Wireworms are formidable but manageable pests

Potatoes damaged by wireworms. (Christine Noronha photos)

(Above photo) Potato varieties being tested for susceptibility to wireworm in Canoe Cove, P.E.I.

by Emily Leeson
“If you have one wireworm, you have wireworms.” That’s the stark reality that Dr. Christine Noronha, an entomologist with Agriculture and Agri-Food Canada in Prince Edward Island, presented at the Kentville Research Station in Kentville, N.S., on May 12. Wireworms are the larvae of click beetles and they’re increasing in number worldwide.
“You can never get rid of them, but you can keep the population low enough that the damages are acceptable,” said Noronha.
Keeping that damage low is what she’s been researching and it’s likely research that will continue for a long time. She’s been at it for 14 years and there’s no end in sight. There’s just so little known about this pest — what it does, what it’s drawn to, and what will keep it away. 
Species in the Atlantic provinces are European in origin. And although not all click beetles are considered pests, the ones that are can do significant damage to a harvest. They affect a wide variety of crops and can specifically render root crops unmarketable. They feed on oats, wheat, barley, clover, corn, carrots, lettuce, onions, peas, potatoes, parsnips, and cabbage.
With five-year life cycles, the damage wireworms can do increases each year as they grow bigger. In their larvae form, they are yellowish-brown, 1-3 cm long, and unlike maggots or grubs they are hard-bodied.
The mature beetles lay 100-200 eggs and the larvae hatch mid-June, then feed on seedlings all summer. Into the fall, they feast on root crops, rising up from the lower depths of the soil to feed before returning below. They are photo-negative, avoiding the light by staying deep within the soil whenever they can. How far down they go, what they are doing, and what signals them to surface is still largely unknown.
Their long life cycle makes them a unique challenge to study and get rid of.
“Once you have a population in your field, you are going to continue to,” said Noronha. “It’s very hard to get rid of them. It takes 10 years to see significant numbers of adults. (An infestation) creeps up on you.”
Sod fields, particularly those that have not been cultivated in several years, appear to be particular hot spots for infestation. Infestations are common in under-seeded fields and pasturelands, but the adults may even lay eggs in bare soil.
While other insects can be tracked by their movements in relation to sunlight, wireworms are different because they exist primarily underground. They seem to be drawn toward moisture, they love sandy soil, and they move faster as the soil heats up in the spring and summer. Unfortunately, while a crop may fall victim to a dry year, wireworms appear to be able to withstand extended periods of drought. 
“The dry weather isn’t good for the crop but it doesn’t matter to the wireworms,” said Noronha.
The pests are aggressive survivalists that can weather even drastic seasonal change. They have no problem surviving the winter. In the lab, Noronha has frozen them and thawed them out, and found them “all OK.” She’s tried starving the larvae for a few months to see how they react, but they survived. In fact, they were fine. 
Less is known about the adult click beetles.
“We don’t even know what they eat,” said Noronha. “We think wildflowers. If I can find out what adults eat in the field, then maybe I can find something to attract them to a certain spot and then kill them there.”
There’s just not a lot known at this point. Wireworms are studied in the lab but it takes five years to study an adult, which is a significant length of time in terms of a lab study.
Determining a particular field’s wireworm population is the first step in addressing the problem. Noronha believes the best time to monitor crops for wireworm infestations is in the fall when they approach the soil surface to feast on crops and fatten up for hibernation.
Simple bait probes fashioned out of piping and filled with carrots can be set throughout a field. Wireworms have an aggregated distribution, meaning some areas of a field may have a large amount of them and other areas of the same field may not.
Determining wireworm populations in the fall allows producers to plan for the next season. At that point, Noronha said producers have to ask themselves: “What am I going to put in this field based on this population?” 
There are several tools available to producers to mitigate losses due to wireworms. There are insecticides available to help with wireworm control when planting. And in recent trials, a combination of insecticides and a new crop rotation model have garnered good results.
A producer Noronha has been working with has not rid himself of his farm’s wireworm infestation, but he has significantly reduced wireworm damage such that no crop has been entirely lost.
The strategy of planting buckwheat or Brown mustard as rotation crops is working to reduce wireworm damage in root crops. The glucosinolates found in these plants offer a natural defence against pests. When the plant is physically threatened, the glucosinolates combine with other released enzymes in the plant tissue to create a natural pesticide.
The new prescribed rotation involves planting either buckwheat or Brown mustard in June of the first year, discing the cover crop into the soil as it reaches the seed-forming stages, followed by a second planting in August. Those two plantings are repeated in the second year, and the main crop of root vegetables is planted in the third year.
Alternatively, producers can clip the first crop of mustard in August and allow the plants to continue to grow, monitoring for any seed production and clipping when necessary. In the case of the latter option, the wireworms feed on the living roots, and thereby instigate the production of the natural pesticide.  
While the crop rotation system seems to be working, there’s still much more understanding to grapple with. “We know it works,” Noronha said. “But how it’s working, why it’s working … to tease that out is really difficult.”