Pathogenicity of Verticillium lecanii to different developmental stages of the silverleaf whitefly, Bemisia argentifolii

Thirty-five strains of Verticillium lecanii which originated from different hosts and geographical locations were tested as potential biocontrol agents against silverleaf whitefly, Bemisia argentifolii Bellows & Perring. All strains were tested for their pathogenicity to third-instar nymphs. Several isolates which exhibited high pathogenicity to nymphs were also tested against eggs, pupae and adults of B. argentifolii. Eggs were found to be immune to infection, but mortality of hatching nymphs reached 95-98%. The rate of hatching nymphs' infection depended on the age at which the eggs were inoculated and the strain's virulence. Mortality of nymphs recorded on day 4 after inoculation varied from 0.5 +/- 0.3% to 83 +/- 2.4%; that of the control ranged from 2.5% to 10.2%. The most virulent strains, with LT50 ranging between 3.2 and 3.8 days, were isolated from aphids in Israel and probably have a similar origin. The pathogenicity of V. lecanii strains to pupae 6 days after inoculation varied between 59 +/- 12.1 % and 72.5 +/- 13.1 %, as compared with natural mortality of 13.5 +/- 4%. The maximum adult mortality caused by V. lecanii strains was between 34.1 +/- 5.1% and 52.6+/- 3.8%.