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Grain yield reductions and losses in grain quality due to insect pests are a constant problem in the Southeast. Control options include cultural practices to prevent or avoid injury, transgenic Bt corn, at-planting insecticides (including seed treatments), and foliar insecticides. Research is being conducted at the Pee Dee and Edisto Research and Education Center to assess the efficacy of Bt corn and insecticidal seed treatments. Commercialized since 1996 targeting the European corn borer, initial Bt traits expressed a single Bt gene (Cry1Ab or Cry1F). More recent Bt traits express two to three Bt toxins for above-ground insect pests, which has benefited growers in the southeastern U.S. by improving efficacy for key insect pests such as the corn earworm (Fig .1) and fall armyworm (Fig. 2).

A recent study at the Pee Dee REC examined how Bt traits affect the life cycle of the corn earworm. Older Bt corn hybrids expressing single toxins are only moderately toxic to corn earworm. Growth and development of corn earworm were monitored in field plots to determine potential changes in susceptibility to this toxin and others over time. Small plots of corn hybrids expressing a range of Bt hybrids and near isoline non-Bt hybrids were planted in Florence, SC (and also in Plymouth, NC with colleague Dominic Reisig) with paired non-Bt hybrids from the same genetic background.

Corn earworm feeding on corn.

Fig. 1.  Corn earworm feeding on corn at Pee Dee REC.

Corn earworm larvae were sampled on several dates from ears. There was no reduction in larval weight, number of insect entering the pupal stadium, pupal weight, time to eclosion, and number of pupae able to successfully eclose to adulthood in the hybrid expressing Cry1Ab compared with a non-Bt paired hybrid. As Cry1Ab affected these in 1996, corn earworm may be developing resistance to Cry1Ab in corn, although these results are not comprehensive, given the limited sampling period, size, and geography.

Non-Bt corn damage by fall armyworm 

 Bt corn with no injury from fall armyworm.

Fig. 2.  Non-Bt corn hybrid (top) with fall armyworm injury and Bt corn hybrid (bottom) with no injury at Pee Dee REC.

On-going trials by Clemson Ph.D. student Thomas Bilbo are examining the fecundity and egg viability of corn earworm moths emerging from transgenic corn hybrids expressing Bt toxins and non-Bt isolines. Fecundity of emerging moths was determined by confining moths from each plot in containers in the laboratory. These data will be used by modelers to refine resistance evolution models that currently lack empirical data and have to make a number of assumptions that can lead to inaccurate predictions.

Thomas’ work also includes evaluating resistance to Bt resistance using purified Bt toxins in laboratory bioassays with populations of corn earworm collected in commercial corn fields in North and South Carolina.

For more information regarding corn research in South Carolina, visit the Edisto Research Education Center corn research page or contact Dr. Francis Reay-Jones.