Synergism of the spore on insecticidal activity of δ-endotoxin of Bacillus thuringiensis against the diamondback moth, Plutella xylostella (Lepidoptera: Yponomeutidae) is not observed at late stage in bioassay

Synergism of spores from several Bacillus thuringiensis serovars and the δ-endotoxin of B. thuringiensis serovar kurstaki HD-1 and HD-73 were evaluated using two bioassay systems (artificial diet and leaf dip) against larvae of Plutella xylostella. The magnitude of the synergistic effect of the live spores from B. thuringiensis serovar japonensis strain Buibui with the δ-endotoxin of B. thuringiensis serovar kurstaki HD-1 was 120 times higher two days after feeding on the artificial diet. However, by 7 days after the treatment, the synergistic effect was lost. Thus, the synergism of live spores with the δ-endotoxin was only observed very early in the treatment. The live spores from kurstaki HD-1 or strain Buibui exhibited a 35% reduction in the killing time of P. xylostella when they were combined with the δ-endotoxin from kurstaki HD-1 or HD-73. No effects were observed when dead spores killed by gamma-radiation were combined with the δ-endotoxin. These results clearly indicated that live spores were necessary to express the synergistic effects on diamondback moth larvae.