No. 7 Article 5/May 9, 2008

Late Corn Planting--How Will Some Key Insects Respond?

Across much of the Corn Belt this spring, the progress of corn planting is considerably behind the 5-year average, and we've received inquiries about the potential effects of the significant delays on insect pests. There is no one simple answer due to the different life cycles, biology, and uniqueness of each pest. Provided here are some thoughts on several insect pests of corn and the effects that late planting may have on their importance this season.

Seedling corn in Champaign County (May 7, 2008).

Corn rootworms. Corn rootworm larvae typically hatch in late May. In some very cool and wet springs, I've witnessed hatch as late as mid-June. If corn planting were delayed until late May, some starvation of larvae would occur, especially of larvae that hatched from eggs early. Generally, early planting (April through early May) tends to favor good establishment of corn rootworm larvae.

European corn borer. With the extensive use of Bt hybrids, this insect has almost become a "forgotten pest" of corn. By late May through early June, we should begin to see moths seeking to lay egg masses on corn plants. Egg laying occurs during the evening hours. Early planting tends to favor good establishment of the first generation of corn borers. Late planting promotes better establishment of the second generation.

Black cutworms. Late planting of corn tends to promote more problems with black cutworms. Fields that are tilled later in the spring and subsequently planted are often prime targets for good establishment of black cutworms. Migrating moths prefer to lay their eggs in fields with abundant winter annual weed cover. Larvae that can go through several molts on weeds may be able to inflict significant injury on some corn seedlings.

White grubs. Years ago, when it was less common to plant as much corn in April as we've come to expect in recent years, annual white grubs (including Japanese beetle grubs) were not considered significant threats to corn production. Producers were advised to look at rasters of white grubs and determine whether annual or true white grubs (Phyllophaga spp.) were present in a given field.

Because true white grubs feed all summer long during the second year of their life cycle, they can cause economic damage to stands of corn. Annual white grubs were considered less of a threat due to the shorter period in which corn seedlings were exposed to root hair pruning. However, corn planting in early April elevates the pest status of annual white grubs. This year, annual white grub injury should be more limited and the focus will return to true white grubs. Cold, wet soils also may delay grub development and result in longer exposure of corn seedlings to root hair pruning by larvae.

Wireworms. Delayed planting and cool, wet soils may result in greater densities of wireworms remaining in the upper soil profile and feeding on root systems of corn seedlings. In addition to direct seed tunneling, wireworms have the potential to feed within the growing point tissue of plants. If this occurs, significant stand reductions may result. This season might be a very good test of the reliability of the insecticidal seed treatments on Bt seed.

Seedcorn maggots. Late planting of corn into cool and wet soils will increase the risk for seedcorn maggot injury to corn seedlings. Fields with an abundance of decaying organic matter, especially those with frequent manure applications, are particularly vulnerable to stand reductions.

Let's hope for some favorable planting weather over the next several weeks. We look forward to learning from readers if significant problems with any of these insect pests begin to take shape this spring.--Mike Gray

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