Abstract: The synergistic effects of Pseudaletia unipuncta granulosis virus (PuGV-Ps) on Bacillus thuringiensis (Bt) were tested by bio-assays employing larvae of the diamondback moth Plutella xylostella. The results indicated that the co-toxicity coefficients (CTC) of Bt combined with PuGV-Ps in different ratios were diverse from 105.3 to 195.0, showing a positive synergistic effect of PuGV-Ps on Bt. Among the mixtures, the most significant effect was in the 4∶1 ratio of Bt and PuGV-Ps, in which the LC₅₀ was 0.039 mg/mL. When environmental temperature was low (16℃ and 20℃), the synergistic effects were statistically significant, while there were no differences under temperatures of 28℃ and 32℃ compared with the treatment of feeding the insects with Bt alone. pH value influenced the synergistic effect gravely, which was elevated along with increase of pH value. In higher pH value of 8 and 9, PuGV-Ps elevated mortalities of P. xylostella larvae by Bt up to 16.67% and 23.33%, respectively. The co-effects of Bt and PuGV-Ps were also varied along with the larval age. Mortalities of the 2nd and 3rd instar larvae increased by 50% and 30.31%, respectively, in the treatment of Bt+PuGV-Ps compared with that of Bt only, but no significant synergistic effects were found in tests with the 1st and 4th instar larvae. Much higher synergistic effect was observed in treatments with oral inoculation of PuGV-Ps 2 h prior to Bt treatment, and the mortality 48 h after treatment increased 66.67% compared with the treatment of oral inoculation with Bt and PuGV-Ps simultaneously. SDS-PAGE analysis showed that in vitro the δ-endotoxin with molecular weight about 130 kD could be degradated by PuGV-Ps into 47 kD, 60 kD and 61 kD activated toxins that resisted further degradation.

Key words: Pseudaletia unipuncta granulosis virus, Bacillus thuringiensis, synergistic effect, Bt toxin, Plutella xylostella