Abstract: In order to find a faster and more effective method to use entomogenous fungus Isaria javanicus to control the diamondback moth, Plutella xylostella (L.), bioassays were carried out to clarify if destruxins can influence mycelia growth production and germination of conidia spores of the strain SP053, and to evaluate their joint toxicity against the 2nd instar larvae of P. xylostella. The results indicated that destruxins had no significant effect (P>0.05) on strain growth and conidia production and germination. However, there was certain synergism between destruxins and conidia spores of the  strain SP053. The mixtures of S50-CD100 (meaning the mixture of conidia spores of the concentration 50×10⁵/mL and crude destruxin of the concentration 100 mg/L; other mixtures indicated in the same way), S25-CD100, S25-CD50 and S12.5-CD100 showed distinct synergism. The S25-CD100 combination showed the highest effectiveness with the cooperative virulence index (c.f.) reaching 52.31 and 31.07 at 48 and 72 h after treatment, respectively. This synergism was verified by virulence bioassay as well. The mixtures contained crude destruxin (CD) from 25 to 100 mg/L had lower LC₅₀ values, for example, the LC₅₀ values of the mixture SP053-CD100 (containing CD 100 mg/L) at 48 and 72 h were 17.45×10⁵ and 10.55 ×10⁵ spores/mL, respectively; meanwhile, those of the control (without destruxins) were >50×10⁵ and 35.85 ×10⁵ spores/mL, respectively. It is so concluded that the mixing of destruxins and the I. javanicus strain SP053 has synergism against P. xylostella. This result may be used to improve biocontrol of the pest.

Key words: Destruxins, Isaria javanicus, Plutella xylostella, joint toxicity, synergistic activity