Features Agronomy Crop Protection
A smarter way to use insecticides for wireworm control in potatoes

“We’re very excited about this delivery system for controlling wireworms,” says Bob Vernon, a research scientist with Agriculture and Agri-Food Canada (AAFC). He and his research team developed this method by drawing on their in-depth knowledge – gained through their many wireworm control studies – of when and how these soil-dwelling larvae feed.

In the spring, about 100 per cent of the wireworm population in a field rises to near the soil surface to feed on plants. Wireworms really like cereals and grasses, but if those aren’t available they’ll eat other crops. In the summer, the wireworms go deeper down to escape hot, dry weather. Then, in late summer, they rise to feed again. As winter arrives, they descend once more. In potato crops, this pattern means wireworms can attack both the mother tubers and daughter tubers. They tunnel into the tubers, reducing marketable yields.

“Wireworms find their host material by following carbon dioxide trails emitted by germinating or respiring plants in the soil,” Vernon explains. “For example, as a wheat seed germinates, it produces carbon dioxide. Since wheat is planted in distinct rows, you’ll get very nice carbon dioxide plumes coming from those rows, which will attract wireworms. Once they get to the carbon dioxide source, they’ll feed on the wheat seeds or roots. The same thing happens with potatoes: after planting, the mother tubers eventually start to germinate and produce carbon dioxide and respire, attracting wireworms.”

Vernon and his research team realized they might be able to use the wireworm’s food detection system against the pest. “The idea that we had going back about 16 years is that you could put a cereal crop such as wheat in with the potatoes, much like putting a granular insecticide with potatoes at planting. Wheat tends to germinate more rapidly than the mother tubers do, so pretty much all of the wireworms in the field will be attracted to the wheat seed and won’t be as inclined to go to the mother tubers. Now, if you put something lethal on the wheat seed, then you’ll kill most of the wireworms early in the season,” he says.

“Because you are drawing the wireworms right to the poison, you can use far less of it than you would, for example, using Thimet [phorate] or Capture [bifenthrin]. With Thimet or Capture, you’re expecting that the wireworms will encounter the insecticide by chance, so you have to use more of the insecticide and it has to be more spread out.”

Vernon’s group started working on this companion planting attract-and-kill approach in the early 2000s. They have tested virtually all of the available insecticides to see which ones would work best with this approach. The method requires an insecticide that kills wireworms; if the product just temporarily knocks out or repels wireworms, the pests might return later to attack the daughter tubers. Vernon notes, “At the present time, of the insecticides registered [for wireworm control in potatoes], Thimet will kill and chlorpyrifos [Pyrinex] will kill. Those are registered on potatoes as in-furrow granular or spray applications. They are not really available to be put onto cereal crops.”

So to demonstrate the attract-and-kill method, his team has used fipronil. This insecticide, with the product name Regent 4 SC, is registered in the United States for use on potatoes and corn.

Vernon’s studies show very low doses of fipronil are quite effective when used with the attract-and-kill method. “For example, we can get the same result as with Thimet 15G but with between about one and five grams of active ingredient fipronil per hectare,” he explains. “To put that in perspective, Thimet is used at 3,250 grams of active ingredient per hectare. So, we’re looking at up to about 3,000 times less active ingredient being put into the soil at planting using the cereal crop attract-and-kill approach with a product like fipronil. And the toxicity of fipronil relative to Thimet is about 100 times less.”

This method gives about 80 to 90 per cent wireworm kill early in the growing season. The small remaining population will cause much less damage to the daughter tubers.

Another positive aspect of this approach is that the risk of other insects and mammals being exposed to the insecticide is reduced. “The treated cereal seeds are about 15 centimetres below the ground surface, and the insecticide is not broadcast over a larger area,” Vernon says. “And if you start with one gram of active ingredient per hectare, then by the end of the summer only about 0.1 or 0.2 grams per hectare would remain in the soil.”

Field experiments show potato yields are very rarely affected by competition with the companion crop. Vernon notes, “We have been able to determine the minimum amount of wheat seed needed to kill wireworms and protect the daughter tubers from blemish damage. We know how much wheat to put in the rows and where to put it so the wheat will not interfere with the growth of the potatoes.”

According to Vernon, growers would need to make some fairly simple, low-cost modifications to their existing planting equipment in order to sprinkle the wheat seeds in with their potatoes.

The one obstacle to adoption of this method at present is that fipronil is not registered for use with potatoes in Canada. When
Vernon started working with fipronil, he was hopeful it would eventually be registered here, but that didn’t happen. However, his team is testing new insecticides every year, and he is “cautiously optimistic” they’ll find a product that can be slotted into the companion planting attract-and-kill approach.

Highlights from other wireworm studies
This attract-and-kill work was funded by various agencies over the years, and the research was recently completed as part of a major project on wireworm control strategies. That project is funded under Growing Forward 2 with the Canadian Horticultural Council, and runs from April 2013 to March 2018.

Vernon is the lead investigator on this national project. The collaborating researchers include his AAFC colleagues Christine Noronha, Todd Kabaluk and Ian Scott. The project’s six components are already making substantial progress.

One component, which is taking place in British Columbia and Prince Edward Island, is evaluating new insecticides, including products that are not yet registered, to see how well they control wireworms in potatoes. This component (and several of the other components in the project) are looking particularly at effects on the three introduced European wireworm species, which are causing significant problems in Prince Edward Island and British Columbia. Along with validating the effectiveness of Capture, the researchers have also identified other very promising products that could be candidates for registration.

Another component involves studies in British Columbia, Alberta and Prince Edward Island to assess the efficacy of various new insecticidal seed treatment products to control wireworms in cereal crops grown in rotation with potatoes. The researchers have found several proprietary products that look very promising. They are also testing sprays for killing click beetles – the adult stage of wireworms.

The third component is assessing several ways to use brown mustard for controlling wireworms in Prince Edward Island trials. This research has found that using mustard seed meal as a soil amendment is not practical for field-scale use. However, planting mustard between rows of potatoes shows promise. To improve control strategies, this component also includes a study to learn more about the biology of Agriotes sputator, the invasive European species that is wreaking havoc on Prince Edward Island.

In the fourth component, the researchers are developing a biological control method to attract and kill click beetles. They have invented pheromone granules that can be used to attract beetles to an application of Metarhizium spores. This fungus is highly lethal to click beetles, and the trials have achieved up to 95 per cent mortality. The researchers are working on various aspects to develop this method into a cost-effective, practical option for commercial use. The pheromone granules themselves might also have potential as a way to disrupt click beetle mating.

The fifth component involves development of a trap for monitoring wireworms, which uses carbon dioxide to attract the pests, and development of a method for monitoring carbon dioxide production. The researchers are testing different ways to improve the trap, and they are monitoring wireworms to predict feeding damage. They have also made an apparatus for measuring carbon dioxide production.

The project’s sixth component is the continuation of the national wireworm survey, which started in 2004. Wireworm species information is important because different species can have different responses to control measures. Canada has over 20 wireworm pest species. The species vary from region to region, and multiple species may occur in a single field. AAFC’s Wim van Herk is identifying specimens collected from farm fields across the country and mapping species distribution. Robert Hanner’s lab at the University of Guelph is sequencing the DNA of the specimens to enhance identification.