Abstract: The predatory mite Phytoseiulus persimilis is a specialized predator of Tetranychusspecies and is used worldwide as a biological control agent of the spider mite Tetranychusurticae. Despite its great predatory abilities, the biocontrol efficacy of P. persimilis decreasesunder dry conditions. It has been proven that T. urticae populations have a higher intrinsic rateof increase at low relative humidity and are therefore more difficult to control. However, theeffects of drought on P. persimilis populations have not been investigated in detail yet. Tocontribute to the improvement of spider mite biocontrol on crops grown in arid environments,we focused on the effects of different humidity regimes on P. persimilis adult females and theireggs. We evaluated the survival and oviposition rates of P. persimilis females under constantand variable humidity regimes, as well as the hatching rate of their eggs under low humidity.We observed an intriguing phenomenon: while the eggs laid by P. persimilis females exposedto constant high humidity did not hatch at low humidity, P. persimilis females exposed toconstant low humidity started laying drought-resistant eggs after 24 hours of exposure.Additionally, both the survival and the oviposition rates of P. persimilis females were affectedby the different humidity regimes tested. These results provide essential information about theeffects of drought on P. persimilis, and help to define new research questions, towards theimprovement of spider mite biocontrol in arid environments.