Epizootic potential for aphids of different isolates of the fungus,Verticillium lecanii

Abstract

Large (6.7–8.4 μm in length) and small (3.8–6.7 μm in length) spored strains ofVerticillium lecanii (Zimm) Viegas killed similar numbers of adults of the aphids treated with aqueous spore suspensions. However, when these strains were compared by determining the proportions of progeny aphids acquiring infection from treated adults, i.e. a measure of epizootic potential, large differences emerged; only large-spored strains exhibited as strong an epizootic potential as the standard strain, 1–72, with mean progeny mortality ratios (test/standard) close to 1.0. Further experiments indicated that the speed of germination, fastest for large-spored strains, accounted for the greater epizootic potential of such strains. In addition, the mode of sporulation of large-spored strains on aphid cadavers probably also contributed to the more efficient spread of such strains. A general conclusion was reached that measurement of the effectiveness of a pathogen against a host insect must be based on many factors, in addition to virulence against a single developmental stage.