Compatibility of biological control with Bt maize expressing Cry3Bb1 in controlling corn rootworms

The western corn rootworm, Diabrotica virgifera virgifera, is a serious maize pest in the USA and Europe. Genetically engineered (GE) maize producing insecticidal Cry3Bb1 protein derived from Bacillus thuringiensis (Bt) to control this pest has been commercialized in the USA. One crucial part of the environmental safety assessment of GE plants is the evaluation of potential risks for non-target species, including biological control agents. Toxicity of the Cry3Bb1 protein to non-target species can only occur when the Bt protein is ingested in a biologically active form. The insecticidal activity of Cry3Bb1 expressed in different tissues of Bt maize, contained in maize herbivores, and in spiked soil was confirmed in sensitive insect bioassays using larvae of the Colorado potato beetle (Leptinotarsa decemlineata). Bioassays with the same concentration of Cry3Bb1 per ml diet suggested that nutritional quality of food and degradation processes may influence Cry protein toxicity. A risk assessment for a generalist predator, the spider Theridion impressum was conducted. The spider was found to be exposed to the Bt protein after analyzing its prey spectrum and the Cry3Bb1 concentration in potential prey species collected in a Bt maize field. As feeding studies with Bt maize fed prey and Bt maize pollen did not indicate adverse effects on adult and juvenile spiders, it can be concluded that Bt maize most likely poses a negligible risk for T. impressum. The interaction of an entomopathogenic fungus, Metarhizium anisopliae, with Bt maize and D. v. virgifera was studied in the laboratory. While feeding on Bt maize delayed the development of D. v. virgifera larvae, there was no difference in fungal infection rates between Bt and control maize treatments for larvae and adults. This implies that Cry3Bb1-expressing Bt maize is compatible with biological control by this entomopathogenic fungus. The presented results together with previously published laboratory and field studies on non-target species demonstrate that rootworm-resistant Bt maize contributes to sustainable agriculture by maintaining ecological services including biological control provided by spiders and entomopathogenic fungi.