Late planting's impact on corn and soybean insects

Questions are surfacing about the impact of late planting on key insect pests of corn and soybeans, says Mike Gray, University of Illinois extension entomologist. Based upon the progress of planting this spring, Gray said he can understand the interest.

Western corn rootworms

Western corn rootworm larvae typically begin to hatch during Memorial Day weekend celebrations. Above-normal temperatures for the remainder of May could accelerate this time line, Gray notes. 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 has been progressing at a very impressive clip across many areas of the state," he says. "At this point, unless corn planting is set back again due to a prolonged stretch of wet weather, I don't believe the slow pace of planting will affect western corn rootworm densities this growing season."

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. Gray says that he has seen many fields that match this description this spring.

"Don't be lulled into complacency just because a Bt hybrid has been planted," he warns. "Large infestations of black cutworms (anticipated this spring) can potentially overwhelm certain Bt hybrids. I urge producers to look for early signs of leaf feeding to assess the potential threat of cutting."

European corn borers

Although few growers worry about this pest anymore, Gray says early planting tends to favor the establishment of the first generation. Late planting increases potential problems with the second generation.

Bean leaf beetles

Bean leaf beetle establishment is affected by many factors, including their ability to overwinter as adults beneath plant debris in wooded or sheltered areas, Gray says. Although many areas in Illinois experienced very cold temperatures this past 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 first fly to alfalfa fields," he says. "Early-planted soybean fields are most at risk to early-season bean leaf beetle feeding. 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 to 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 of last year were exceptionally low, Gray says. These sub-economic adult densities led to very few eggs on its overwintering host, buckthorn.

"I anticipate a very weak flight of aphids to soybean fields this spring," he says. "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 occurred and seedlings are subjected to prolonged periods of cool and wet soil conditions, increased levels of root injury by white grubs and feeding on below-ground portions of the stem may occur by wireworms.

"As soil temperatures increase, wireworm larvae typically begin to move deeper into the soil profile and away from the seed zone", Gray explains. "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 three-year life cycle and may injure corn root systems all summer long the second year of their life cycle. Accurate identification of grub species is key to their effective management, Gray says.

Corn earworms, corn leaf aphids, and fall armyworms

Many insects migrate into the Midwest each year. However, it's a bit too early to determine how the planting may affect densities of corn earworms, corn leaf aphids, and fall armyworms, according to Gray.

"Corn may reach the pollination period at a later date in July this season, typically a period of the summer more prone to hot and dry conditions," he concludes. "These insects may reach economic densities on corn plants growing under more stressful conditions this year. Time will tell."

For more information on late planting and insect pests, read the May 12 edition of The Bulletin online at http://bulletin.ipm.illinois.edu/.

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