Replanting and Soil Insects in Corn

May 14, 1999
We have received a few reports that some of the corn planted very early this year has been or will be replanted. Some people have encountered corn soil insect pests as they inspected fields for reasons for the slow growth or uneven emergence. Because corn had been planted early and the seeds and seedlings have been in cool soils for a while, exposure to insect injury has been protracted. If seedcorn maggots, white grubs, or wireworms are present in early planted fields and the corn is growing slowly, injury can be quite noticeable. (Click here to see a calculator for determining the predicted yields at different planting dates and plant populations for corn.)

The most obvious aboveground symptom of injury caused by these soil inhabitants is a reduced plant population. Upon further investigation, you may find seedcorn maggots or wireworms feeding in the seeds, or you may find white grubs or wireworms feeding on roots. Wireworms also will feed on the belowground portion of the seedling stem. Because of the clumped distribution of these insects in most fields, certain areas of a field may be heavily infested, and other areas appear to be fine.

If a grower plans to replant his or her corn because of an inadequate plant population and one or more of the aforementioned soil insects are present, a seed treatment or soil insecticide may be warranted. Although seedcorn maggots likely will pupate soon, white grubs and wireworms may be present for a couple more weeks. A seed treatment will protect the seeds from attack by seedcorn maggots and wireworms; an appropriate soil insecticide will protect corn seedlings from damage by all three insects. Seed treatments for protection against insects were discussed in issue no. 4 (April 16, 1999), and soil insecticides for white grubs and wireworms were listed in issue no. 2 (April 2, 1999) of the Bulletin. However, if a soil insecticide was applied earlier in the year (before or during the first planting), some restrictions about the amount of insecticide that can be applied in a given year may be important. Table 2 lists the corn soil insecticides and the maximum amount that can be applied per season. Do not exceed these amounts.--Kevin Steffey

Author: Kevin Steffey