Identification of two haemolysins in larvicidal activity of Bacillus thuringiensis against the bean bug, Riptortus clavatus

Abstract:  This study characterized larvicidal activity against the bean bug, Riptortus clavatus (Hem., Alydidae), associated with a strain of Bacillus thuringiensis serovar morrisoni (H8ab). Purified crystals and solubilized crystal proteins exhibited only low-level activities, while the supernatant of broth culture contained rapid and strong larvicidal activity. Heating at 100°C for 10 min destroyed the activity. Two extracellular vegetative proteins, with molecular masses of 40 and 45 kDa, were obtained by diethylaminoethyl (DEAE)-cellulose column chromatography from the bacterial culture fluid. Both proteins were related to the known haemolysins of Bacillus cereus , showing strong cytolytic activity against sheep erythrocytes. The bean bug-killing activity was not associated with individual proteins; however, strong activity was induced when two proteins were combined. The combined proteins were toxic to larvae in the early stage of first instar but not against larvae of later instars and adults. Larvae of the diamondbackmoth, Plutella xylostella , were not killed by these proteins.