Time-dose-mortality modelling and virulence indices for six strains of Verticillium lecanii against sweetpotato whitefly, Bemisia tabaci (Gennadius)

Bioassays of six strains of Verticillium lecanii (Zimmermann) Viégas were conducted with the sweet potato whitefly, Bemisia tabaci. For inoculation, batches of third‐instar whitefly nymphs on sweetpotato seedlings were immersed in conidial suspensions of five dosages at concentrations from 10³ to 10⁷ conidia/ml. Each dosage was used to inoculate 150–250 nymphs. The nymphs were maintained at 25°C, under 95% RH, photo phase of 16 : 8 (L : D) and observed daily for mortality. The resulting data were analysed over 8 days by a complementary log‐log (CLL) time‐dose‐mortality model, based on the Hosmer–Lemeshow test, analysing the time‐dose trends for five concentrations of six strains of V. lecanii simultaneously. The parameters from the model were used to estimate the virulence indices (the values of LC₅₀) of six strains against B. tabaci. Based on the time‐dose‐mortality relationships fitted and the virulence indices, the virulence of the six strains of V. lecanii for B. tabai was compared. Results indicated that the strain Vl6063 imported from Canada and the domestic strains V3450 and Vp28 derived from B. tabaci and a scale insect, respectively, were more virulent than the others with LC₅₀ values of 2.57 × 10⁵, 6.03 × 10⁵ and 6.03 × 10⁵, respectively.