Host plants influence susceptibility of whitefly Bemisia tabaci (Hemiptera: Aleyrodidae) to the entomopathogenic fungus Isaria fumosorosea (Hypocreales: Cordycipitaceae)

We quantified the tritrophic effect of host plant on the susceptibility of the sweetpotato whitefly Bemisia tabaci (Genn.) to a fungal pathogen in the laboratory. Second-instar whiteflies reared on cucumber, eggplant, tomato and bean plants for six generations were exposed to conidial suspensions of Isaria fumosorosea isolate IF-1106. Our results did not detect differences in response (proportional survival or median lethal time, LT50 days) among insect populations derived from different plants that were treated with 10(7) conidia/ml. However, at concentrations <= 5x10(6) conidia/ml, whiteflies reared on bean and tomato died significantly more quickly (i.e. LT50 of 4-5 days) compared with cucumber and eggplant reared populations (5-7 days). Bean and tomato-reared populations were also more susceptible to mycosis (LC50 approximate to 6 x 10(5) conidia/ml) compared with those reared on cucumber (1.9 x 10(6) conidia/ml) and eggplant (1.5 x 10(6) conidia/ml). A separate study confirmed that this differential response of whitefly populations to I. fumosorosea was not explained by differences in deposition rate of conidia on leaf surfaces (i.e. a dosage effect). Our findings show that host plants affect the pathogenicity and virulence of a herbivore pathogen, but depend on the rate of exposure (inoculum) applied.