Abstract
The enhancement of the insecticidal activity of δ-endotoxin from Bacillus thuringiensis serovar kurstaki HD-1 by several microorganisms and spores from six serovars of B. thuringiensis was investigated. Fourth instar larvae of the diamondback moth, Plutella xylostella were fed on 5g of artificial diet containing 1ml of test preparation. The LC50 values of the δ-endotoxin combinations with spore or bacterial cells (108/ml) were: B. thuringiensis serovar kurstaki: 0.05μg/ml, serovar aizawai: 0.06μg/ml, serovar kenyae: 0.21μg/ml, serovar israelensis: 0.53μg/ml, serovar entomocides: 1.10μg/ml, serovar sotto: 2.00μg/ml, living cells of Lactobacillus rhamnosus: 0.42μg/ml, Saccharomyces sake: 0.77μg/ml, Bacillus subtilis: 2.50μg/ml, Zygosaccharomyces soya: 2.10μg/ml, and Bacillus coagulans: 3.40μg/ml. Whereas the LC50 of δ-endotoxin was 5.80μg/ml. Higher correlations were observed between the enhanced insecticidal activity and proliferation of microorganisms in the insect hemolymphs. Germination of spores or proliferation of bacterial cells in the insects were observed when only δ-endotoxin was applied. This suggests that: (1) spore germination in the intestine of the diamondback moth is responsible for the lethal precipitate action, (2) spores or living cells of some microorganisms act synergistically with the δ-endotoxin toxicity.
