University of Illinois

No. 8/May 16, 1997

The Subterraneans

As soon as we had written our article about the soil insects that injure corn early in the season, especially during cool weather, we started receiving and reading reports about subterranean pests reducing plant stands. Dave Dimmick with IMC Agribusiness observed a field in Sangamon County in which the plant population had been reduced from 30,000 to 16,000 plants per acre. Noel Troxclair, Extension educator in IPM at the Marion Extension Center, readily found wireworm larvae about 3 inches deep as he was placing wireworm bait stations in a field in Wayne County. Entomologists at the University of Kentucky have reported seedcorn maggots feeding on both vegetable and field crops, and Ed Billingsley, Extension educator in horticulture at the Marion Extension Center, reported in Illinois Fruit and Vegetable News that some squash fields in southern Illinois are infested with seedcorn maggots.

With so much corn in the ground and growers worried about its emergence in the cool soil, discovery of subterranean insect pests as people examine plant stands is inevitable. As long as the soil remains cool, the insects will have a longer period during which they can feed on and injure corn seeds and/or seedlings. A good burst of warm weather that encourages corn growth will allow struggling corn seedlings to escape or compensate for some of the injury. However, continued cool weather will make growers and consultants nervous as they wait for the plants to become established.

Continue looking for insects that might cause stand reduction for the next couple of weeks. Again, we remind you that "rescue" insecticide applications are not effective for insects like wireworms, white grubs, and seedcorn maggots. The only recourse for controlling these insects is to apply an insecticide seed treatment or a soil insecticide if the plant population has decreased so drastically that replanting is necessary. However, don't be hasty with either the decision to replant or the decision to use a soil insecticide. If replanting is necessary but happens after wireworms or seedcorn maggots have done their damage, an insecticide may not be necessary. As the soil warms up, wireworms will stop feeding on the corn and descend in the soil for the summer. Seedcorn maggots will complete their development and pupate, no longer posing a threat.
Kevin Steffey, Extension Entomology, (217)333-6652