Wireworms include the larvae of several species of click beetles (beetles that flip into the air with an audible click when turned upside down). These larvae feed primarily on grasses and grass crops such as corn. However, they have a broad host range that includes beans, beets, cabbage, carrots, ginseng, lettuce, onion, peas, potatoes, radishes, as well as herbaceous ornamentals.

Appearance

Wireworms are thin, shiny, jointed yellow to reddish-brown, worm-like larvae. They range in length from ¼ – 1½-inches and are approximately 1/8-inch wide. Different species are distinguished by the ornamentation on the last segment of their bodies. Wireworm larvae can be distinguished from ground beetle larvae by a lack of curved mandibles and extra appendages at the end of the abdomen, which are present on ground beetle larvae. Adults are hard-shelled, brown or black “streamlined” beetles that right themselves from an overturned position with a clicking sound.

Symptoms and effects

Only the larval stage of click beetles can cause damage. The wireworm feeds on seeds, preventing germination, or on the underground roots and stems of the plants, causing them to wilt and their growth. Dead spots scattered throughout a planting may indicate wireworm activity. If you dig up the seedlings in affected areas, you will find them riddled with holes. Larvae may also be found feeding on the roots of wilted plants. Wireworms tend to cause the most damage 1-4 years after plowing up sod in poorly drained lowlands, but they are not exclusive to those areas. Wireworms can ruin the tubers of potatoes by burrowing small, round tunnels.

Life cycle

Wireworms have an extended life cycle, taking from 1-6 years to complete 1 generation. They overwinter as either adults or larvae. Larvae inhabit the upper 6 inches of soil where they migrate only short distances and feed on seeds and plant roots. Larvae are sensitive to moisture and prefer moist, cool, heavy soil. They may burrow deeply into the soil in dry conditions.

Adults become active in spring and begin laying eggs. Adult females live 10-12 months, spending most of this time in the soil where they may lay up to 100 eggs in soil and grassy weed infestations in row crops. Eggs hatch over a period of several days to several weeks. The tiny larvae immediately begin to feed on the roots of grasses, weeds, and other crops. Because of their extended life cycle, larvae of some species will feed for two to three years before pupating. Adults that emerge from these pupae remain the pupal chambers until the following spring.

Scouting

Scheduled scouting is not recommended, and no thresholds have been developed for wireworms. If you suspect wireworm damage, dig up several ungerminated seeds or damaged plants along a 4- to 6-inch core of surrounding soil. Check for wireworms in and around the roots, or in the underground portion of stems.

Bait stations buried in fall or spring are another way to check for wireworms. One bait method involves soaking untreated corn and wheat seed in water for 24 hours to encourage germination. Seed is then placed into ½-foot deep holes in fields that are several inches wide. Cover the seed with dirt, and a black plastic bag, topped with clear a plastic bag. Germination is sped up by the plastic which will enhance solar radiation. Place some soil around the corners of the top bag to ensure its stability. Dig up after one week. At least ten bait traps per field are needed to provide an accurate estimate of wireworms. Control is recommended if there is an average of 1 or more wireworm per trap.

Management

Cultural control

Clean cultivation and crop rotations that avoid susceptible crops may reduce wireworm numbers. Some species of wireworms thrive in poorly drained soil and can be controlled by improving drainage. Clean summer fallowing of infested fields has been effective in some areas. Seeds are more susceptible if to wireworm damage if they take longer to grow. As a result, planting in warmer soil can speed up germination and reduce the amount of time that wireworms can cause damage.

Chemical control

Insecticides registered for wireworms are rarely recommended, since outbreaks are infrequent. If treatment is necessary, make applications at the time of planting and incorporate them into the soil prior to planting.

Refer the the UW-Extension publication Commercial Vegetable Production in Wisconsin (A3422) for recommended insecticides and management practices.