Abstract: 【Objective】The objective of this study is to explore the effect of extracellular enzymes and their virulence during entomopathogenic fungi degrading the integument of scale insects and to provide evidence for biological control by applying entomopathogenic fungi. 【Method】 The strains of the entomopathogenic fungi, Lecanicillium lecanii V3.4505, V3.4504, L. fungicola HEB02, and Fusarium incarnatum-equiseti HEB01 were used and the two species of scale insects, Ceroplastes japonicus Green and Rhodococcus sariuoni Borchsenius, as the target were studied. The cuticle of the two scale insects was used as the sole carbon source in the medium for fungal culturing. The activities of four extracellular enzymes, including lipase, protease (Pr1), chitinase, and N-acetyl-D-glucosaminidase (NAGase), produced by the four strains were determined, and based on this, the function of the extracelluar enzymes during the strains penetrating the integument of scale insects was analyzed. Meanwhile, the cumulative mortalities of the two scale insects infected with the four strains for eight days were assayed to evaluate the virulence of the strains and the extracellular enzymes. 【Result】 The activities of the four enzymes all changed significantly during the scale insect integument-degrading process, their changing trend all exhibited rising in the first few days and falling in the later. The activity peak of lipase appeared earliest that was at two days after inoculation, and the lipase activity of the strains cultured on the cuticular medium of C. japonicus was obviously higher than that on R. sariuoni. The activity peak of Pr1 came after 3-4 d while the activity peak of chitinase and NAGase appeared at 6 d and 4-5 d, respectively. Among the four strains, V3.4505 strain caused the highest mortality rate of 73% for C. japonicus and 81% for R. sariuoni, respectively, and that showed significant difference with other two strains, HEB02 and HEB01. The average values of Pr1 activity of four strains were significantly related to the cumulative mortality of the two scale insects, and the linear equations were y=0.082x+5.822 (R²=0.823) for C. japonicas and y=0.119x+14.75 (R²=0.764) for R. sariuoni. Similarly, the average chitinase activity of four strains were also significantly related to the cumulative mortality of the two scale insects, the linear equations were y=-0.148x+15.89 (R²=0.645) for C. japonicas and y=0.095x+10.46 (R²=0.762) for R. sariuoni. 【Conclusion】 The four extracellular enzymes worked together to degrade the wax and the integument of the scale insects in the infecting process. The lipase degraded the wax on the scale insects and its peak came earliest, and its activity of the four strains cultured on the cuticular medium of C. japonicus was obviously higher than that on the R. sariuoni because C. japonicus has a thicker wax layer. Among other three enzymes, Pr1 worked first to degrade the protein in the procuticle of scale insects then chitinase and NAGase worked together to degrade the chitin in the integument. The activity peak and level of each enzyme were corresponding to the distribution and content of its substrate in the integument of scale insects. The correlation analysis on the enzyme activities of the strains and mortality of the scale insects indicated that Pr1 and chitinase should be used as a virulence indication for the strains. Among the four strains, L. lecanii V3.4505 showed better pathogenic characteristics, which means this strain possesses higher virulence against scale insects and it can be considered as a better pathogenic fungus for biological control.

Key words: entomopathogenic fungi, extracellular enzymes, Ceroplastes japonicus Green, Rhodococcus sariuoni Borchsenius, integument, virulence