| In the studies of biological control of aphids, the virulence ofBeauveria bassiana to aphids and its utilization have received considerable attention in recent years. However, most studies up to date have concentrated on the evaluation of virulence of B. bassiana to aphid adults, the virulence to the nymphal stages has been rarely studied, and the potential of B. bassiana as a biological control agent of aphids has not been examined quantitatively at the population level.In this study, we chose a systemic insecticide, imidacloprid, an isolate of B. bassiana highly virulent to aphids, BBSG8702, and the green peach aphid, Myzuspersicae, as the target pest, and conducted the research in the following three interconnected steps. First, the effect of B. bassiana on the nymphal development and survival and the subsequent fecundity of these inoculated nymphs after they had become adults was examined, in order to analyse the potential of the fungus as a control agent for populations of the green peach aphid; next, the lethal and sublethal effects of imidacloprid on the green peach aphid were tested, to provide the necessary data for the combined utilization of imidacloprid and B. bassiana; and finally, the population increase and the change of population structure were investigated when B. bassiana alone or B. bassiana combined with imibacloprid were applied. Our purpose was to further understand the potential of B. bassiana as a biological control agent of aphids at the population level, to find out new ways for enhancing the potential of the fungus, and to provide information for developing new aphicide formulations with B. bassiana.In the first part of the study, when first or fourth instar nymphs of the green peach aphid were inoculated with B. bassiana, the durations to the first moult following inoculation were slightly decreased, and the levels of cumulative mortality at the 12l day after inoculation reached 7.2% and 58.7% respectively. Deaths of the infected aphids inoculated with the fungus at the fourth instar occurred after they had become adults, and the peaks of mortality occurred at 7-10th day after inoculation. When the nymphs inoculated with the fungus at the fourth instar had developed to adults, the fecundity of the adults infected by the fungus was slightly decreased, while the fecundity of uninfected adults remained unaffected, compared to that of the adults in the control. When the nymphs inoculated with the fungus at the first instar had developed to adults, their fecundity did not differ from that of the aphids in the control.In the second part of the study, the test results showed that O.lu.g/ml imidacloprid significantly increased the development time of the first instar nymphs of aphid, and when these treated nymphs had developed to adults, their fecundity was significantly lower than that of the adults in the control. Although 2ug/ml and 4u.g/ml imidacloprid applied to nymphs at the fourth instar did not increase the development time of that instar, the fecundity of these treated aphids after they developed to adults was much lower than that of the adults in the control. When B. bassiana and 0.4(ig/ml imidacloprid were both applied to nymphs at the second instar, the mortality induced by the fungus was significantly increased. However, when B. bassiana and 0.5|ig/ml imidacloprid were bothapplied to nymphs at the third instar, the mortality induced by the fungus did not increase.In the third part of the study with experimental populations of the green peach aphid, when the cohorts were started with apterous adults and B. bassiana was applied alone, at the sixth day after inoculation the number of adults in the treatment with the fungus was significantly lower than that in the control. Although the numbers of individual instars did not differ significantly between the control and the treatment, the total number of aphids in the treatment with the fungus was lower than that of the control. However, at the 13th day after inoculation, the total number of aphids in the treatment ha... |
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