Prey-mediated effects of Bacillus thuringiensis spray on the predator Chrysoperla carnea in maize

Bacillus thuringiensis (Bt) bioinsecticides are generally considered safe to beneficial insects. However, negative effects of transgenic Bt-expressing maize on an important predator, Chrysoperla carnea, have previously been reported. Here, we make an ecological assessment of the effects of Dipel, one of the most widely used Bt-sprays in agriculture, on C. carnea larvae. Indirect effects due to a reduction of prey were tested by rearing three prey species of C. carnea (the aphid Rhopalosiphum padi, the spider mite Tetranychus urticae, and Lepidoptera larva Spodoptera littoralis) on either maize plants sprayed with Dipel (at the recommended field concentrations) or on control plants. Effects of Dipel on C. carnea were assessed by performing greenhouse experiments in which chrysopid larvae were kept on Bt-sprayed or control plants and fed with herbivores reared on Bt-sprayed or control plants. Dipel had no effect on aphids; however, negative effects on spider mites were observed. Spider mites reared on Bt-sprayed plants had a significantly lower intrinsic rate of natural increase compared to those reared on control plants. Similarly, S. littoralis larvae were significantly affected by Dipel as the developmental time required by larvae which were fed Bt-sprayed plants was prolonged when compared to larvae on untreated plants. Negative effects on C. carnea larvae were also shown through prey-mediated exposure to Dipel. A significant increase in mortality, a prolonged developmental time and a slight decrease in weight was observed for C. carnea fed with ‘Bt-contaminated’ S. littoralis larvae. The effects of Dipel on C. carnea larvae shown in this study, are comparable to earlier published results obtained with Bt maize. The ecological relevance of these results is discussed in comparison with the possible risks that the deployment of Bt-expressing maize plants pose to the predator C. carnea.