The pathogenicity of two isolates of Verticillium lecanii towards non-target invertebrates and six species of aphid was tested in bioassays. There was no evidence of infection in any of the 20 non-target invertebrates. All target aphid species were susceptible to at least one isolate. The level of susceptibility varied between aphid species, with Myzus persicae and Brachycaudus helichrysi showing the greatest susceptibility.

Studies on initial spore deposition, retention and germination on two susceptible aphid species and four resistant non-target insects revealed differences in the behaviour of the pathogen between the two groups. Spore density mm−2 was greater on susceptible insects both immediately post-inoculation and following 24 h incubation. There was no loss of spores from M. persicae over 24 h. All other species showed more than 50% spore loss in the same period. However, even after 90% spore loss, large numbers of spores remained on the bigger insects such as Agonum dorsale. Germination and germ-tube growth were possible on such resistant non-target insects, indicating that resistance to infection occurs after this stage. There was little difference in rate of germination on the two groups. However, differences in germ-tube growth were evident, with the production of longer and narrower germ-tubes on the resistant insects.