Resistance management for Bt maize and above-ground lepidopteran targets in the USA: from single gene to pyramided traits.

Abstract Since first being commercialized in 1996, transgenic maize expressing Bacillus thuringiensis (Bt) proteins has gained widespread acceptance in the world. In 2013, nearly 50 Mha of Bt maize were planted in 15 countries. In the same year, growers in the USA alone planted c.30 Mha of Bt maize, which accounted for 76% of the total Bt maize area of the country. Up to now, Bt maize technology can be classified into two generations. The first generation of Bt maize contains only a single Bt gene for a target. In 2010, the second generation of Bt maize became commercially available and this expresses two or more pyramided Bt proteins. Currently, the pyramided products are predominant in the USA. The major lepidopteran targets of Bt maize in the USA are corn borers (Crambidae), the corn earworm, Helicoverpa zea, and the fall armyworm, Spodoptera frugiperda. To counter the threat of insect resistance, two resistance management strategies for Bt maize, 'high dose/refuge' and gene pyramiding, have been implemented. The long-term use of Bt maize against the major agricultural pests in North America provides a good opportunity to analyse the effectiveness of the adopted insect resistance management (IRM) plans. Analysis of the available data shows that all corn borer species remain susceptible to Bt proteins and that no field resistance has occurred after nearly two decades of intensive use of Bt maize in the continent. Pyramided Bt maize is effective in controlling corn earworm and fall armyworm, although recent studies indicate that field resistance to single-gene Cry1F maize in the fall armyworm has occurred in the south-east coastal areas of the US mainland. Knowledge of the resistance management gained from the USA should be useful for other countries in their sustainable use of Bt crop technology.