In the transgenic era, the most important insect management decision a corn producer makes is deciding which type of corn hybrid to plant — a Bt hybrid or a non-Bt hybrid? This decision is typically made in the fall or early winter, well before planting ensues. In a sense, a producer who elects to plant a Bt hybrid takes out an insurance plan against a wide range of insect pests for the upcoming growing season. I have referred to this in previous Bulletin articles as — insurance pest management, the new and most popular form of IPM across the Corn Belt. According to the USDA Economic Research Service, 80% of corn grown in the United States during 2014 was a Bt hybrid.
Once again, we have Professor Chris DiFonzo, Department of Entomology, Michigan State University, to thank for preparing an updated Handy Bt Trait Table. Within this table Professor DiFonzo provides information on trade and event names, specific Cry proteins expressed, refuge requirements, herbicide tolerance characteristics, and targeted insect species. Producers are encouraged to pay particular attention to whether or not a specific Bt hybrid offers control or suppression of a given insect pest. Many of the Bt hybrids now offer a wide range of insect protection above ground (e.g. European corn borer, black cutworm, fall armyworm, corn earworm, stalk borer, western bean cutworm) and below ground (corn rootworm). In addition, producers need to ensure that they are deploying the proper refuge with their Bt hybrid of choice. Although seed blend refuges (5% and 10%) are becoming more common, use of some Bt hybrids requires a structured refuge. The level of complexity regarding refuge requirements for the wide variety of Bt hybrids has increased in recent years.
Recently, bioassays conducted by Dr. Joe Spencer, an entomologist with the Illinois Natural History Survey, have confirmed western corn rootworm resistance to the Cry3Bb1 protein in several Illinois counties (Henry, Kankakee, Livingston, McDonough, Mercer, Sangamon, and Whiteside). A segment of the western corn rootworm population has developed cross resistance to the mCry3A protein and resistance to crop rotation in Kankakee and Livingston counties. Producers who have not been satisfied with the level of root protection afforded by a Bt hybrid and suspect resistance, should consider planting a pyramided Bt rootworm hybrid — a hybrid that expresses more than one Cry protein targeted at corn rootworms. Crop rotation also should be considered by producers in fields with a history of continuous corn production. For producers who elect not to rotate crops, nor use a pyramided Bt rootworm hybrid, the planting of a non-Bt hybrid along with a planting-time soil insecticide is an option. The key to successful long term management of corn rootworms is utilization of an integrated approach — alternating tactics and thus hopefully avoiding resistance development. Unfortunately, the history of western corn rootworms is one of repeating past mistakes and development of resistance to nearly every management strategy.
A final note — black cutworm moths are migrating into the state of Illinois and growers who elect not to plant a Bt hybrid offering control against this pest should remain especially vigilant for early signs of leaf feeding once corn seedlings begin to emerge. Fields at most risk include those heavily infested with winter annual weeds.
Mike Gray, Professor & Extension Entomologist, Department of Crop Sciences, University of Illinois