They live quietly in your fields for as long as two or three seasons, silently but slowly damaging your crops and undermining your farm’s profitability.
Their top hosts are grassy crops such as corn, but they’ve even been known to feed on potatoes, scarring the tubers with long feeding holes.
Wireworms damage grassy plants in several ways.Immediately after planting, they can either eat the seed or cut off seedlings below ground level. A few weeks later, after the plants have grown a bit more, they will tunnel into the underground portion of the stem and cause the plant to wilt and die. Even later, they’ll feed on roots, eating deep tunnels into them.
After a busy growing season chowing down on your profitability, the most mature larvae pupate into beetles in the soil during the late summer. The following summer the adult female will disperse hundreds of eggs throughout the field.
It’s a pernicious pest, and it might surprise you, because the last time it was a major pest problem, the Second World War was still recent news.
Wireworms are a group of related pests that are a growing source of concern for farmers across Canada, according to one research scientist.
Bob Vernon, based at AAFC’s Pacific Agri-Food Research Centre in Agassiz, B.C., has spent much of the past 15 years watching this once-banished pest make a comeback, for some interesting reasons.
This group of insects is thought to do better under zero- or minimum-till scenarios and continues to benefit from the fading impact of organochlorine insecticides such as DDT, banned in the 1970s. As populations bounce back, the wireworm issues of the 1920s and ’30s are threatening to re-emerge and Vernon says British Columbia was the first province to develop serious problems. He says modern crop rotations involve a lot of wireworms’ preferred food crops such as cereals and grasses, but chemical controls aren’t nearly as persistent or lethal.
“Wireworms are a very difficult insect group to control because you can’t see them and you’re looking at one generation every four or five years, which all may overlap in the field,” he says. “You also have between 20 and 30 economic species, and different species with different life cycles may be more susceptible to certain controls than others.”
This means that in a field, there can be up to five different species and many newer products may only control two or three. Vernon says in order to combat these multiple defences, he’s had to search for a silver bullet.
Past studies seemed to suggest there were a lot of products to work with when it came to wireworm control but because wireworms can’t be observed underground in the field, researchers didn’t know the wireworms weren’t being killed by the chemicals and only repelled, or sometimes induced into recoverable comas.
“If you put neonicotinoids, for example, on corn or wheat seed, for the most part when the wireworms come to feed on the seed, they get cold-cocked for a month or two and then they recover,” Vernon says. “If you go from cereal crops into, say, potatoes, the population in the cereal crop will carry over.”
Even now he says, they suspect the higher rate of neonicotinoids used on corn offer more mortalities — but they don’t know for sure. It’s not like the old days when organochlorines such as DDT killed and even gave effective control in following years.
Those products came very close to the silver bullet designation, but there’s nothing like them registered in Canada today. However, in the U.S., there is fipronil.
“What we found with fipronil, which is very interesting, is it kills wireworms dead,” says Vernon. “At higher doses it kills them very quickly but at extremely low doses, it doesn’t kill them right away, but does so maybe 200 days down the road.”
Based on what he has seen in the lab, Vernon’s idea is to knock out the wireworms with a low rate of neonicotinoids such as 10 grams of active ingredient per hectare or so, rather than the usual 30, and then add some fipronil for an ultimate kill strategy.
“So the wireworms might come out of their coma but you’ve preserved your crop because of the neonicotinoid and they might be able to survive for a little bit but eventually they die,” Vernon says. “The neat thing is that you kill the wireworms before the next crop even gets there.”
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Vernon says he has colleagues who are working on alternatives to his chemical control strategies. On the West Coast, Todd Kabaluk is studying fungal pathogens and his work is showing some promise for limited wireworm control in the field but it’s not as potent as the leading chemical controls. A considerable challenge has been that the effectiveness of the pathogen varies between wireworm species in some cases, shifting Kabaluk’s focus toward control of the adult beetles instead. On the East Coast, research scientist Christine Noronha, of Agriculture and Agrifood Canada’s Crops and Livestock Research Centre in Prince Edward Island, has been looking at different rotation crop options, and one which seems to work really well is brown mustard.
“If you plant it for two years before you plant potatoes, it’s giving really good control,” Noronha says. “The number of holes per tuber is way down to two holes per tuber compared to 20 holes per tuber associated with conventional barley rotations, and there was also a higher percent of tubers with no damage.”
As the problem continues to worsen and spread into previously unaffected parts of the island, Noronha has been recommending farmers grow brown mustard as a rotation crop in fields that already have wireworm issues. Noronha says she is also working to control wireworms in carrots, rutabagas and recently transplanted fields of cabbage.
“There’s a lot of grower interest in either using it for two falls or just one, depending on the populations,” Noronha says.
She also says they are still working to establish thresholds to help farmers determine if they can get away with only one year of cover because there is no financial gain by growing the crop. Of course, some farmers have explored the option of growing a full season for seed but risk facing it again the next year as a weed.
“Mustard is good in the fall because it’s quite resistant to the cold and it will flower, but the seeds don’t mature,” Noronha explains. “It’s actually better if you cut and cultivate it because the glucosinolates are a lot higher if you don’t let it go to seed.”
Glucosinolate is the chemical found in the mustard crop which reacts with an enzyme when plant material is crushed or damaged to produce the fumigant known as isothiocyanate. Noronha says brown mustard also has a secondary means of attack against wireworms, storing another type of toxic chemical in roots which it increases production of in the event of insect feeding. It is so deadly that given the choice, wireworms won’t touch the plant and that is why the cost can’t be offset by adding other seed for cover-cropping purposes.
“If all they have is mustard in the field, they have to eat it to survive, so I wouldn’t recommend it as a blend,” she says. “Growers say the mustard is kind of expensive but on the other hand, so is losing a good potato crop.”
With their last chemical control option due to become deregistered, farmers are willing to try unproven alternatives despite the cost. Vernon says a combination of cultural and chemical controls may be the ultimate solution eventually, but there are still too many unknowns to identify it. “A lot of work needs to be done with these alternative strategies,” he says. “Once you know whether an individual strategy is going to work, how it works and what species it works on, then you can start to think about integrating them.”
If fipronil doesn’t get registered in Canada, Vernon says they do have a system now to substitute future fipronil-like chemicals into their management strategies, because he says multinational companies are very aware of wireworms.