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.