Wireworms

Wireworms are the soil-dwelling larvae of click beetles. Several wireworm species occur in western potato soils, but the most common are the Pacific coast wireworm, the sugarbeet wireworm, and the dryland wireworm. Several species of the genus Conoderus have also been encountered in some production areas.

Adult wireworms are slender, reddish brown to black click beetles that are 0.25 to 0.5 inch long. The larvae are wirelike, having hard bodies that are slender, cylindrical, yellowish to brown in color, and about 0.75 inch long when full grown. Common wireworm species require 3 to 4 years to complete their life cycle. Most of the time is spent in the larval stage, but all stages may be present at once during the growing season.

Adults do not damage potatoes, but the larvae, or wireworms, may damage seed pieces and young root systems during stand establishment, resulting in poor stands. More commonly the damage is seen as shallow to deep holes in the potatoes, caused by wireworms burrowing into the tuber while feeding. Wireworms bore perpendicularly or diagonally to depths up to 0.5 inch, but do not tunnel all the way through the tuber.

The best time to manage wireworms is before planting. Check for wireworms by observing the field during plowing or discing, or by baiting. If wireworms are present, monitor by taking soil samples to determine the need to treat.

In recent years, wireworms have been most common in the northern mountain areas in fields that have been in weedy alfalfa or pasture for several years before potatoes. Avoid planting potatoes in fields immediately following clover, grass, pasture, or weedy alfalfa. Summer fallow will reduce wireworm numbers by drying the soil.

Use cultural controls on an organically certified potato.

The most direct way to detect wireworms in a field is by general observation during plowing or discing of a field, particularly where old alfalfa, clover, or pasture is being taken out. Wireworms can also be detected by baiting, using carrots, packets of untreated corn and/or wheat seed, or ground whole wheat flour, provided they are used when soil temperatures are 50°F at 4 to 6 inches deep. Baiting does not give a good estimation of the density of the population. If baiting shows the presence of wireworms, take soil samples to estimate the wireworm density. Use a 6-inch post hole digger and a shaker/sifter to sample. Take samples in spring when soil temperatures are 45°F or higher at the 6-inch level or in late summer at the 18-inch level. The following guide is used in some production areas.

Preplant broadcast treatments have provided adequate control in limited field tests conducted in California. Band treatments are used in some areas, but have not been evaluated under California conditions. In areas where potatoes are planted in late fall and winter, soil-applied insecticides tend to break down before wireworms become active when the soil warms in spring.