AF CONTRIBUTOR |LETHBRIDGE
Wireworms are one of the oldest and most common crop pests but remain strangers in our midst. Agriculture Canada entomologist Bob Vernon wants to change that – but he needs help from farmers.
Wireworm damage shows up as bare patches in newly emerged cereal and forage crops, as well as harming other seedling crops ranging from strawberry transplants to potatoes. A seed treatment that included the insecticide Lindane was so effective that it was only needed every three or four years. But that insecticide is no longer registered, and so alternative seed treatment may be needed every year.
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Wireworms are slim caterpillars, often reddish brown but sometimes yellow or white, with a fairly hard shell. The larvae of click beetles spend only a few weeks of their three-to five-year lifespans as adults
Mostoftheresearchthat hasbeendonehaslooked atspeciesotherthanthose we’vestudied.”
BOB VERNON
living above ground, which makes them difficult to study.
Click beetles lay their eggs around midsummer, a few inches below the soil surface. Larvae, called neonates, are around three millimetres long and grow in the soil for three to five years, reaching a length of about 38 millimetres before pupating. The adult beetles crawl out of the soil to mate above ground and lay eggs in the soil.
Wireworms are attracted by the CO2 seeds give off when germinating. They follow a CO2 trail and then head down the row, munching as they go. Lindane worked so well, says Vernon, because two-thirds of residents and 85 per cent of neonates died. The treated crop was protected and, with very few neonates growing into residents and reaching damaging size, there was no visible damage until a new crop of neonates grew to almost full size.
Vernon has been studying the impact of newer insecticides on various species of wireworms (there are many) in the lab. After feeding on seed treated with neonicotinoid insecticides, wireworms look dead. But under a microscope, it’s apparent they are just immobilized, with only their mandibles moving. They remain in this state for several weeks, but they revive and then, says Vernon, “they’re hungry as heck.” Even neonates are not killed by the treatment, so there’s no control of the population.
By that time, midsummer for spring-seeded crops, plants are big enough to withstand wireworm attack. For potatoes though, the problem isn’t that plants can’t withstand their feeding, it’s that the tubers are downgraded and wireworm feeding gives fungi and bacteria access to the potatoes. Wireworms that encounter seed treated with a synthetic pyrethroid are repelled by it but not killed.
Fipronil kills wireworms, but it’s a slow-acting toxin. At acceptable concentrations for a seed treatment, it takes about four weeks to kill wireworms. Vernon is hoping to get a combination of the neonicotinoiod, thimethoxam, and fiprinil approved for registration. That would be ideal, providing crop protection and high activity against wireworm populations.
The effectiveness of neonicotinoids fades over time, says Vernon. Conditions like cold soils don’t favour rapid germination and may leave the crop without protection. If you suspect high populations of wireworms, he advises seeding under ideal germination and emergence conditions to get good protection from a neonicotinoid seed treatment.
Vernon wants to know which of Canada’s 30 species of wireworm are most important in various regions. He’s asking farmers who collect significant numbers in bait balls to send them to him.
“Most of the research that has been done has looked at species other than those we’ve studied,” says Vernon. “And, insecticides work better on some species than on others.”
Catching the worm
Bait balls, at least 20 per quarter, can attract wireworms if they’re responsible for emergence problems. Wim Van Herk, Vernon’s assistant, puts out hundreds to collect wireworms every spring.
“Bury bait balls about six to eight inches deep, a week or two before seeding,” he advises. “Put them anywhere in the field except in low areas and use as many as you can and mark each with a flag.”
Van Herk uses four-inch pots that hold about a cup of soil, and puts a mixture of sawdust or vermiculite with wheat and soaks it for about 20 minutes before putting it in the pot. Leave the bait balls in the soil a week or 10 days, or longer if it’s cool. As the wheat germinates it will give off CO2 and attract wireworms. Van Herk takes an old dinner plate to check bait balls, poking them apart to find wireworms.
If you find significant numbers of wireworms, put them in pill bottles or similar containers with no bait but some soil. Only put two or three in a container – Prairie species are cannibals. Send them with details of field history and legal description to:
Dr.BobVernonAgricultureandAgri- FoodCanada.6947#7Hwy.P.O.Box 1000Agassiz,B.C.V0M1A0 1-604-796-1708 ( [email protected])
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“Insecticidesworkbetteronsomespeciesthanonothers.”
BOB VERNON
