Phenotypic variation in egg survival in the predatory mite Phytoseiulus persimilis under dry conditions

The predatory mite Phytoseiulus persimilis is widely used for augmentative biological control, as an effective predator of the spider mite Tetranychus urticae. However, the biocontrol efficacy of P. persimilis decreases under dry conditions. One of the reasons for this decline concerns P. persimilis’ eggs, which are sensitive to low humidity. In this study, we investigated the possibility to select for a strain of P. persimilis adapted to dry conditions. To understand the potential sources of phenotypic variation in egg survival under dry conditions, we tested the effects of genetic and environmental factors on variation in this trait. We compared egg hatching of five P. persimilis strains, under constant as well as variable humidity conditions, at 25 °C. The results show no intraspecific genetic variation among the five tested strains in egg hatching under constant and variable humidity conditions. In all five strains, less than 20% of the eggs hatched when they were exposed to constant low (60% RH) humidity conditions. However, when eggs were exposed to successive cycles of low and high humidity, a common pattern observed in the field, significantly higher hatching rates were observed. Under variable humidity conditions, more than 73% of the eggs hatched successfully, even when exposure to high humidity was limited to only 13% of the egg developmental time. Although P. persimilis eggs suffered from a high rate of water loss under constant dry conditions, they were able to compensate for this water loss when exposed to high humidity conditions for a few hours during their development. A decreased biocontrol efficacy of P. persimilis under dry conditions may be explained by a higher egg mortality when relative humidity is constantly low. Yet, when relative humidity exhibits diurnal variation, periods of high humidity may mitigate the effects of periods of low humidity during development of P. persimilis eggs.