University of Illinois

No. 2/April 4, 1997

The Standard Wireworm Article

There's really nothing new under the sun for wireworm control. However, some growers may be putting former CRP land back into production, so some concern about wireworms exists. Usually fewer than 1% of the corn acres in Illinois are affected by wireworms each year. However, land that has not been in crop production for several years could harbor significant numbers of wireworms and a few other critters.

Wireworm larvae are cylindrical, obviously segmented, and wirelike in appearance. They range in size from 1/3 to 1/2 inch long and are usually hard-bodied and yellow to brown, although some species are soft-bodied and white. Their three pairs of legs are short, and the last abdominal segment is either notched or conelike. Adults are called click beetles because they make an audible click when they flip their bodies into the air, attempting to right themselves when turned upside down. They are hard-shelled and streamlined in appearance (tapered toward both ends); and they are brown, black, or gray.

Click beetles become active in the spring. The adults live 10 to 12 months, although most of their time is spent in the soil. Female click beetles burrow into the soil to lay eggs, primarily around the roots of grasses. The larvae live in the soil for 2 to 6 years, depending upon the species. Generations of the long-lived larvae overlap, so wireworms of all sizes and ages can be found in the soil at the same time. Larvae are most active early in the spring when soils are relatively cool. As the summer progresses and soil becomes hot and dry, wireworms descend in the soil.

Wireworms cause early season injury to corn by boring into seeds before or during germination, preventing germination or killing seedlings before they emerge. Larvae also tunnel into the base of plants below the soil surface, injuring or killing young plants. Wireworm injury is often most severe in corn planted after longstanding meadows, pastures, small grains, and other forage crops. Corn fields with chronic infestations left uncontrolled for several years also may suffer heavy damage.

Attempts to control wireworms with an insecticide rescue treatment after the damage appears are not successful. Therefore, management strategies must be implemented before or at planting time. Wireworms are difficult to detect because their damaging stage (larva) occurs below ground. However, a baiting system has been developed to evaluate the potential for a wireworm infestation before planting time. Entomologists at the University of Missouri recommend placing two bait stations per acre 2 to 3 weeks before planting. However, most practitioners use fewer bait stations (5 to 10 bait stations per field may be practical). Regardless of the number of bait stations established, be sure that the baiting program adequately represents all areas of the field.

Follow this procedure for baiting:
1. Dig a hole about 2 to 4 inches deep and 6 to 9 inches wide at the soil surface.

2. Place cup of an equal mixture of untreated corn and wheat seeds into the hole; fill and slightly mound with soil.

3. Cover the mound with an 18-inch square of black plastic; then cover this sheet with a 3-foot square of clear plastic. The plastic collects solar heat and speeds germination of the corn and wheat, which attracts overwintering wireworms. Cover the edges of the plastic with soil to prevent wind from blowing the plastic away.

4. Mark each station with a flag or stake. A cross-section of am wireworm solar bait station is shown in Figure 2.

5. A few days before planting, remove the plastic and soil covering the bait, and count the number of wireworms found within and around the bait.

If you detect an average of one wireworm per bait station, an economic infestation is likely. In some instances, several wireworms may be found in one bait station and none in others. Wireworm infestations tend to concentrate in some locations. It may be possible to limit treatment to areas where the concentration of wireworms is heaviest.

A lindane or lindane + diazinon planter-box seed treatment may deter wireworms from attacking the seeds but will not protect the seedlings. Heavy infestations justify application of an insecticide at planting time. The following insecticides are labeled for control of wireworms in corn: *Aztec 2.1G (6.7 oz) in a band or in furrow; *Counter CR (6 oz) in a band or in furrow; *Force 1.5G (8 to 10 oz) or *Force 3 G (4 to 5 oz) in furrow; *Fortress 2.5G (6 oz) or *Fortress 5G (3 oz) in furrow; *Furadan 4F (2.5 fl oz) in furrow; and *Thimet 15G (8 oz) or *Thimet 20G (6 oz) in a band. The rate (oz) in parentheses indicates the amount of product per 1,000 feet of row. An asterisk (*) indicates a restricted-use insecticide. In-furrow application of products labeled for in-furrow placement usually is most effective. Lorsban 4E can be broadcast and incorporated before planting at a rate of 4 pt per acre.

Kevin Steffey, Extension Entomology, (217)333-6652