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

Thirty-five strains of Verticillium lecanii which originated from different hosts and geographical locations were tested as potential biocontrol agent s 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 aga inst eggs, pupae and adults of B. argentifolii. Eggs were found to be immun e to infection, but mortality of hatching nymphs reached 95-98%. The rate o f hatching nymphs' infection depended on the age at which the eggs were ino culated and the strain's virulence. Mortality of nymphs recorded on day 4 a fter inoculation varied from 0.5 +/- 0.3% to 83 +/- 2.4%; that of the contr ol 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 d ays 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%.