Can Pyramids and Seed Mixtures Delay Resistance to Bt Crops?

The primary strategy for delaying the evolution of pest resistance to transgenic crops that produce insecticidal proteins from Bacillus thuringiensis (Bt) entails refuges of plants that do not produce Bt toxins and thus allow survival of susceptible pests. Recent advances include using refuges together with Bt crop ‘pyramids’ that make two or more Bt toxins effective against the same pest, and planting seed mixtures yielding random distributions of pyramided Bt and non-Bt corn plants within fields. We conclude that conditions often deviate from those favoring the success of pyramids and seed mixtures, particularly against pests with low inherent susceptibility to Bt toxins. For these problematic pests, promising approaches include using larger refuges and integrating Bt crops with other pest management tactics.

TrendsConditions in the field often deviate substantially from those promoting success of the refuge strategy for delaying insect pest resistance to pyramided Bt crops, particularly in pests with low inherent susceptibility to Bt toxins.Phasing out plants that produce only one toxin effective against target pests could increase the durability of Bt crop pyramids.Evolution of pest resistance to Bt crops could be slowed by using combinations of toxins that are structurally distinct, such as Cry and Vip toxins, or Cry toxins with low amino acid sequence similarity in domain II.Gene flow between Bt and non-Bt corn plants in seed mixtures produces a mosaic of Bt and non-Bt kernels in ears of non-Bt corn plants, which could accelerate the evolution of resistance in pests feeding on ears.In some regions of the USA, where western corn rootworm has evolved resistance to Cry3Bb and mCry3Aa, all pyramided Bt corn hybrids targeting this pest are effectively single-toxin crops.Many conditions favoring success of the refuge strategy deviate from the ideal for western corn rootworm, implying that the risk of resistance in this pest is high for all currently available Bt corn hybrids in the USA.The refuge strategy has been successful for delaying resistance to Bt crops in pests with high inherent susceptibility to Bt toxins, but larger refuges are needed and Bt crops must be integrated with other pest management tactics to sustain their efficacy against pests with low inherent susceptibility to Bt toxins