Bt maize expressing Cry3Bb1 does not harm the spider mite, Tetranychus urticae, or its ladybird beetle predator, Stethorus punctillum

There is concern that Bt maize varieties expressing Cry3Bb1 against larvae of the Western corn rootworm (Diabrotica virgifera virgifera; Coleoptera: Chrysomelidae) may also harm non-target beetles. We conducted studies to assess the prey-mediated effects of Cry3Bb1-expressing Bt maize (Event MON88017) on the ladybird beetle Stethorus punctillum Coleoptera: Coccinellidae). This species is specific to spider mites that are known to contain high amounts of Bt toxin when feeding on Bt maize. The developmental and reproduction life-table parameters did not differ for two-spotted spider mites, Tetranychus urticae (Acari: Tetranychidae), fed Bt maize or non-Bt maize. Similarly, larval survival and development, adult survival, and adult dry weight did not differ for S. punctillum fed with T. urticae reared on Bt or non-Bt maize for over 2 months. Interestingly, female beetles exposed to Cry3Bb1 had a significantly shorter pre-oviposition period, increased fecundity, and increased fertility relative to females that were not exposed to the toxin; the reasons for these effects are unclear but are likely due to unidentified differences in plant characteristics. T. urticae contained 56% of the Cry3Bb1 concentration in Bt maize leaves. Beetle larvae and adults that had consumed Bt maize-fed T. urticae contained Cry3Bb1 protein concentrations that were six and 20 times lower, respectively, than Cry3Bb1 levels in their prey. Thus, the toxin was diluted at higher trophic levels. The results indicate that S. punctillum is not harmed by feeding on spider mites containing Cry3Bb1. Consequently, the biological control function provided by this predator should be retained in Cry3Bb1-expressing Bt maize fields.