Transovarial toxicity matters: lethal and sublethal effects of hexythiazox on the two-spotted spider mite (Acari: Tetranychidae).

Abstract

The effects of hexythiazox on life-history traits and demographic parameters of Tetranychus urticae Koch (Acari: Tetranychidae) were evaluated using the age-stage two-sex life table (in fecundity-based and fertility-based variants), with emphasis on its transovarial toxicity. Hexythiazox was applied when T. urticae females were either in the preovipositional period or in the first day of oviposition. In the F0 generation bioassay, treatments with concentrations of 50, 12.5 and 3.125mg/l significantly reduced the longevity of females and their fecundity. These effects were mostly the result of mortality of treated females (18-23%) over the 24-h exposure period. Even though the net reproductive rate (R0) decreased significantly, the intrinsic rate of increase (r), finite rate of increase (λ) and doubling time (D) were not significantly different from the control. The strongest transovarial toxic effect occurred within the first 4 days following treatment, when 52-89% of the eggs laid by treated females (96% in control) hatched. Fertility was significantly reduced by concentrations of 50, 12.5, 3.125, 0.781 and 0.195mg/l. These concentrations caused significant reductions in R0 (34-54%), r (12-24%) and λ (3-5%), whereas D was extended for 0.4-0.7days. In the F1 generation bioassay, 50, 12.5, 3.125, 0.781, 0.049 and 0.012mg/l caused significant reductions in R0 (34-92%), r (10-68%) and λ (3-17%), whereas extending D for 0.3-5.6days. These effects were mostly the consequence of transovarial toxicity. Application of the fecundity-based life table underestimated population-level effects of hexythiazox on T. urticae.