Entomologist Questions High Usage Of Bt Hybrids

Ninety-five percent of the producers who participated in the regional 2012 Corn and Soybean Classic meetings last January said they planted a Bt hybrid in 2011. This very high use rate has been common for several years across Illinois in spite of low numbers of key insect pests such as the European corn borer and the western corn rootworm.

Intense use of Bt hybrids is also anticipated for the 2012 growing season. “I have questioned the wisdom of applying such intense selection pressure on insect populations when many of the pest species are well below economic levels in most producers’ fields,” said Michael Gray, University of Illinois professor, Crop Sciences Extension Coordinator & Assistant Dean for ANR Extension Programs. Nonetheless, this pattern is not expected to change.

When Bt hybrids entered the market place in 1996 and for many years thereafter, the use of a 20% refuge was the standard protocol for the Corn Belt, based upon the use of Bt hybrids aimed primarily at the European corn borer, which express a high dosage level of Cry proteins.

In 2003, Bt hybrids were commercialized for corn rootworms, and similar refuge requirements were implemented across the Midwest, even though the Bt hybrids targeted at corn rootworms were not high dose and the mating characteristics, along with dispersal patterns of adult corn rootworms, are different than those of corn borers.

Why were the refuge requirements similar for such distinctly different insects?

“Because of familiarity, convenience, and thus, the greater likelihood of implementation of the 20% structured refuge by producers rather than tailoring refuge requirements to the unique biological characteristics of corn rootworms,” said Gray.

Today producers have more flexibility with respect to the type of refuge they implement. Although more than half of the producers at the 2012 Classics indicate they intend to use the 20% structured refuge this growing season, the seed blend (refuge-in-a-bag) strategy is gaining popularity as more pyramided Bt hybrids enter the marketplace.

At the 2012 Classics, producers were asked if they planted a refuge in 2011. On average, 83% of producers said they had established a refuge. The proper establishment of refuges will become increasingly important as more acres are planted to Bt hybrids, selection pressure increases, and the threat of the development of western corn rootworm resistance looms.

Approximately 37% of the producers who took part in the 2012 Classics will use a seed blend as their refuge and hedge against insect resistance development.

“From a convenience angle, it’s easy to see why this approach will increase in popularity,” said Gray. Of concern is the anticipated reduction in the volume of non-Bt seed produced by the seed industry as refuge requirements drop from 20% levels, which could make it more difficult for producers to purchase elite germplasm from non-Bt product lines. Access to non-Bt hybrids by producers is important if the industry wants to maintain an integrated approach to pest management across the Corn Belt.

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