Early instar response to plant-delivered Bt-toxin in a herbivore (Spodoptera litura) and a predator (Propylaea japonica)

Propylaea japonica is a significant predator in cotton fields in China. To assess the ecological effects of Bt-cotton cultivars (GK-12 and NuCOTN 33B, producing the fused Cry1Ab/Ac toxin and Cry1Ac toxin, respectively), the development of Spodoptera litura on transgenic Bt-cotton, the intake of Bt toxins, and the effects of Bt-cotton reared S. litura on young larvae of P. japonica were evaluated. Based on enzyme-linked immunosorbent assay (ELISA), the Bt-toxin concentrations in newly molted second-instar S. litura were 978.0 and 720.0 ng g−1 when fed on GK-12 and NuCOTN 33B cotton, respectively. The survival rate of S. litura was decreased, the time required to reach the second-instar was prolonged, and the body mass was lowered, when reared on NuCOTN 33B compared to those on DPL5415. However, there were no significant differences between GK-12 and Simian 3 (Parental non-Bt-transgenic for GK-12). After feeding on GK-12 or NuCOTN 33B -fed S. litura neonates for 72 h, young larvae of P. japonica contained more Bt-toxin than cotton leaf tissue, but less than respective Bt-cotton fed S. litura  . A decrease (P=0.061P=0.061) in body mass was observed when P. japonica young larvae were fed for 72 h with 24 h old NuCOTN 33B -reared S. litura larvae, compared to those fed with DPL5415 -reared S. litura; no significant differences were found when prey was fed with GK-12 vs. Simian 3 plants. Significantly fewer P. japonica larvae molted into second-instar when fed with S. litura reared on NuCOTN 33B, compared to ones fed prey from DPL5415 plants. No such effects were observed when feeding on GK-12 vs. Simian 3 -reared prey. These results suggested that the Cry1Ab/Ac fusion toxin had no direct effect on young larva of P. japonica, and a combined interaction of poor prey quality and Cry1Ac toxin may account for the negative effects observed on P. japonica development when fed NuCOTN 33B -reared S. litura.