Modeling and biological implication of time-dose-mortality data for the entomophthoralean fungus, zoophthora anhuiensis, on the green peach aphid myzus persicae

The entomophthoralean fungus Zoophthora anhuiensis frequently causes epizootics in populations of the green peach aphid, Myzus persicae, infesting cruciferous crops in the middle and lower regions of the Changjiang River, China, during late autumn and early winter. In this study, a bioassay was conducted by exposing aphids on detached leaves to varying doses of Z. anhuiensis conidia discharged from in vitro cultures. Ten doses (1.5-198 conidia/mm2) were used, with each dose including 64-120 aphids. The aphids were maintained at 18 degreesC under a photophase of 12:12 (L:D) and observed daily for mortality. The resulting time-dose-mortality data fitted well to a conditional mortality probability model based on the Hosmer-Lemeshow test (C = 9.52, df = 8, P = 0.70), resulting in a cumulative mortality probability model. The parameters from the latter model were used to estimate the lethal dose (LD50) and time (LT50) for the fungal species against the pest. The LD50 values were 87, 44, and 34 conidia/mm2 on days 5, 6, and 7 after exposure, respectively. The estimates of the LT50 values decreased from 6.7 days at 37 conidia/mm2 to 4.5 days at 198 conidia/mm2. The results indicate that Z. anhuiensis could be a promising aphid pathogen for microbial control competitive with other Zoophthora species. The model is recommended for use in the analysis of time-dose-mortality data for fungus-insect associations. The biological implications of the parameters and the advantages of the modeling technique over the conventional method of probit analysis are discussed. Copyright 1998 Academic Press.