by Rachael Cheverie
Wireworm – old pest, new problems.
We have seen a resurgence of wireworm populations in Nova Scotia (and surrounding provinces) in the last 10 years. It used to be that wireworm was only an issue in fields that had been in sod for many years and only for a year or two after they started being used for row crops. However, we are now seeing wireworm issues in many fields that are in regular crop rotation for a variety of field and horticulture crops.
The reasons for this pest’s resurgence may be attributed to a change in the species distribution to a more aggressive species (Agriotes sputator) and the deregulation of many older pesticides that had long half lives in the soil and were much more effective in controlling wireworms than many of the insecticides available today.
The wireworm is the larval form of the Click beetle (Coleoptera: Elateridae), and there are a number of species of agronomic importance found in Nova Scotia, mainly Agriotes sputator, Agriotes lineatus, and Agriotes obscurus.
Wireworms can affect a wide variety of crops, as they will feed on many different species in the spring and may cause significant stand reductions. Their damage tends to be even more severe in root and bulb vegetables in the late summer and fall, rendering them completely unmarketable.
Click beetles pupate in the soil in the spring and emerge to mate and lay eggs in the spring and early summer. Each female beetle can lay up to 200 eggs in the soil. Those larvae will hatch and grow for up to four years in the soil before pupating and emerging as adults to repeat the cycle.
At any given time, you may have one-year-old, two-year-old, three-year-old, or four-year-old larvae present in the soil, which makes it very difficult to use crop rotation as a method for managing this pest.
Wireworm larvae are attracted to carbon dioxide gradients in the soil, which come from actively growing roots or from decaying plant material.
It is important to recognize that plowing down green manure crops in the late fall or early spring may keep wireworm larvae busy in the soil early in the spring so that they go undetected in bait stations. This can also prevent them from reaching treated seed or soil-applied insecticides.
There are a number of methods for monitoring wireworm and adult Click beetles. Adults can be monitored using pheromone traps, pitfall traps, or the Noronha Elaterid Light Trap. The light trap is a new method developed by AAFC research scientist Christine Noronha in Charlottetown that may actually contribute to reducing the population of adult Click beetles.
You can use bait monitoring stations in fields where you suspect wireworm larvae are already present. This can be done the fall prior to planting or in the spring when the soil warms up to 5 to 10 degrees C. It works best if the field is currently fallow (with no green manure plow-down) so there is no competition for other food sources and the wireworms are drawn to the bait stations.
You can use a variety of baits. An easy one I have found to be successful is to dig a hole in the soil about six inches deep and cut up half a raw carrot into the hole, cover with soil, and mark the spot with a flag. Return in seven to 10 days, dig up the carrot and surrounding soil, and look for the presence of wireworms or feeding damage.
A good resource for monitoring wireworms published by the P.E.I. government can be found by visiting www.princeedwardisland.ca and searching “Wireworm and Click Beetle Fact Sheet.”
If you suspect you have wireworm issues in your field and are unsure of how to manage the problem, please feel free to reach out to Perennia for more information.
(Rachael Cheverie is a horticulture specialist with Perennia Food and Agriculture Inc. based in Bible Hill, N.S. She provides expertise and advice to cranberry, greenhouse, haskap, grape, and vegetable producers. Her particular strengths include pest management, site assessments for new plantings, and production information.)