Relative toxicity of the maize endosperm ribosome-inactivating protein to insects

The relative toxicity of proenzyme and protease-activated forms of maize seed ribosome-inactivating protein (b-32) to several insect species was determined. Only the protease-activated form had significant toxicity to any caterpillars when fed in diets at 1 mg/g of diet. Activity ranged from 70% mortality to cabbage looper (Trichoplusia ni) to no effect to Indian meal moth (Plodia interpunctella). Neither form of the protein showed activity against larvae of the Freeman sap beetle, (Carpophilus freemani). However, the proenzyme and protease-activated forms were approximately equally deterrent in choice assays to other sap beetles and maize weevils (Sitophilus zeamais), with relative feeding rates reduced by up to 6-fold. Because this protein can naturally occur at the 1 mg/g endosperm range in the endosperm of Opaque-2 (normal) plants versus 2 orders of magnitude lower in opaque-2 mutants, it is likely that this RIP plays a natural defensive role against insects. However, some insects appear to have adapted to this protein.