Insecticide-resistant potato beetles spur calls to boost product stewardship
By Vicky Boyd
A small but increasing number of pockets of Colorado potato beetles resistant to a widely used insecticide should be a warning to practice sound resistance management, experts say.
Although the fields involve potatoes, entomologists say resistance could develop in any crop involving nearly any insect pest where growers make repeated applications of neonicotinoids without rotating modes of action.
Based on monitoring programs, entomologists have identified CPB populations in Maine, Delaware, Pennsylvania, Michigan and Long Island that can withstand imidacloprid at higher-than-label rates. Imidacloprid is marketed as Gaucho and Genesis seed treatments, Admire Pro, Leverage and Provado by Bayer CropScience and is a member of the neonicotinoid chemistry class.
Officials also are concerned because a small percentage of beetles appear to be cross-resistant to chemicals within the same chemistry class. Among those are thiamethoxam, marketed as Platinum, Cruiser and Actara by Syngenta Crop Protection and acetamiprid, marketed as Assail by Cerexagri.
In addition, laboratory tests conducted by University of Maryland entomology professor Galen Dively have shown that a few beetle populations are less sensitive to spinosad, marketed as SpinTor and Success by Dow AgroSciences, and abamectin, marketed as Agri-Mek by Syngenta.
Although the chemicals belong to different classes than imidacloprid, they work on the same parts of the beetle’s nervous system, so the findings don’t surprise Dively.
The resistant pockets remain small, but they point to the potato beetle’s innate ability to overcome toxic substances, including insecticides.
“It’s more than a small red flag,” says Ed Grafius, a Michigan State University entomology professor in Lansing. “I think it’s a huge red flag. This is the first time we’ve seen a problem in the Midwest.”
A call to action
The findings also have caught the attention of Bayer officials, who are stepping up their efforts to educate growers and consultants about product stewardship.
“Throughout Maine, and for that matter, all across the country, there has been a very high reliance on neonicotinoids,” says David Rogers, Bayer product manager for insecticide development in Research Triangle Park, N.C. “We took steps, beginning two years ago, to strengthen the resistance-management guidelines on the labels.”
Among those guidelines is making only one soil-applied neonicotinoid application per season, using full rates and rotating crops.
“In southern New Jersey, for example, you have eggplant, some tomatoes and some potatoes. The pest spectrum would all include Colorado potato beetle,” Rogers says. “But the most important part of this is really beyond Bayer’s control and to a certain extent, the growers’, and that is to rotate crops away from Colorado potato beetle infestations.
“As we have found, the beetles that are from the West and Canada, for that matter, remain some of the most sensitive beetles we’ve ever seen.”
Lab monitors tolerance
For the past seven years, Bayer has sponsored laboratory testing to monitor levels of imidacloprid sensitivity, Rogers says. Beetles representing 50 U.S. and Canadian populations were collected and sent to the University of Maryland in College Park during the 2005 season.
To gauge tolerance, Dively feeds the larvae an artificial diet treated with specific rates of imidacloprid.
The results are reported as LC50s, or the lethal concentration of the pesticide that will kill 50 percent of the population. An LC50 of less than 4 parts per million is considered sensitive. An LC50 of 4 to 8 ppm is considered semi-tolerant to tolerant. And an LC50 of more than 8 ppm is considered resistant.
Bayer was already aware of a population in southern Maine resistant to imidacloprid, Rogers says. Growers were reporting difficulty controlling Colorado potato beetle with Admire in 2002 and 2003.
Bayer representatives asked the growers, who farm about 850 acres in an isolated valley, to stop using imidacloprid in 2004. They also conducted a trial with the grower in 2005 to determine whether beetle sensitivity to the insecticide could be restored. The project had some success and will be continued this year.
During 2005, populations from central Maine and Aroostook County also showed imidacloprid tolerance. Growers reported that control was breaking down before the end of the season, says Andrei Alyokhin, a University of Maine assistant professor of applied entomology in Orono.
“We had growers here that were still able to get good control, but later in the season where there were substantial populations, they had to spray their fields with a non-neonicotinoid,” Alyokhin says. “[Galen Dively] tested them in the lab, and they were either tolerant or resistant.”
The Aroostook County discovery was surprising because growers there hadn’t reported any performance problems in seasons past, Rogers says.
Dively’s testing again confirmed a resistant population from two fields near Little Creek, Del., this season, Rogers says.
On Long Island, baseline sensitivity trials conducted before Admire was first used in 1996 showed Colorado potato beetle was resistant.
“The Long Island beetles had an LC50 of 4 before it was even used,” Rogers says. “So to call them resistant beetles or having developed resistance isn’t exactly correct.”
Midwestern concerns arise
MSU’s Grafius has been conducting lab bioassays gauging beetle sensitivity within his state. During the 2004 season, a consultant from central Michigan reported Colorado potato beetle control problems after using Genesis seed-piece treatment.
Grafius conducts his tests differently from Dively and sprays specific rates of imidacloprid directly on the beetles. He then compares those results to those from susceptible beetles.
“In our lab bioassays last year, it took more than 20 times the dose of insecticide to kill the beetles from one of the fields compared to the susceptible strain,” Grafius says. “This year, from one of the fields at that site, it took more than 100 times, so it’s a very large level of resistance.”
Grafius blames the problem on growers relying heavily on Admire since it’s registration in 1995, but he also admits they have few alternatives.
“In fact, when Admire was first labeled in 1995, many growers had lost all other available chemicals due to resistance,” he says. “Most potato growers in the East and Midwest went through some very serious resistance problems in the early 1990s and clearly remember the control problems and yield losses that can happen when resistance becomes serious.”
Grafius is also conducting laboratory bioassays looking at how sensitive Colorado potato beetle from the troubled areas in Michigan are to thiamethoxam, a compound related to imidacloprid. Although the beetles are more sensitive to thiamethoxam than imidacloprid, they are nonetheless growing more tolerant to thiamethoxam.
One of the keys to breaking the resistance cycle, Grafius says, is to rotate to crops that aren’t Colorado potato beetle hosts and move potato fields more than 1/4 mile from last season’s locations. But he also realizes that it may be easier said than done.
Rotation, rotation, rotation
Although the central Michigan grower with the resistance problem rotated potato fields, he only moved them 100 yards away.
“This is a good rotation from a soil standpoint, but not far enough for potato beetle,” Grafius says.
Working with the grower’s crop consultant, Grafius recommended the grower plant eight rows of potatoes around corn in the troublesome fields. The potatoes acted as a trap crop, attracting and concentrating potato beetles in a small area.
Based on counts, Grafius estimates the trap crop attracted 50,000 to 100,000 beetles this spring.
Then the grower could use an insecticide that would have been too expensive if he had to treat the entire acreage. The grower alternated Agri-mek, Spintor and Rimon, an insect growth regulator from Crompton.
Before the larvae could pupate and become adults, the grower disked the trap crop several times. The effort appears to have significantly reduced the beetle population, and more important, resistant beetles, Grafius says.
Neonicotinoid resistance management as easy as 1, 2, 3
Follow these three key steps to help minimize the chances of Colorado potato beetle developing resistance to neonicotinoids, classified as group 4 insecticides by the Insecticide Resistance Action Committee.
1. When you apply a group 4 systemic product (Admire, Gaucho, Genesis or Platinum) to the soil at or near planting time, do not use a group 4 foliar spray (Provado, Actara, Assail or Leverage) in the same season.
2. When you apply a group 4 systemic product (Admire, Gaucho, Genesis or Platinum) to the soil, do not use a group 4 systemic on the next year’s potato crop unless it is located ¼ mile or more from the previous use site.
3. When you apply a group 4 foliar spray (Provado, Leverage, Assail or Actara) in the absence of a group 4 systemic, do not make consecutive applications.
--Courtesy of the University of Wisconsin Department of Entomology
National Potato CouncilNeonicotinoid Insecticides: A Grower Approach to Resistance Management for Colorado Potato Beetle and Green Peach Aphid in Potatoes
Insecticide Resistance Action Committee modes of action classifications: