Side effects of commercial Bacillus thuringiensis insecticides on micro-colonies of Bombus terrestris.

BACKGROUND: Bacillus thuringiensis (Bt) and its protein crystals are used worldwide, either as a spray or when expressed in transgenic crops, for the control of pest insects. However, due to their intensive use, there exists a debate regarding the involvement of this microbial insecticide in bee colony losses. In this study, we evaluated in a tiered approach using laboratory microcolonies the potential lethal and sublethal hazards on colony reproduction and foraging behavior of workers of the bumblebee Bombus terrestris of two commercial Bt strains: kurstaki (Dipel®) and aizawai (Xentari®). Bumblebees, like honeybees, are intensively used in modern agriculture for pollination and fulfill a crucial role in natural ecosystem.
RESULTS: Exposure to bumblebees dermally or via treated pollen to either of the two Bt formulations at their field recommended rates (0.1%) caused no reduction in survival. However, when applied in the feeding sugar water Xentari® killed all workers at a concentration of 0.1%, but this lethal effect was lost at 0.01%. With respect to reproductive effects, Dipel® was harmless, while Xentari® at 0.1% delivered in the feeding sugar water and pollen reduced reproduction by 100 and 31%, respectively. Lower doses of 0.01% Xentari® in the sugar water showed no more effect. In addition, Dipel® at 0.1% and Xentari® at 0.01% in the feeding sugar water did not impair the foraging behavior, resulting in normal nest colony performance.
CONCLUSION. The results with Dipel® and Xentari® demonstrated that, in general, the Bt strains are safe to B. terrestris bumblebees, although in some cases there were detrimental effects that depended on strain and route of exposure. In addition, we believe that to draw firm conclusions regarding the hazards of Bt to bumblebees, we need more information on relevant concentrations of Bt products in the environment. Hence, routine testing for lethal and sublethal effects is recommended to ascertain combined use of Bt products and bumblebees in modern agriculture.