Reproducing during Heat Waves: Influence of Juvenile and Adult Environment on Fecundity of a Pest Mite and Its Predator

SIMPLE SUMMARY: Heat waves experienced early in the lives of arthropods can affect their sensitivity to heat stress as adults. Thus, juvenile acclimation may influence fine-tuned predator–prey relationships since the two opponents respond differently to heat stress. Here, we exposed heat wave-acclimated and non-acclimated females of the predatory mite Phytoseiulus persimilis and its prey, the spider mite Tetranychus urticae, to extreme and mild heat waves and assessed their reproductive performance on bean leaves. Additionally, ovipositing prey was exposed to predator cues during heat waves. Our results showed that juvenile acclimation decreased the tendency of both species to escape from the leaves, but younger and more fecund predator females left the leaves than prey females under extreme heat waves. Reproduction in both species increased under extreme heat waves but was not affected by juvenile acclimation. Additionally, predator cues lowered the oviposition rates of prey, but the effect was marginal compared to the strong positive effects of heat waves. Our results indicate that control of spider mites by P. persimilis may become less efficient under extreme heat waves, partly because the predators suffer more than their prey from high temperatures and partly because escaping predators risk dying before they find a suitable place to feed and oviposit. ABSTRACT: The thermal history of arthropod predators and their prey may affect their reproductive performance during heat waves. Thus, a matching juvenile and adult environment should be beneficial as it enables the individuals to acclimate to extreme conditions. Prey fecundity, however, is also affected by a second stressor, namely predation risk. Here, we assessed the impact of extreme and mild heat waves on the reproductive output of acclimated (juvenile and adult heat wave conditions are matching) and non-acclimated females of the biocontrol agent Phytoseiulus persimilis, a predatory mite, and its herbivorous prey, the two-spotted spider mite Tetranychus urticae, on bean leaves. Their escape and oviposition rates and egg sizes were recorded over 10 days. Additionally, ovipositing prey females were exposed to predator cues and heat waves. Acclimation changed the escape rates and egg sizes of both species, whereas fecundity was only influenced by the adult thermal environment via increased egg numbers under extreme heat waves. Acclimation reduced predator and prey escape rates, which were higher for the predator. Pooled over acclimation, both species deposited more but smaller eggs under extreme heat waves. Acclimation dampened this effect in prey eggs, whereas acclimation resulted in smaller female eggs of the predator. Prey deposited larger male and female eggs. Predator cues reduced prey oviposition, but the effect was small compared to the large increase gained under extreme heat waves. We argue that the success of predators in controlling spider mites during heat waves mainly depends on the fates of escaping predators. A permanent absence of predators may result in the numerical dominance of prey.