April 2007
Imidacloprid Resistance Appearing After 11 Years

There’s no need for growers to panic, but it’s true. Colorado potato beetles are becoming resistant, in some production areas, to the leading insecticide that has been knocking them dead for 11 years.

There are several reasons why this does not mean growers will return to the bad old days of the early 1990s. At that time, beetles became so resistant to the existing insecticides that growers had to turn to physical methods of control. Chemistry failed, and growers were literally killing the beetles by flaming them to death, trapping them in plastic-lined trenches and letting them fry in the sun or spraying them with abrasive substances that wrecked their skin.

Then along came imidacloprid (Admire), the insecticide from Bayer CropScience. It was a miracle product, and beetles died in droves.

In the years that followed, the furrow-applied application method was augmented by seed treatments and foliar sprays. Related neonicotinoid compounds were discovered, and growers began to feel more comfortable.

Two university entomologists who carefully watch the Colorado potato beetle (CPB) see no great harm in growers feeling comfortable ¬â€“ as long as they don’t get complacent.

Ed Grafius, Michigan State University, and Galen Dively, University of Maryland, warned growers from the start that CPB has not changed its stripes. It’s a model insect” when it comes to its ability to consume toxins with impunity ¬â€“ and to develop resistance to new insecticides within years after they appear.

They knew it would happen with imidacloprid as well.

Grafius and Dively have been working together to monitor populations of CPB to detect shifts in their susceptibility to imidacloprid and to examine possible cross-resistance with other neonicotinoid insecticides and to spinosad, a different class of insecticide but one that acts at the same genetic receptor site.

Because, as they said, they knew the beetles would behave as they had in the past and would develop resistance, both Dively and Grafius began monitoring imidacloprid performance as soon at it hit the field, when beetles were 100 percent susceptible. At the start, Dively said, when exposed to spray concentrations of 0.4 parts per million of imidacloprid, half the beetles would die.

Now, they are finding populations in which the 50 percent lethal concentration (LC50) is 12 parts per million. That means it takes more than 30 times as much active ingredient as it once did to kill beetles.

Growers who are reporting some control problems are usually dealing with beetles that can tolerate up to 4 parts per million, 10 times what used to kill them.

Populations of these resistant beetles have been found in southern Maine, Delaware, Maryland, Michigan, Minnesota and Long Island, where resistance usually shows up first and did again with imidacloprid.

“Resistance can be found in areas throughout the Northeast as far west as Pennsylvania,” Grafius said. “We’ve found resistance on a farm in Michigan and we’re concerned about that situation.”

As has long been the case, resistance is not an issue in the West or in Florida.

“These areas have never had the insect pressure or the resistance problems,” Grafius said.

That means the West has fewer problems and more materials, since the beetles have never developed resistance of old standbys like Temik, Vydate and Disyston. Resistance is an issue up and down the East Coast and across the Midwest and the Red River Valley.

Besides using the LC50 measure of susceptibility, Dively looks at other resistance aspects. Typically, he said, “the first thing you see is a reduction in residual action, a slipping of control.”

While furrow-applied imidacloprid once gave 90 days of control nearly a full season as resistance develops, growers will start seeing live beetles in the field perhaps 30 days after planting.

How growers react to these beetles is important. Many times, they will make a foliar insecticide application. If they do that, he said, it is important they not choose to use imidacloprid again, exposing beetles with growing resistance to a second shot of the same chemical in the same season. Provado is the foliar version of Admire, Leverage is Admire plus Baythroid, and Assail and Actara also are neonicotinoids.

“In New England, repeated use of imidacloprid at planting has been a common practice,” Dively said.

In Delaware, where potato land is in short supply, growers are not able to adequately rotate land. Dively recommends that growers not grow potatoes after potatoes and move fields at least a quarter mile away. CPB are not strong fliers and they find potato fields by walking to them, he said. A quarter mile is a long hike for a beetle.

Growers in Delaware used imidacloprid (Admire) from 1995 on, but when resistance began to appear, many shifted to thiamethoxam (Platinum). That wasn’t a good decision either, Dively said, one that “probably exacerbated the problem.” Platinum is another neonicotinoid.

Molecule for molecule, thiamethoxam is five times as active as imidacloprid, but its labeled use rate is half, not one fifth. So, in making the shift, growers are effectively more than doubling the dose, Dively said. They may get more effective control, but not for the reasons they think. Resistance may develop faster.

Growers in Delaware are now rotating modes of action by using spinosad (SpinTor) and novaluron (Rimon).

In Maryland, Dively said, growers were using a very effective program in which they used in-furrow treatments of imidacloprid only on the perimeter of fields. They were creating a toxic barrier 100 to 150 feet wide on the sides and ends of fields while leaving some 70 percent of the interior untreated.

This was effective, he said, but growers get nervous when they see damage and some beetles did survive and do damage in the interior. They abandoned that approach, even though it was less costly and the damage was not economically significant.

On Long Island, CPB showed insensitivity to imidacloprid from the onset “due to a general predisposition (of beetles there) to resist toxic actions of all insecticides,” Dively said. Growers saw residual activity decline after only three years of use.

“Realizing the urgent need for resistance management, Long Island growers started to shift away from soil treatments in 1998 and began using foliar treatments of abamectin, cryocide and spinosad to alternate modes of action,” he said.

Grafius also researches control methods that rely less heavily on chemicals. Last year, he started a study to see if trap crops could be used to help manage resistant beetles. A 6-foot-wide crop of potatoes was planted around a 20-acre field in which resistance problems had occurred the year before. The main part of the field was planted to corn.

“The trap crop attracted about 100,000 beetles during late May and June, and the grower used non-neonicotinoid insecticides (SpinTor, Rimon and Agri-Mek) to achieve control at a modest cost,” Grafius said. “Only two acres need to be sprayed.

“The trap crop was disked repeatedly and destroyed before larvae matured and entered the soil for pupation. Very few beetles were found in the potato field (nearby).”

Grafius does not know how effective the method was. Beetle behavior is believed to be such that after emerging from the pupal stage, they walk to the nearest potato field and settle in, Grafius said. What proportion flew away or walked to a field further away wasn’t studied.

One additional piece of research Dively has run on resistant beetles is a “fitness” test. In general, when insects develop resistance to a chemical, there are “fitness costs” associated with that. They are usually less “fit to survive” than those chosen by natural selection.

This appears to be the case in imidacloprid resistance. The resistant beetles laid fewer eggs and survival over winter was slightly less, meaning that given time the beetles will probably revert to become a susceptible population, Dively said.

David Rogers, the product development manager for Bayer CropScience, said Bayer has supported the entomologists’ monitoring work but is, of course, not happy to see resistance to Admire. But Bayer’s financial interest is, he said, best served by helping growers make the right kinds of decisions so the product does not lose its overall usefulness.

He and representatives from seven other chemical companies serve on the neonicotinoid subcommittee of the Insecticide Resistance Action Committee (IRAC). IRAC is an industry group that provides guidelines on insecticide resistance management.

Formed in 2003, this subcommittee has developed resistance management recommendations for growers using the neonicotinoids (post-synapse acetylcholine receptor agonists) that include acetamiprid, clothianidin, dinotefuran, imidacloprid, thiacloprid and thiamethoxam. They are all assigned to the IRAC Mode of Action Classification Group 4A.

The guidelines say:

1. To reduce selection pressure, apply insecticides only when pest control is warranted, based on insect presence or historical pest pressure.

2. For pests with a high propensity for resistance development:
a. Limit the number of neonicotinoid applications per crop or per season.
b. Use only one long-lasting soil application per crop or per season. Soil applications include in-furrow, sidedress, drench and chemigation applications or as delivered by seed treatment.
c. If the crop has already been treated with a soil-applied neonicotinoid and additional insect control is necessary, use products with a different mode of action.
d. Minimize the number of generations exposed to the same chemistry, ideally a single generation. Do this by limiting the number of sequential foliar applications of neonicotinoids and alternate with effective labeled insecticides of a different mode of action in a “windows” approach. Windows of two to three neonicotinoid applications should be alternated with non-neonicotinoid applications providing similar length of control and removing neonicotinoid exposure to a similar number of generations.

3. Tank mixes: For resistance management, rotation between insecticides with different modes of action is considered a more sustainable practice than tank mixtures.

4. Integrated pest management (IPM): Applications of insecticides, including those containing neonicotinoids, should be integrated into an overall insect management program. Cultural practices known to reduce insect pest development should be followed.

5. Product performance: Determine the efficacy of insecticides used in the pest management program against the target pest. If a Group 4A mode of action insecticide (a neonicotinoid) appears to be less effective against an insect that it previously controlled or suppressed, contact the manufacturer representative, local Extension specialist or certified crop adviser.

The National Potato Council has developed similar resistance management recommendations specific to neonicotinoid insecticides available at www.nationalptoatocouncil.org or through state potato associations.

While growers and the companies involved have a stake in the continued good performance of neonicotinoid insecticides, the situation is not as dire as it appeared before 1995, when no new insecticides were appearing on the horizon.

This fall, Dively said, BASF will probably bring a new insecticide on the market that has shown itself to be effective against CPB. Tested under the name BAS-320, it is “very active,” he said. Foliar-applied, it appears to be a good alternative with a totally different mode of action, one that is still not well defined, he said. DuPont also is working with a new product.”

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