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."
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."