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Field trials of insecticide efficacy carried out in Prince Edward Island in 2014 provided data supporting Capture’s suppression of wireworm damage. Photo: Christine Noronha, AAFC.

What’s After Thimet?

Thimet’s phaseout puts the spotlight on Capture as a viable alternative for controlling wireworm in potatoes. Researchers, industry stakeholders and growers say it delivers.

There’s a growing sense of unease among Canadian potato producers as the countdown continues on the deregulation of Thimet 15-G. In the face of increased wireworm pressure across the country, especially in Prince Edward Island, growers aren’t the only group troubled by the phasing out of the only insecticide registered in Canada for use on potatoes that kills wireworms.

“Growers need a product that kills wireworms. They are concerned about it and we are concerned about it as well,” says Christine Noronha, a research scientist and pest control specialist with Agriculture and Agri-Food Canada.

However, last year’s eleventh-hour registration of bifenthrin, the active ingredient in FMC’s Capture insecticide, presented an opportunity for P.E.I. growers to test drive this new tool for suppressing wireworm damage of potatoes in large-scale field trials.

“We evaluated tubers from farmers who used Capture on their farms. It seemed to work. Overall, I think farmers were satisfied with it,” says Noronha.

Mark McMillan, FMC’s business manager for Eastern Canada, was also pleased with the results. “I believe it performed as expected. It gave similar control of wireworm damage to Thimet, the current standard product. There was statistically no difference between the two products’ performance with respect to wireworm damage suppression,” he says.

“The first season is always a concern because it’s a new product, a new application method, a new use pattern for growers — so it’s a learning curve. But I think after the first couple of years, growers will get used to the product and will use it in the most effective manner.”

However, due to bifenthrin’s late-spring registration last year, growers scrambled to carry out field trials, which made scientific evaluation difficult for researchers as some growers did not have time to establish control plots or assess wireworm pressure before applying the insecticide.

Despite these challenges, researchers were able to gather some scientific data, says Noronha. “There was one field where it was done properly, and it showed results. They had a check, they had Thimet and Capture. Capture seemed to work on that field.”

Although scientific data from large-scale field trials is limited at present, Capture has been tested and evaluated in Canada for many years in experimental plots with good results, says Noronha.

Tuber exhibiting wireworm damage. Photo: Bob Vernon, AAFC.

In addition, field trials of insecticide efficacy carried out in 2014 also provided data supporting Capture’s suppression of wireworm damage. For example, marketable yield was 26.9 metric tonnes per hectare from Capture-treated plots, 28 metric tonnes per hectare from Thimet-treated plots and 23.25 metric tonnes per hectare from untreated check plots (see Table 1). All tubers were evaluated based on processing industry standards.*

Of the seven insecticides evaluated in the field trials by Noronha’s team, only one other experimental product showed significant wireworm damage suppression in addition to Capture and Thimet. Marketable yield of plots treated with Ethoprop, an organophosphate insecticide not currently registered for use in Canada, was 26.6 metric tonnes per hectare.

Bifenthrin is a pyrethroid insecticide that acts by creating a protective zone around the mother and developing daughter tubers. Bifenthrin remains active in the soil for an extended period of time, protecting tubers for the growing season. The insecticide has been used for many years and on millions of acres in the United States and other countries.

According to McMillan, Capture’s use this season will expand dramatically from the approximately 5,000 acres put down in Prince Edward Island in 2014, he says, to include potato-growing areas in Western Canada, Ontario and Quebec.

Don’t Dribble

Meanwhile, whether or not bifenthrin will be the answer for wireworm damage control in 2016 and beyond is going to depend on producers, says Bob Vernon, who heads up AAFC’s national wireworm research project and is one of Canada’s leading wireworm experts.

“Bifenthrin may, in fact, work differently in its effectiveness to Thimet, in that bifenthrin has a large repellent component to it, which might be driving wireworms away from the planted rows of potatoes,” he says. “That opens up a certain amount of variability with bifenthrin use — farmers will be in control to a certain extent of the effectiveness of bifenthrin, more so than with Thimet.”

Dribble application of Thimet is well established and consistent from farmer to farmer. Bifenthrin, on the other hand, should not be dribbled in. It must be sprayed on both sides of the furrow at planting, and the variability in bifenthrin’s application — width of furrow, amount of water applied, coverage area — makes all the difference to its effectiveness.

“The wider the furrow at planting, and the better coverage of that larger, open furrow with bifenthrin, the better chance farmers will have of repelling wireworms from the area where the daughter tubers will ultimately be formed,” says Vernon.

The idea, he says, is to set up what is essentially a force field that is as wide as possible at the time of planting. Narrower furrows will result in smaller force fields, which the daughter tubers can grow outside of, and are then susceptible to wireworm damage.

“Give it a good shot this year. Apply the bifenthrin in a wide furrow with lots of water, spray on both sides and use that as your comparison with Thimet. If you’re not willing to apply it as prescribed, then don’t bother, because you’ll lose potatoes,” says Vernon.

Tuber containing mature larvae of the Pacific Coast wireworm, Limonius canus. Photo: Wim van Herk, AAFC.

Stunning Evidence

Although Capture protects marketable yield with comparable results to Thimet, one important question remains — does it kill wireworms? Laboratory results conducted by Noronha indicate Capture does not kill wireworms, but stuns them while they are in contact with the insecticide.

“Capture paralyzes them, and as soon as you take them out of the soil with Capture in it, they come back to life and start moving again pretty quickly,” she says.

“It does reduce damage for processing, it does increase marketable yield, so there is suppression of the number of holes and scars per tuber. It’s not necessarily controlling the population.”

As growers get acquainted with bifenthrin as new tool in their wireworm suppression arsenal, researchers will continue to test and evaluate new insecticide products in search of one that decreases wireworm populations.

“In the United States, farmers have options that kill wireworms. We’re testing them. It’s tricky because it depends on the species, or sometimes a chemical will work really well in one area but not in another, or even the same field. It will work against one species but not the other, and sometimes you have mixed populations in your field,” says Noronha.

“The chemicals need to be tested here to see if they will work against our species and under our climatic conditions and soil type.”

Canadian producers also need access to more than one control product for the rotation of insecticide chemicals in order to prevent resistance from developing.

Breeding for Less Susceptibility

Although the search for new chemical control products is a strong and vital focus, researchers are opening up new avenues of exploration for suppression of wireworm damage. Recently, Noronha and her team evaluated wireworm damage on 21 different potato varieties. No chemicals were used, so any damage suppression would be directly related to the variety.

The results indicated a level of innate suppression at work within certain varieties. For example, chipping varieties, which averaged eight holes per tuber, fared better than Norland, Eva, AC Chaleur and Shepody, which averaged between 23 and 25 holes per tuber. Russet Burbank had roughly 18 holes per tuber while Goldrush and Atlantic averaged 12 holes per tuber in the tests.

“There’s something going on there,” says Noronha. “The varieties themselves are showing some level of less susceptibility. This information can be given to a breeder and perhaps a variety like Russets can be bred to have traits in it that will help it be less susceptible to wireworm damage.”

Put the Integrated Back in IPM

In the short term, wireworm damage will likely continue to trouble Canadian potato growers, and as far as Noronha is concerned, dependence on one control mechanism is not the way to go.

“Growers are going to have to change the way they do things. They are going to have to manage this insect,” says Noronha.

“We’re so used to using one technique and it works, but we have to put the integrated back into integrated pest management. Not just monitoring and spraying — that is not the option with these insects. You really need to integrate all techniques and manage them.”

In addition to chemical control, techniques such as crop rotation (with crops such as brown mustard and buckwheat) and ploughing down green material in the fall (as opposed to the spring when this material will attract and keep wireworms below the soil surface) will help keep wireworm populations in check.

“We’re trying to put as many tools as we can in the toolbox. Right now, we have chemicals, we have rotation and we have plough/no plough, but we’re continuing to look at the biology of this insect to see what else we can do to control it and get the population down,” says Noronha.

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