Asymmetrical thermal responses to heat stress in Phytoseiulus persimilis and its prey Tetranychus urticae lower the predation success

Abstract: The predatory mite Phytoseiulus persimilis is used globally to control spider mites
(Tetranychus urticae) both in the field and greenhouses since several decades. Due to its high
capacity for population increase, the predator suppresses spider mites very efficiently under
optimal thermal conditions. However, when developing under extreme heat waves, the female predator-prey size ratios shifted in favor of prey, which may result in reduced predation success. Alternatively, the predator might be more aggressive under heat stress to compensate for water loss via increased predation. We evaluated these assumptions by rearing the predator and prey separately under mild (daily Tmax: 32 °C) or extreme heat waves (daily Tmax: 38 °C) from egg to adulthood, placed then single female predator and prey couples on small bean leaf discs, and exposed them to constant temperatures (32 °C, or 38 °C) corresponding to the Tmax values during their juvenile development. The mites were videotaped over 90 min and the heat stress effects on prey survival and maximum velocities of predator and prey were analyzed. Prey survival was significantly higher under extreme heat stress (38 °C). Furthermore, maximum velocity of prey was not affected by heat and time, whereas maximum velocity of the predator decreased over time. Compared to prey, the maximum velocity of the predator was only higher within the first 15 min under 38 °C, but was then considerably lower after 45 min. These species-specific shifts in velocity increased the ability of prey to escape an attacking predator. Consequently, we argue that these behavioral modifications in conjunction with morphological shifts (prey body size increased, predator body size decreased under extreme heat waves) were responsible for the low predation success under extreme heat stress. Whether these effects also endanger the biological spider mite control under heat stress in crops, should be verified in population experiments.