

Managing Colorado potato beetle requires various techniques.

The most destructive potato crop pest in Canada, the Colorado potato beetle (Leptinotarsa decemlineata), shows no sign of slowing down. Indeed, since the 1950s, Colorado potato beetle has developed resistance to 52 different compounds belonging to all major insecticide classes, and resistance is growing to neonicotinoid chemistries.

According to Dr. Tracy Shinners-Carnelley, vice president research and quality with Peak of the Market in Winnipeg, Man., there is an ongoing increase in the amount of Colorado potato beetle (CPB) activity in many parts of the country.

“When I look at our Manitoba situation, we have certainly seen an increase in beetle pressure over the last couple of years,” she says. “That is not the general situation in all of Manitoba, but certainly there are more and more observations of growers and agronomists seeing more beetle activity. And talking to colleagues across the country, some provinces are seeing an uptick in the level of beetle activity.”

Colorado potato beetle adults overwinter in the soil and along hedgerows and emerge in early spring, when they lay clusters of yellow eggs on the underside of leaves of newly emerged potato plants. The eggs hatch within a week and the larvae begin feeding on the leaves, passing through four larval stages before pupating in the soil. The beetles emerge from the pupa as new adults and they feed on the leaves before moving to hibernating sites. Both larvae and adults feed extensively on potato leaves, which can result in complete defoliation of the plants if populations are left uncontrolled.

Since the early 1990s, control of this pest has become more complicated because of CPB resistance to insecticides in many parts of Canada, especially from Manitoba east to P.E.I. One reason chemical resistance has developed is because, historically, rotating chemicals with different mode of action sites was not encouraged to a great extent.

“Colorado potato beetle has developed resistance to every chemistry that it’s been exposed to,” says Shinners-Carnelley. “To date, we’ve been very fortunate to keep the efficacy of the neonicotinoids we have. But certainly, we are seeing that decrease in the duration of control that we’re getting with seed treatment or in-furrow applied neonics.”

According to Dr. Ian MacRae, professor and extension entomologist with the University of Minnesota, the Colorado potato beetle is “the poster child for insecticide resistance”.

“Colorado potato beetle has extremely effective detoxification mechanisms,” he notes. “Of every single mode of action that’s been tried against Colorado potato beetle somewhere, the population has developed resistance to that mode of action.”

When neonicotinoids were introduced in the 1990s, they looked like very promising products at the time. They had relatively mild mammalian toxicity, they were translaminar (penetrating leaf tissues), they were systemic. “You could put them down at planting time, and they’d be taken up by the plant and would be available for a period of time,” says MacRae. “Everyone who was growing potatoes at that time remembers how effective they were at controlling Colorado potato beetle.”

Shinners-Carnelley maintains potato growers have been fortunate the neonics insecticides have lasted for as long as they have. Even though there is reduced susceptibility or resistance in some cases, the neonics still have a role within insect management in potatoes and still offer some benefit for beetle control.

But the potato industry is still scrambling to find alternate methods of controlling CPB today and into the future.

One way is to continue the use of foliar insecticides, such as Delegate and Exirel. “These are both newer classes of chemistry with different modes of action,” says Shinners-Carnelley. “But the challenge in front of us is, in the short term, how do we use those foliar insecticides in a way that we don’t invite resistance?”

In addition, there is the new concern that CPB are emerging at different times, over a longer period. In grower data studied by Dr. Russell Groves, professor and vegetable extension specialist with the University of Wisconsin-Madison, CPB are emerging over much longer periods of time.

“We have to now deal with Colorado potato beetle populations that are not just emerging over a discrete window of 10 to 14 days, but rather these populations are emerging over 20 to 25, or even 30 days,” says Groves. “Fortunately, we have several reduced-risk tools that have recently become available to us. However, most of these are not considered broad-spectrum in nature, and only affect specific stages of the beetle’s development. More specifically, these are tools that can kill only during very discrete life stages; some that will work on the eggs, whereas some work on just the early stages of larvae, such as the first and second instar larvae.”

In this way, it is critical for growers to understand which are the predominant stages in the field, and what the best choice of product should be.

Most of the damage by Colorado potato beetle is due to feeding by the largest (fourth) stage of larvae, when they are about 1/4 inch (8 mm) long. If foliar insecticide applications are needed, these should be targeted when the oldest larvae are in the third stage of growth 1/8 inch (about 5 mm long). Spraying too early means fewer eggs will have hatched and more applications may be needed.

Shinners-Carnelley says it’s important to read the insecticide label carefully for information about when to spray. For instance, some insecticides work best when applied to small larvae, while some insecticides do not work well under certain weather conditions, such as pyrethroid insecticides which do not work well when temperatures are above 25C. Spraying when conditions favour optimum effectiveness of the insecticide will also increase the level of control.

“When we’re trying to maximize the amount of control that we can get using those chemistries towards potato beetle, we definitely see rate response. So growers need to be using the highest labelled rate for the neonics, and that will give them the longest duration of control in the field. Which means it helps to delay the need to consider foliar insecticide during the summer months,” she says.

“The other thing that we’ve seen from three years of research is that when we compare the same insecticide, whether it’s applied as a seed treatment or an in-furrow, we’re getting longer duration of control with the seed treatment. But we also have to be very cautious in how we use foliar insecticides that are active ingredients of relatively newer insecticides because they are working well right now. But we have to make sure that we try and keep those insecticides effective as long as we can.”

Growers should also ensure they’re not going in too quickly and applying insecticides. Instead, growers should become familiar with seeing Colorado potato beetle during the growing season and getting comfortable with a certain amount of defoliation or beetle presence before they need to go in and apply a foliar insecticide.

There is no one sure threshold for Colorado potato beetles. And that’s because different potato varieties have different tolerances. “For instance, for longer-season varieties such as Russet Burbank, it’s going to have a different threshold than what a short-season variety would have like Red Norland,” notes Shinners-Carnelley. “My gut feeling is that potatoes can actually tolerate a lot more defoliation and beetle presence than what growers feel comfortable with.”

Last year (2017) was the final year of a large insecticide trial conducted in Winkler, Man., by Shinners-Carnelley and colleagues Darin Gibson and Debbie Jones. The objective of the study was to evaluate insecticide management strategies, including a combination of registered seed treatments, in-furrow and/or foliar insecticides. The study looked at a number of insect management strategies, including one using just foliar insecticides, and others using seed treatment or in-furrow applications and then one or two foliar insecticides. What the study found was there was no significant difference in yield amongst any of the different insecticide strategies.

Some conclusions: Verimark treatments appeared to break by July in all years; Exirel performed well (used in 2017 only); Delegate foliar insecticide continues to perform well; the neonicotinoid treatments provided good control, although many neonic treatments provided control only until mid-July or longer, then a foliar insecticide was needed; the highest labelled rate generally resulted in fewer CPB than lower rates; and, seed treatment application generally provided longer duration of control compared to the in-furrow application.

“We got up to a maximum of about 30 per cent defoliation in some of the insecticide treated plots,” notes Shinners-Carnelley. “So that’s a significant amount of defoliation, but yet we weren’t seeing any significant differences in yield. What this tells me is we’re far away from knowing what the exact threshold is, but we have a little bit more tolerance for defoliation than what we feel comfortable with when we see them in the field.”

Crop rotation is also an important tool that can be used to control CPB which overwinter as adults within the field of origin, and within uncultivated areas near the field. Cool spring weather often forces overwintered beetles to walk to a new food source, so the farther a potato field is from overwintering sites, the less the probability that it will be colonized by migrating beetles. This can reduce the need for insecticide applications.

“Some of the things that have been known for years are practiced less and they’re not always very feasible for growers, but rotation over distance is one thing that can work,” says Groves. “If you can move potatoes a quarter to a half mile away from where the potatoes were last year, you can go a long way in knocking beetles out. It can work, and it can work very successfully. But logistically, it’s hard to do and growers are not going to start thinking about these kinds of approaches until they really have to.”

According to Groves, there are also plenty of natural controls, generalist predators of CPB. Some carabid beetles that consume egg masses, for instance, or predatory stink bugs that eat early larvae. But he notes they’re not always in abundance in potato, and they often can’t keep up with the CPB population.

“There are often generalist predators, like ladybird beetles and ant lions and other things that will feed on the eggs of CPB,” says Groves. “But the reproductive potential and fecundity of this insect [CPB] – that’s the oviposition rate per female – just overwhelms the system.

“So yes, there are natural enemies, it’s just natural enemies cannot be relied upon in every system,” he adds. “We plant potatoes on such a large scale, in large monocultures, that the number of generalist predators that can arrive and become established in these crops is difficult. They may work in your vegetable garden (as natural predators), but often not sufficiently on a commercial scale.”

As we near another year of cropping, Shinners-Carnelley admits that the toolbox for beetle control doesn’t really contain a lot of effective options. But growers can still manage well with the options they do have.

“We have to try and control them the best that we can with the neonics,” she says. “But we also have to be very cautious in how we use foliar insecticides that are active ingredients of relatively newer insecticides because they are working well right now. We have to make sure that we try and keep those insecticides effective as long as we can.”

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