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Wet Weather, Delayed Planting, and Insects

May 10, 2002
Most farmers are not focused on insect problems at the moment. Farmers fortunate enough to have been able to plant corn (mostly in the western and central counties) are witnessing slow emergence and slow growth of corn seedlings; much of the emerged corn is yellow. The cool, wet soils are not contributing to rapid, healthy corn growth. On the other hand, many farmers have planted very little, if any, corn. They're waiting for the rains to cease, just to get started or to resume planting after having planted only a few acres here and there. As of May 5, the Illinois Agricultural Statistics Service estimated that 26% (Southwest Crop Reporting District) to 52% (Central Crop Reporting District) of the corn had been planted in western and central counties. In eastern counties, 6% (Southeast Crop Reporting District) to 32% (East Crop Reporting District) of the corn had been planted.

Although the focus is on the weather right now, the weather may have a dramatic impact on the potential for insect problems. The cool, wet soils and delayed planting may represent either bad news or good news, depending on the crop and the insect. Following is an overview of how our current spring weather conditions may affect the potential for certain insect pests to cause problems (or not):

· Corn seeds and seedlings in cool, wet soils are more likely to be injured by subterranean insects such as grape colaspis, seedcorn maggot, white grubs, and wireworms. Corn seeds and small seedlings remain susceptible to feeding by these insects when both emergence and growth are slow. Slow-growing corn seedlings can't "grow away" from the damage. White grubs are the primary insect concern among many farmers right now; I continue to receive reports of very large densities of white grubs. Thus far, all samples received have been Japanese beetle grubs. Although no injury has been reported, we'll have to remain vigilant.

· On the other hand, later planting may enable corn to escape damage caused by these same subterranean insects. These insects are most problematic early in the season. Many of these insects will be finished feeding for the season by the time corn roots are available.

· Delayed planting increases the potential for black cutworm problems. Black cutworm females seek vegetation in which to lay eggs, and delays in fieldwork have allowed many fields to get pretty fuzzy with weeds. When corn is planted into these fields, black cutworm larvae will stop feeding on the weeds when they are killed with herbicides and will begin feeding on corn seedlings.

· Slugs survive better during cool, wet springs. Entomologists in Ohio recently observed slugs hatching from eggs in northern counties. We should anticipate some occurrences of slugs any time now. No-till fields are most likely to harbor populations of slugs.

· Planting later reduces the risk of injury caused by first-generation European corn borer. Moths of European corn borers that will lay eggs to begin the first generation seek tall (usually early-planted) corn plants on which to deposit eggs. If corn is not available at the time of egg laying, the first generation won't develop. The first capture of a European corn borer adult in Illinois this year occurred in Pulaski County on April 30. From this first capture we can predict when first instars (larvae) hatch from eggs--212 degree-days (base 50 deg F) accumulated after the first capture. Bob Scott, Illinois State Water Survey, has predicted that 212 degree-days will be accumulated on May 15. Without much corn in the area, egg-laying females will have little success.

· Planting later reduces the risk of injury caused by corn rootworm larvae. Although corn with small root systems could be damaged severely by rootworm larvae, many of the larvae will starve to death because of the lack of availability of food.

· Planting later reduces the risk of injury caused by bean leaf beetles. Bean leaf beetles that have emerged are "hanging out" in fields of alfalfa and clover, waiting for soybeans to emerge. They will have a long wait in many areas. Consequently, many bean leaf beetles will perish without laying eggs.

· Planting later increases the potential for infestations of second-generation European corn borers and southwestern corn borers (in southern Illinois). Assuming any of these borers survive this spring, later-planted corn will be highly attractive for moths that lay eggs to begin the second generations of both species.

· Later-planted corn also is highly attractive to corn rootworm adults. The fresh silks and pollen will provide a great food source during the heat of the summer, and females often lay large numbers of eggs in late-planted cornfields.

In some areas of Illinois, there will be a mixture of early- and late-planted corn, giving some insects obvious choices. Consequently, people in these areas will have to remain very alert for the development of insect problems. When targeting fields for scouting, make certain you understand the relationship between planting time and the potential for insect problems.--Kevin Steffey

Author: Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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