Prey-mediated effects of Cry1Ab toxin and protoxin and Cry2A protoxin on the predator Chrysoperla carnea

Laboratory feeding experiments were carried out to study prey-mediated effe cts of artificial diet containing Bacillus thuringiensis proteins on immatu re Chrysoperla carnea. Activated Cry1Ab toxin and the protoxins of Cry1Ab a nd Cry2A were mixed into standard meridic diet for Spodoptera littoralis (B oisduval) larvae at the following concentrations; for Cry1Ab toxin, 25, 50, 100 mu g g(-1) diet were used; for Cry1Ab protoxin, the concentration was doubled (50 mu g g(-1) diet, 100 mu g g(-1) diet and 200 mu g g(-1) diet) t o give relative comparable levels of toxin concentration. Cry2A protoxin wa s incorporated into the meridic diet at one concentration only (100 mu g g( -1) diet). For the untreated control, the equivalent amount of double disti lled water was added to the meridic diet. Individual C. carnea larvae were raised on S. littoralis larvae fed with one of the respective treated merid ic diets described above. The objectives were to quantify and compare the r esulting effects on mortality and development time of C. carnea with those observed in two previous studies investigating prey-mediated effects of tra nsgenic Cry1Ab toxin-producing corn plants and the other studying effects o f Cry1Ab toxin fed directly to C. carnea larvae. Mean total immature mortal ity for chrysopid larvae reared on B. thuringiensis-fed prey was always sig nificantly higher than in the control (26%). Total immature mortality of C. carnea reared on Cry1Ab toxin 100 mu g g(-1) diet-fed prey was highest (78 %) and declined with decreasing toxin concentration. Cry1Ab protoxin-expose d C. carnea larvae did not exhibit a dose response. Prey-mediated total mor tality of Cry1Ab protoxin-exposed chrysopid larvae was intermediate (46-62% ) to Cry1Ab toxin exposed (55-78%) and Cry2A protoxin (47%) exposed C. carn ea. In agreement with the previous studies, total development time of C. ca rnea was not consistently, significantly affected by the Bt-treatments exce pt at the highest Cry1Ab toxin concentration. However, both highest mortali ty and delayed development of immature C. carnea raised on Cry1Ab toxin 100 mu g g(-1) diet-fed prey may have been confounded with an increased intoxi cation of S. littoralis larvae that was observed at that concentration. At all other B. thuringiensis protein concentrations S. littoralis was not let hally affected. Comparative analysis of the results of this study with thos e of the two previous studies revealed that in addition to prey/herbivore b y B. thuringiensis interactions, also prey/herbivore by plant interactions exist that contribute to the observed toxicity of B. thuringiensis - fed S. littoralis larvae for C. carnea. These findings demonstrate that tritrophi c level studies are necessary to assess the long-term compatibility of inse cticidal plants with important natural enemies.