Make wireworm decisions before planting

Wireworms, the immature stage of a click beetle, damage corn plantings and cause localized stand loss during the seedling stage. They range in length from 1/4 to 1 1/2 inches and are found most often in grasslands or fields with a recent history of sod.

Wireworm and damage to a corn seed.
The click beetle (adult stage of wireworm) does not damage corn.

This soil insect likely will have increased significance as more land, set aside as part of the Conservation Reserve Program (CRP), returns to corn production. The presence of this pest in Iowa was recently demonstrated by a study that found wireworms in 45 percent of 89 Iowa CRP (grassland) fields sampled. The study also showed that wireworms are more likely to occupy well-drained soils commonly found on ridgetops or hillsides. Wireworms can spend several years in the immature stage during which they feed on newly-planted corn seeds as well as roots of established corn. There is no evidence that wireworms reduce yield in soybean. Wireworm damage usually is detected after the opportunity for making preventive pest management choices is past, so scouting for this pest prior to planting would be a wise decision.

Scout for wireworms with a bait trap at least one week before planting. Wireworms are more likely to be near the soil surface as the soil temperature increases in the spring, so scouting close to planting will increase the likelihood of finding wireworms. To scout, place 1/2 cup of a 1:1 corn/wheat seed mixture in a hole that is 4 inches wide and 10 inches deep, and cover with soil. The seed mixture should be soaked in water 24 hours before use to facilitate germination. Then cover the soil over the bait with a black plastic trash bag to warm the surface and speed germination. Be sure to cover the edges of the bag with soil to prevent wind from blowing away the plastic.

Four traps per acre have been shown to be a reliable sampling plan for wireworms, but this trap density may be unrealistic because of the time and effort involved in the scouting process. As a compromise, at least 10 traps should be used in each field. Collect the samples after one week, count the number of wireworms in each bait and determine the average.

You have several pest management options if you find wireworms in the traps. If you find more than one wireworm per trap, there's a high probability you'll experience damage to corn seeds and seedlings from this pest. Use a granular soil insecticide with corn at planting, placing the insecticide in the furrow, or rotate to a nonhost crop such as soybeans. If you find less than one wireworm per trap, the probability of wireworm damage is low but still possible. A seed treatment containing both diazinon and lindane should be used to protect corn seed in this situation, but seed treatments protect only the seed and will not protect the seedling. If no wireworms are found, the probability of wireworm damage is very low, but still possible; wireworms may be present but for some reason were not attracted to the bait traps.

For seed treatments to be effective, all seeds must be coated with the chemical. Be sure to follow all label directions when you use either a seed treatment or soil insecticide.

This article originally appeared on pages 30-31 of the IC-478 (4) -- April 14, 1997 issue.

Updated 04/13/1997 - 1:00pm