As crop threats go, wireworm is one of the most difficult to predict. Its fickle behaviour, from feeding choices and patterns to life-span, makes it near-impossible to determine populations or geographic distribution. What we do know is that wireworm problems are increasing.
Wireworms are among the most significant early-season stressors for many crops in Western Canada, posing a threat to early stand establishment. They seem to prefer cereal crops, but also have a taste for pulse crops and canola.
What makes these tricky little worms so difficult to understand? To begin with, there are a lot of them. There are more than 30 species of wireworm, all similar in appearance and behaviour. Correct identification can be difficult, but is important to building a control strategy.
Wireworms are the larval form of click beetles. They can live in the soil for several years, seemingly indifferent to adverse conditions. This makes them very hard to spot and harder to manage. Adult click beetles overwinter in the soil, emerging in the early spring to lay between two and 400 eggs by early June. Larvae hatch within a three- to seven-week period and can live for three to five years by feeding on roots or germinating seed.
The larvae are usually reddish-brown, although sometimes white or yellow. They are slender with hard, smooth, jointed bodies and three pairs of legs behind the head. Larvae go through a pupation period of about one month, the white pupae contained in earthen cells in the soil. Full-grown larvae vary in size from 1/2 to 1-1/2 inches. They emerge as adult click beetles, ready to overwinter in the soil and perpetuate the cycle. Adults range from one-quarter to one-half inch in length, black-brown in colour and hard-shelled. The adult click beetle does not damage crops.
“Not all wireworms cause serious problems,” says Shane Thomas, agronomist for GMac’s Ag Team at Kindersley, Sask. “Out of about 30 species, just a few of them can be economically significant. The problem is that if you have more than one species, it’s likely to include the one that can hurt you. It’s best to be prepared.”
The wireworm’s ability to survive in the soil over several years means an infested field can host several generations at different life cycles at one time.
As the soil warms, wireworms migrate to within a few inches of the surface to feed on fledgling plants. Hot, dry weather will drive them into the soil, as deep as three feet. Damp, cool weather pulls them to the surface; colder weather drives them back down.
Soil conditions that favour wireworms are diverse and variable. Cereal crops are susceptible, as is recently broken sod. Well-drained, silty soil is very attractive, and wireworms love the safe haven of summer fallow, with higher levels. Soil with high levels of carbon dioxide as organic matter decomposes, as in heavily tilled soil, attracts wireworms to the surface. Anything that has been tilled for 10 years or more will see higher levels of damage.
“Lindane was the only treatment that actually killed them. Following the Lindane ban, other products came along that will suppress them for a few weeks, allowing the crop to advance beyond the vulnerable stage. You’ll never totally get rid of them, but the goal is to keep them under control until your crop gets past them.”
Feeding activity is influenced by availability. Their love for carbon dioxide released by germinating seeds pulls them to the surface to feed just as crops are emerging. They will move back down into the soil in times of poor food supply, surviving for up to two years.
Their underground nature can make it hard to identify wireworm damage. It is often mistaken for dry conditions, cutworm damage or a miss with the seeder. Wireworms feed on germinating seed, roots and seedlings, killing some plants and damaging others and making them vulnerable to disease.
Some signs include wilted discoloured plants that still have a root, stems that are shredded, but not severed and dead seedlings or hollowed-out seeds. Beware of row sections missing in stands that appear healthy in all other respects, because wireworms have a tendency to feed along crop rows. Thin stands or plants with green outer leaves but dead central leaves might also signal wireworm damage.
Waiting to spot the damage in the emerging crop may indicate you’re too late to attack the problem.
“Seeding good quality seed with high germination, maybe increasing the seeding rate, that’s a good line of defence,” says Thomas. “Treat the seed, and focus on quick, even emergence. The faster that crop can get a good start, the less damage will be done.”
Some feeding is required for the worms to ingest a lethal dose of insecticide, so some feeding damage will be evident as the treatment does its work.
If you suspect populations, dig into the soil to look for them. They’ll move quickly away from the light, but if soil temperatures and germinating seed have attracted them to the surface, you’re likely to spot them by digging at the right time.
Bait balls are useful for assessing populations. Mix one to one and a half cups of oatmeal or wheat flour with two tablespoons of honey and another half cup of water until the mixture forms a ball. Put the bait balls into the ground directly, or in an old sock or mesh bag.
Bury the bait balls four to six inches deep, and be sure to mark the spot so you can retrieve them later. It will take about a week for the balls to decompose and create the carbon dioxide that will attract the wireworms. Check back in seven to 10 days to see what you’ve got. To get a good read, you’ll need about 20 bait balls, evenly spaced over an acre.
Once in a field, wireworms are there to stay. But they can be controlled. †