Abstract: A Pieris brassicae derived Zoophthora radicans isolate, R₀, and its host-passage isolates R₁-R₅, obtained by repeatedly passing through Plutella xylostella larvae, were bioassayed against the second instar larvae of the new host in the laboratory. The host-passage isolates tended to exhibit higher infectivity to the new host than the original isolate as the number of host passages increased. Based on the time-concentration-mortality modeling, the LC₂₀ (a lethal concentration to cause 20% mortality) on days 1-6 after exposure to spore showers was estimated as 14.7, 14.5, 9.0, 7.1, 6.0 and 5.5 spores/mm² respectively for R₀; 9.6, 5.0, 4.2, 3.6, 3.1 and 3.0 spores/mm² for R₁; 4.6, 2.9, 2.8, 2.5, 24 and 2.2 spores/mm² for R₃; and 5.2, 3.7, 3.2, 2.8, 2.6 and 2.6 sproes/mm² for R₅ The phenoloxidase activities were significantly lower in the larval hemolymph stimulated by the infection of the host-passage isolates. There was also a positive correlation between the log₁₀ (LC₂₀) and the detected phenoloxidase activities among the isolates (0.852<0.95). Apparently, the increased infectivity to the new host may result from an ability of the original Z. radicans isolate to avoid or overcome its immune defense in the course of the repeated new host passages.

Key words: Zoophthora radicans, Plutella xylostella, host passage, phenoloxidase activity, infectivity