Wireworms are the larval stage of click beetles and are persistent pests for potato growers. These soil-dwelling creatures can spend years underground, causing damage that severely reduces the quality and marketability of potato crops. Successfully getting rid of them requires a combination of immediate strategies and careful, multi-season soil management.
Identifying Wireworms and Characteristic Damage
The wireworm is a slender, cylindrical larva that looks somewhat like a piece of stiff, segmented wire, giving it its name. They are yellowish-brown to tan, possess a hard shell, and can grow up to 1.5 inches in length. A single generation spends between two and six years feeding in the soil before pupating into an adult click beetle.
The most noticeable sign of an infestation is the specific damage caused to developing potato tubers. Wireworms bore small, clean, circular entry holes into the potato’s surface, typically no wider than a pencil lead. This entry leads to tunnels throughout the tuber’s flesh, destroying its internal structure and allowing for secondary fungal or bacterial infections. This damage is distinct from the larger, shallower cavities left by slugs.
Mechanical and Cultural Control Strategies
Immediate, hands-on methods can significantly reduce the wireworm population during the current growing season. Pre-plant bait trapping is an effective technique that takes advantage of the pests’ attraction to concentrated food sources. Bury untreated pieces of carrot or potato, skewered on a small stick for easy retrieval, a few inches deep about two weeks before planting.
Dig up these baits every few days, collect and destroy the wireworms gathered on them, and then re-bait the area. This process monitors the infestation level and manually removes a large number of larvae. Tilling the soil prior to planting also helps, as mechanical cultivation injures the larvae and exposes them to natural predators, such as birds, and to desiccation.
Wireworms move up and down in the soil profile in response to moisture and temperature changes. Intentional flooding of the planting area for a short period before the season can drown or force the larvae to the surface for removal. Proper sanitation is another step, requiring the prompt removal and destruction of all crop debris, especially infected tubers, immediately after harvest to eliminate a food source.
Long-Term Soil Management and Prevention
The long life cycle of the wireworm means that a sustained, multi-year approach to soil management is necessary for lasting control. Crop rotation is a primary preventative measure, particularly avoiding planting potatoes after grass, sod, or grains like wheat and barley, which are favored egg-laying sites. Instead, rotate potatoes with non-host crops such as onions, buckwheat, or certain mustards, which have biofumigant properties that suppress wireworm populations.
Adjusting the soil environment can also make the area less hospitable to the pest. Wireworms thrive in acidic conditions, so gradually raising the soil pH above 6.0 through the addition of lime can help deter them over time. Incorporating nitrogen-rich organic amendments, like blood meal, into the soil is another method, as the decomposition byproducts are toxic to the larvae.
Timing the potato crop’s growth to avoid the pests’ peak feeding periods is an effective preventative strategy. Wireworms are most active and damaging in the late summer and early fall when they seek tubers for moisture, especially in dry conditions. Planting early-maturing varieties or simply lifting the crop earlier in the season before the main feeding period can significantly reduce the damage seen at harvest.
Biological controls offer a sustainable option, utilizing beneficial entomopathogenic nematodes. Specific species, such as Steinernema feltiae or Heterorhabditis bacteriophora, are applied as a soil drench, actively seeking and infecting the larvae with a lethal bacteria. These microscopic roundworms are safe for plants and humans but require ample soil moisture and the correct temperature range for successful application.