Intraspecific Competition of an Insect‐Resistant Transgenic Canola in Seed Mixtures

Abstract

Seed mixtures are recommended as a strategy to minimize or avoid having insects develop resistance to insect‐resistant transgenic crops. The objective of this study was to evaluate a canola, Brassica napus L., transgenic for a Bacillus thuringiensis cry1Ac gene for its resistance against diamondback moth, Plutella xylostella L., and for its competitive ability with nontransgenic canola in seed mixtures. Transgenic and nontransgenic canola were planted either as pure stands or in mixtures of 75:25, 50:50, and 25:75 in plastic trays in greenhouse experiments or in field experiments at three locations during the 1996 through 1998 field seasons. The trays and plots were either infested with diamondback moth neonates or left without any insect infestation. In diamondback moth‐infested treatments, transgenic plants had low levels of damage both as a pure stand and in mixtures. Nontransgenic plants in diamondback moth‐infested trays and plots suffered high levels of defoliation and produced less biomass and seed yield compared with transgenic plants. Relative crowding coefficient (RCC), a measure of competition between the two plant types, ranged from 0.6 to 1.1 in plots where there was no diamondback moth infestation and 1.1 to 12.8 in plots where there was diamondback moth infestation. No competitive advantage was observed for either plant type in seed mixtures when there was no diamondback moth infestation. Transgenic canola because of its high level of resistance was competitively superior in seed mixtures when there was diamondback moth infestation.