Abstract: 【Aim】 Based on our previous screening tests, this study aims to compare the synergism of surfactants on the infection of Isaria fumosoroseus strain PF904 on the 3rd instar larvae of Plutella xylostella， and to screen the suitable adjuvant to improve the control efficacy of I. fumosoroseus PF904. 【Methods】 After the 3rd instar larvae of P. xylostella were sprayed with the spore suspension of I. fumosoroseus PF904 added with the surfactants PEG-12 dimethicone (OFX-0193) (final concentration 125 mg/L) and naphthalenesulfonicacid, dibutyl-, sodium salt (Nekal) (final concentration 250 mg/L), respectively, the number of attached conidia of I. fumosoroseus PF904 on the larval body surface and the infection rate and pathogenicity of I. fumosoroseus PF904 against the larvae were determined by observation with scanning electron microscopy and laboratory pathogenicity test. 【Results】 Adding 250 mg/L Nekal and 125 mg/L OFX-0193, significantly increased the number of attached conidia of I. fumosoroseus PF904 at various structural areas on the body surface of the 3rd instar larvae of P. xylostella. The numbers of conidia attached to spinous structural area, gentle structural area and acanthoid structural area on the body surface of the 3rd instar larvae of P. xylostella exposed to the spore suspension of I. fumosoroseus PF904 with 250 mg/L Nekal were 2.19, 1.78 and 1.94 times as high as that of the control (spore suspension added with water), respectively, indicating that Nekal increases the adhesion rate of I. fumosoroseus PF904 spores. Adding 250 mg/L Nekal and 125 mg/L OFX-0193, significantly reduced the conidial germination time, promoted the formation of appressorium and increased the infection rate of I. fumosoroseus PF904. The results of laboratory pathogenicity test showed that adding 250 mg/L Nekal and 125 mg/L OFX-0193, significantly increased the pathogenicity of the spore suspension of I. fumosoroseus PF904 against the 3rd instar larvae of P. xylostella. After exposure to the spore suspension of I. fumosoroseus PF904 with 250 mg/L Nekal, the corrected mortality rate of the 3rd instar larvae of P. xylostella was 79.2% and the median lethal time (LT₅₀) was reduced to 2.71 d, indicating that Nekal has a significant synergistic effect. 【Conclusion】 Nekal at the concentration of 250 mg/L is a suitable adjuvant for I. fumosoroseus PF904 to improve its control efficacy against P. xylostella larvae.

Key words: Isaria fumosoroseus, surfactant, scanning electron microscopy, number of attached conidia, pathogenicity