Intestinal bacteria affect growth of Bacillus thuringiensis in larvae of the oriental tea tortrix, Homona magnanima diakonoff (Lepidoptera: tortricidae).

Spores and parasporal crystals of a Bacillus thuringiensis serovar aizawai were fed to fifth instar larvae of the oriental tea tortrix, Homona magnanima, that had been reared aseptically or that had been reared normally. Viable cell numbers of B. thuringiensis and other bacteria in H. magnanima larvae were estimated by homogenization of samples and dilution plating on peptone-polymyxin agar medium for B. thuringiensis cells and on nutrient agar medium for the other bacterial cells. B. thuringiensis did not grow in the larval cadavers of normally reared H. magnanima while bacteria other than B. thuringiensis grew rapidly. In contrast, B. thuringiensis within the larval cadavers of aseptically reared H. magnanima grew and increased 20 times. The bacteria other than B. thuringiensis from the sample homogenates of normally reared larvae that were fed on B. thuringiensis-treated diets had the same characteristics as the bacteria isolated from the guts of healthy H. magnanima larvae, which were putatively identified as Streptococcus spp. and Staphylococcus spp., typical intestinal bacteria of insects. The results strongly suggest that intestinal bacteria influence the growth of B. thuringiensis in the larvae.