关键词: 球孢白僵菌, 桃蚜, 若蚜, 脱皮, 毒力

Abstract: The effects of moulting by nymphs of the green peach aphid, Myzus persicae on the virulence of Beauveria bassiana BBSG8702, was bioassayed in the laboratory. At 21℃ and with a conidial suspension of 1×10⁶/mL, aphids inoculated with the fungus at the 1st, 2nd, 3rd or 4thinstar suffered 10.1%, 2.1%, 3.1% and 40.2% cumulative mortality respectively, significantly lower than that observed in the aphids inoculated at the adult stage (98.4%). Among the individuals that eventually died of the infection following inoculation at the 1st, 2nd, 3rd or 4th instar, 8.1%, 50.0%, 44.4% and 98.0% of them developed to adulthood and produced healthy offspring prior to death. Mortality of the aphids inoculated with the fungus at the nymphal stages was closely related to the time interval between inoculation and first moult post-inoculatIon; the earlier the moult, the lower the mortality. A small proportion of infected nymphs, especially those infected at early instars, died before the next moult post-inoculation. Detailed observations were made on the development, survival and reproduction of aphids inoculated with the fungus during the very arly stage of the 1st or the 4th instar, stages when nymphs were most susceptible to fungal infection. Compared to the duration of normal nymphal development, nymphs inoculated with the fungus had slightly shorter (<5%) development times for the instar in which they were inoculated. Pre-adult mortality caused by the infection was only 4.1% and 0 when the aphids were inoculated with the fungus at the early 1st and 4th instar respectively. Moreover, the reproductive rates of those Infected aphids that eventually died at the adult stage due to the fungus during the first 8 days of their adult life were similar to controls not inoculated. These findings demonstrate that moulting by this aphid, especially multiple moulting by the aphids following infection with B. bassiana at an early nymphal stage, Is an effective mechanism for avoiding or delaying infection by this fungus, and thereby reduces the virulence of the fungus.

Key words: Beauveria bassiana, Myzus persicae, nymphs, moulting, virulence