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Profit thieves

A persistent and insidious crop pest could be chewing away at your profits year after year


What’s three-quarters of an inch long, hard-bodied, yellowish brown, lives in the soil, and may be secretly chewing away at your profits?

Wireworms… even their name is descriptive. Wireworms are the larval stage of the adult click beetle, and while the beetles do no damage, the same can’t be said about their offspring. The click beetles lay eggs around the roots of grasses and grains and the emerging wireworm larvae then feast on seeds and seedlings by boring into stems and roots and by tunneling into tubers such as potatoes. Crops attacked by wireworms have reduced plant populations and just don’t seem to thrive, since the wireworms continue to feed upon the small roots of many plants throughout the season.

Wireworms are relatively easy to identify. They are darker than most maggots or grubs you usually see in the soil and tend to have hard, tan to orange bodies. While there are over 800 species of wireworm worldwide, only 30 of these are pests in Canada

What makes wireworms a particularly difficult problem is their persistence in the soil. The larvae can thrive in the same field for two to six years, and can overwinter even in our Canadian soils. Wireworms move up and down in the soil profile as soil temperature and moisture changes. During the spring growing season, the wireworms migrate into the top few inches once soil temperatures reach 10 C, and remain there until it gets too hot. Once it gets really hot (>25 C) or if the soil becomes dry, they burrow deep into the soil.

Wireworms are typically an issue in fields that have been planted to, or are coming out of, grasses, cereals, or grassy pasture. Since they live up to six years in the soil, they can be an issue for any crop planted after a grassy crop. Crops that are potentially affected by wireworms include small grains such as wheat and barley, clover, corn, potatoes and other root crops, and various vegetable crops. Heavy, wet soils also seem to be the preferred home for wireworms and damage is often worst in cool, wet spring weather.

From the Grainews website: Cereal, pulse treatment cleared for on-farm application

How do you know if you have a wireworm problem? You have to look, and there are a couple of recommended ways to accomplish this. The first involves taking soil samples six inches (15 cm) deep from several spots across the field. Sift the soil to reveal the wireworms. The best time to do this is when soil temperatures have warmed to approximately 10 C in the spring. My research indicates that one wireworm per shovel of soil means there is a population of more than 20,000 wireworms per acre. More than two wireworms per 10 shovels of soil means treatment may provide an economic return.

Another method is to dig several holes on the diagonal across the field and place a carrot or freshly cut potato about four inches (10 cm) deep. Cover the hole with soil, mark the spot and dig it up about three days later. If one or two wireworms are found per sample, you have a problem.

What are your options to control these nasty little worms? There is nothing registered to control wireworms once the crop is out of the ground, so you need to think about either treating the seed or the soil where the seed is placed. I suspect that most will want to treat the seed rather than the soil, although there are five soil treatments available. If you prefer to attack wireworms with a seed treatment, you have what might seem like a wide choice of 16 wireworm seed treatment products available, although every one of these is a Group 4 (neonicotinoid) product. That’s not a bad thing, since the neonics are very effective, but it does shed a spotlight on just how vulnerable we are if the concern for bee mortality results in restrictions or a ban on neonics.

The bottom line is that like many small critters in the soil, wireworms can easily go unnoticed since they rarely destroy an entire crop, but rather steal yield and profit in a more insidious manner.

As long as neonics continue to be available as seed treatments, there is no need to allow these hungry little worms to eat your profits. For my money, unless I was sure that I did not have a wireworm problem, I would include a neonic insecticide as part of my fungicidal seed treatment program. It’s easy to do and effective not only on the wireworms, but also on several other soil insects that like to dine on your crop, and your wallet.

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