The time-dose-mortality model of a Paecilomyces fumosoroseus isolate on the diamondback moth,Plutella xylostella

The diamondback moth, Plutella xylostella, is an important insect pest of cruciferous vegetables in south China and has developed serious resistance to insecticides used. In an effort to search for alternative control methods, the pathogenicity of an isolate (SCAU-PFCF01) of the fungus Paecilomyces fumosoroseus, originally derived from Coptotermes formosanus, against the diamondback moth larvae, was studied in the laboratory by immersing larvae of different instars into serial concentrations of 10~3-10~7 conidia/mL of the fungus. The mortality of the diamondback moth caused by infection of P. fumosoroseus increased with the concentration of conidial suspension and the time after treatment. The highest concentration (10~7 conidias/mL) treatment caused 96%, 85%, and 80% mortality for 2nd, 3rd, and 4th instar larvae on day 7, respectively. A time-dose-mortality model was used to analyze the bioassay data and the model fitted the data well, resulting in parameters for estimating the time and dose effects. The pathogenicity of P. fumosoroseus to the diamondback moth differed among various larval stages. The susceptibility of the diamondback moth larvae to P. fumosoroseus decreased in the sequence of 2nd instar, 3rd instar, and 4th instar. The estimated LC_ 50 values of 2nd instar on day 7 after treatment, 3rd instar on day 5 after treatment and 4th instar on day 4 after treatment were 1.17×10~4, 1.44×10~4, and 5.21×10~4 conidia/mL, respectively; while LC_ 90 of 2nd, 3rd, 4th instar larvae were 1.98×10~6, 3.82×10~7, and 1.29×10~8 conidia/mL, respectively. The median lethal time of P. fumosoroseus to the diamondback moth larvae differed at various concentrations. Values of LT_ 50 of all tested instars were shortened with concentration. The LT_ 50 values of 2nd, 3rd, and 4th instar were 3.16-1.72 days, 3.21-1.83 days and 3.69-2.04 days, respectively, in the range of 1×10~5-1×10~7 conidia/mL. The results suggested the potential of the isolate SCAU-PFCF01 for use in microbial control of P. xylostella.