University of Illinois

No. 9/May 23, 1997

Several Reports of Wireworm Problems

We have received several reports of wireworms causing damage and reducing plant populations in cornfields in several areas of the state. Most of the calls have come from producers, consultants, and pesticide- or seed-industry reps in southern and western counties. Some of the wireworms have been the typical red-yellow to red-brown, hard-shelled wireworms with which most people are familiar. However, some observers have found "white" wireworms. These wireworms usually are creamy white, with a dark brown head and first thoracic segment (the segment immediately behind the head). The last abdominal segment is slightly darker than the body and is notched. Due to their "atypical" color, these wireworm species sometimes are confused with ground beetle larvae. However, "white" wireworms have short legs and are slow moving, and their mouthparts are the standard chewing mouthparts of plant feeders. The predatory ground beetle larvae have relatively longer legs and are usually quick moving. Their mandibles are sickle-shaped, to enable them to capture prey.

As we have indicated in previous issues of this Bulletin, the cool weather has slowed corn growth, making the seedlings susceptible to injury by subterranean insects like wireworms for a longer period of time in the spring. As soon as the weather warms up, the plants will become more vigorous and might be able to endure some of the injury.

As you scout fields of seedling corn to assess the plant population and look for problems, look for wilted plants or gaps in rows. If wireworms are the culprits, you should be able to find them with relative ease near the wilted plants or beginning to feed on adjacent healthy plants. Remember, wireworms feed on the seeds and on the underground portion of the stem. The latter feeding injury often results in "dead heart," a symptom in which the center leaves of the plant are dying or dead. Keep in mind that other insects like black cutworms and stalk borers may cause "dead heart," so accurate diagnosis is important.

"Rescue" treatments for wireworms are ineffective, so don't let anyone convince you otherwise. However, if plant populations have been reduced sufficiently to warrant replanting (an agronomic and economic decision), application of a seed treatment or a soil insecticide labeled for wireworm control is probably justified.

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