Abstract: In order to provide theoretical proofs for the application of cationic adjuvants, the synergism of six cationic adjuvants to avermectin on 3rd-instar larvae of the diamondback moth, Plutella xylostella, were studied by dipping method and contact and stomach method in the laboratory. The effect of adjuvants on physical properties of avermectin dilutions and activities of three protective enzymes of P. xylostella larvae were also tested. The results showed that all cationic adjuvants had no direct biological activity alone under the concentration of 400 mg/L, but they could dramatically improved avermectin bioefficacy against P. xylostella. The synergism ability with dipping method was more obvious than with contact and stomach method. Among the six adjuvants, copper abietate, 1427 and 412103 had 3.71, 2.82 and 2.72-fold synergism ratio, respectively. Although all the six adjuvants could decrease the surface tension and contact angle of water dilutions of avermectin EW and increase their depositing rates on the leaf of cabbage, they showed no significant difference either in different treatments or in different concentrations of the same adjuvant. These results suggested that the change of properties of dilutions by cationic adjuvant were not the main cause of synergism. The activities of peroxidases (POD) and superoxide dismutase (SOD) increased dramatically 2 h after being treated by avermectin dilutions which were added with copper abietate, 1427 and 412103. This implied that the three adjuvants had higher permeabilities on P. xylostella larvae.

Key words: Plutella xylostella, avermectin, cationic adjuvant, synergism, protective enzymes