The primary organism targeted by control measures is the wireworm, which is the larval stage of the click beetle. The adult click beetle is a slender, dark-colored insect named for its ability to snap its body to flip itself over. Adults generally cause little economic damage to plants, but females lay hundreds of eggs in the soil, often in grassy areas, which hatch into the destructive larvae. Wireworms are hard-bodied, cylindrical larvae that can live in the soil for two to six years, continuously feeding on seeds, roots, and tubers. Control strategies must focus on eliminating these subterranean larvae and making the environment unsuitable for adult egg-laying.
Cultural and Environmental Management
Killing wireworms begins with modifying the soil environment to disrupt their life cycle and expose them to natural threats. One effective long-term control is incorporating a comprehensive crop rotation plan. Wireworms thrive in fields previously planted with grasses or continuous cereal crops, so alternating these with non-host crops like legumes or mustards can starve the population over time.
Changing soil conditions can also make the habitat inhospitable to the pests. Intensive, repeated tillage during late spring and early summer can physically injure larvae and expose them to predatory birds and sunlight. For areas with the right soil type, deliberately flooding the soil for two weeks or more when temperatures are warm can significantly reduce wireworm numbers. Reducing excessive soil moisture through drainage improvement is also important, as many wireworm species prefer damp conditions.
Targeted Non-Toxic Control Methods
For active intervention, several non-chemical methods exist that directly kill or capture the pests. Simple bait traps are an effective way to monitor the population and remove small numbers of wireworms before planting. This involves burying slices of raw potato or a mixture of soaked corn or flour four to six inches deep, which attracts the larvae as they search for food. Checking these traps every few days allows for manual collection and disposal before they can damage a new crop.
Introducing beneficial organisms into the soil is a targeted approach that avoids synthetic chemicals. Certain species of entomopathogenic nematodes, specifically Steinernema carpocapsae and Heterorhabditis bacteriophora, actively seek out and parasitize wireworms. These microscopic roundworms enter the larvae and release symbiotic bacteria, which multiply and cause the host’s death within days. Successful application requires the soil to be moist and the treatment timed for when wireworms are near the soil surface, typically in the early spring or late summer.
Another physical control method involves the application of Diatomaceous Earth (DE), a finely ground powder made from the fossilized remains of ancient aquatic organisms. This powder kills by mechanical action rather than chemical toxicity. The microscopic, abrasive edges of the DE particles scratch the wireworm’s waxy outer layer (cuticle) when they crawl over it. These abrasions allow the DE to absorb the fats and moisture from the insect’s body, leading to fatal dehydration. However, DE’s effectiveness is reduced in wet or highly humid conditions, as moisture negates the drying action.
Chemical Insecticide Applications
When wireworm populations are high and non-toxic methods have not been sufficient, chemical insecticides may be necessary, often applied preventatively. Most insecticides for wireworm control are designed to be applied to the soil before or at the time of planting, as post-emergence treatments are ineffective against the subterranean pests. Products typically come in granular or liquid formulations that are incorporated into the top layer of soil where the larvae feed on seeds and roots.
Common active ingredients available to homeowners for soil treatment include compounds like bifenthrin or pyrethrins, which are contact insecticides. These applications are intended to protect seeds and seedlings during their most vulnerable stage. It is necessary to follow all label instructions precisely, paying attention to application rates and safety warnings. Many effective products are restricted-use and intended only for commercial agriculture, so home gardeners must confirm that a product is explicitly labeled for residential use on food crops. Over-application or improper use can pose environmental risks, including toxicity to beneficial soil organisms and local waterways.