Virulence and horizontal transmission of by the adults of the greenhouse whitefly (Hemiptera: Aleyrodidae) and the efficacy of oil formulations against its nymphs.

The pathogenicity of dry conidia and fungal suspensions of 16 entomopathogenic fungal isolates (10 and six ) was evaluated against adults and second instar nymphs of the greenhouse whitefly, respectively. All the tested isolates were pathogenic to and caused mortality of 45-93% against the adults and 24-89% against the nymphs. However, strains showed higher virulence to both developmental stages as compared to strains. The three most virulent isolates that caused high mortalities in adults were ICIPE 18, ICIPE 62 and ICIPE 69, with cumulative mortalities of 82, 91 and 93%, and median lethal times (LT) of 5.20, 5.05 and 4.78 days, respectively. These isolates were further assessed for spore acquisition and retention by the adult insects at 0, 24, 48 and 72 h after exposure to dry conidia spores. There was no significant difference among isolates on their acquisition by the insects, although the effect of time on the number of spores retained by each insect was significant. For ICIPE 62 and ICIPE 69, spore number was significantly higher immediately after exposure at 0 h than at 24, 48 and 72 h, whereas for ICIPE 18, the spore number remained constant for all the days. The infected "donor" insects were able to horizontally transmit the acquired spores to uninfected "recipient" insects causing high mortality rates in both donor and recipient groups. ICIPE 7, ICIPE 18 and ICIPE 62 were the most virulent isolates against the nymphs in aqueous formulation during the first screening with >80% mortality. However, in 2% (v/v) oil formulations at 1 × 10 conidia/ml, canola formulated ICIPE 62, ICIPE 18 and olive formulated ICIPE 18 were the most effective, resulting in 87.8, 88.1 and 99.4% nymphal mortalities respectively and with lower LT. Oil formulations significantly enhanced the efficacy and virulence of the isolates against the nymphs compared to aqueous formulations.