Late planting's impact on corn and soybean insects

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Lately Mike Gray, University of Illinois, has received quite a few emails and calls regarding the potential impact of this season's late planting on corn and soybean insects. He can understand the interest. According to the USDA NASS Illinois field office in Springfield, only 10 percent of corn had been planted as of May 1, and just 1 percent had emerged. This compares with a 5-year average of 46 percent planted by May 1. In sharp contrast, last year 85 percent of Illinois corn was planted by May 1. He provides information for some key insect pests of corn and soybeans regarding the potential impact of late planting on economic infestations.

Western corn rootworms. Larvae typically begin to hatch Memorial Day weekend. Above-normal temperatures for the remainder of May this year could accelerate the timeline. In general, late planting tends to reduce the likelihood of economic infestations of corn rootworms. During the past few days (May 7 to 11), corn planting is progressing at an impressive clip across many areas of Illinois. Unless planting is set back again by a prolonged stretch of wet weather, he doesn't believe the slow start to planting will affect western corn rootworm densities this growing season. However, he has witnessed declines in western corn rootworm densities the past several seasons, for reasons he discussed in previous issues of the Bulletin. He intends to conduct surveys in producers' fields during 2011 to more accurately assess western corn rootworm adult levels.

Black cutworms. Fields that have been tilled and planted late this spring are more susceptible to black cutworm injury. Corn is at most risk when planted into fields that have supported dense populations of winter annual weeds, and we have certainly seen many such fields this spring. Don't be lulled into complacency just because a Bt hybrid has been planted. Large infestations of black cutworms -- anticipated this spring -- can potentially overwhelm certain Bt hybrids. He urges producers to look for early signs of leaf feeding to assess the potential threat of cutting.

European corn borers. Does anyone worry about this insect anymore? Early planting tends to favor the establishment of the first generation of European corn borers. Late planting increases potential problems with the second generation.

Bean leaf beetles. Establishment of bean leaf beetles is affected by many factors, including their ability to overwinter as adults beneath plant debris in wooded or sheltered areas. Although many areas in Illinois experienced very cold temperatures last winter, snowfall was abundant and likely provided a blanket of insulation. As bean leaf beetles break their dormancy and begin emerging from their overwintering sites, they often fly first to alfalfa fields. Early-planted soybean fields are most at risk for early-season feeding by bean leaf beetles. Because soybean planting will be later this season, I don't anticipate large economic infestations of this insect this spring. Any early-planted and isolated field of soybeans located near a wooded area is always at risk for bean leaf beetle injury.

Soybean aphids. Soybean aphid densities were very low throughout Illinois in 2010. In fact, suction trap counts in September and October were exceptionally low. These sub-economic adult densities led to very few eggs on the overwintering host, buckthorn. He anticipates a very weak flight of aphids to soybean fields this spring. The late planting of soybeans will further contribute to a downward spiral of aphid densities early in the season. If the summer of 2011 is mild, aphid densities could certainly rebound by late season. The reproductive power of this insect is impressive. A hot summer may result in another no-show for this insect.

White grubs and wireworms. In general, delays in corn planting negatively affect densities of these soil insect pests. If planting has been done and seedlings are subjected to prolonged cool and wet soil conditions, increased root injury by white grubs and feeding on below-ground portions of the stem by wireworms may occur. As soil temperatures increase, wireworm larvae typically begin to move deeper into the soil profile and away from the seed zone. Corn that is planted later in the month into soils that are becoming progressively warmer may not experience as much wireworm injury. Annual white grubs, such as Japanese beetle grubs, typically complete pupation by late May and early June. Consequently, their potential to injure late-planted corn is greatly diminished the further planting is delayed in May. However, true white grubs have a 3-year life cycle and may injure corn root systems all summer long in the second year of their life cycle. Accurate identification of grub species is key to their effective management.

Corn earworms, corn leaf aphids, and fall armyworms. Quite a few insects migrate into the Midwest each year. It's a bit too early to determine how the timing of planting may affect densities of corn earworms, corn leaf aphids, and fall armyworms. However, corn may reach pollination at a later date in July this season, typically a period of the summer more prone to hot and dry conditions. These insects may reach economic densities on corn plants growing under more stressful conditions this year. Time will tell.

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