Wireworms, the larval stage of click beetles, are a common and destructive pest found in garden soil. They pose a significant threat to garden productivity, especially to crops that develop underground or rely on strong root systems. Wireworms feed on seeds, roots, and tubers, often causing plant death or rendering harvested produce unmarketable.
Identifying the Threat and Vulnerable Crops
Wireworms are slender, segmented larvae with a hard, shiny, yellowish-brown body that gives them a “wire-like” appearance. They range in size from one-quarter of an inch up to over one inch long, as their life cycle can last for several years in the soil. The adult click beetle is elongated and dark brown, named for its ability to snap its body to flip itself over when placed on its back.
The damage is often first observed as wilting or stunted seedlings failing to thrive in the spring. In root crops, they bore small, round holes into the surface of tubers or roots, creating narrow tunnels as they feed. Highly susceptible garden crops include:
- Potatoes
- Carrots
- Corn
- Sweet potatoes
- Onions
The larvae may also feed on the seeds of beans and peas, preventing germination.
Cultural and Preventive Soil Management
One of the most effective long-term strategies for managing wireworms involves modifying the soil environment to make it less hospitable. Wireworms are often attracted to areas recently in grass, sod, or pasture, as grass roots provide food and a preferred egg-laying site for adult click beetles. Avoiding planting susceptible crops directly following a grassy area helps reduce infestation risk.
Improving soil drainage is another preventative measure, as most wireworm species prefer moist, moderate-temperature soil conditions. Reducing soil moisture can discourage their presence. Intensive tilling during late spring and early summer can be highly effective because it exposes eggs and young larvae to desiccation and natural predators like birds. Tillage should specifically target the upper soil layers during warm, dry periods to disrupt the wireworms’ habitat.
Crop rotation is a simple but powerful tool, rotating highly susceptible crops with less susceptible ones, such as flax, mustard, or buckwheat. Planting a cover crop of brown or hot mustard and then incorporating it into the soil can act as a biofumigant, releasing natural compounds that suppress wireworm populations. These cultural practices focus on breaking the wireworm’s multi-year life cycle and reducing population density.
Active Non-Chemical Removal Methods
When an active wireworm infestation is detected, physical removal methods can significantly reduce their numbers without chemical treatments. A practical method involves using bait traps made from pieces of potato or carrot, which are highly attractive to the larvae. Cut a raw potato or carrot in half and bury the piece a few inches deep near affected plants, marking the spot.
The wireworms will be drawn to the starchy bait. The trap should be dug up and checked every few days, with collected larvae destroyed and the bait replaced. For small garden areas, soil solarization can be employed by covering the infested soil with a clear or black plastic sheet during the hottest part of the summer. This process traps solar energy, raising the soil temperature high enough to kill the wireworms and their eggs through heat and desiccation.
Another option for localized infestations is intentional soil flooding, which forces the wireworms to the surface for collection and removal. Saturating the soil for at least two weeks, particularly when soil temperatures are above 68°F, can reduce populations, as most wireworm species cannot tolerate prolonged submersion. This method is effective in well-draining soil where alternating flooding and drying cycles increase mortality.
Biological and Targeted Controls
Biological controls offer a low-impact, environmentally friendly approach by introducing natural enemies into the soil. Beneficial entomopathogenic nematodes (EPNs), specifically Heterorhabditis bacteriophora or certain Steinernema species, can be applied to target the larvae. These microscopic roundworms are watered into the soil, where they actively seek out and parasitizes the wireworms, releasing a lethal bacterium.
Successful application of nematodes requires the soil to be moist and the temperature to be above 50°F (10°C) for the organisms to remain active. Applying EPNs in the fall or spring when wireworms are closer to the soil surface can maximize their effectiveness. Targeted soil treatments, like planting biofumigant crops such as brown mustard, also suppress pest numbers. These plants naturally release compounds toxic to wireworms when the green material is chopped and incorporated into the soil.