Should I stay or should I go: the Milker-Killer dilemma in a predatory mite

The predatory mite Phytoseiulus persimilis is a biological control agent commonly used to suppress spider mite (Tetranychus urticae) infestation in crops and ornamentals in greenhouses and fields. Although much is known about this predator and its ability to control spider mite populations under the economic threshold, there is a gap in the knowledge regarding its dispersal behaviour and effects of dispersal on prey exploitation. In theory, predator dispersal reduces predation pressure and increases local prey population growth, yielding a delayed benefit of more prey to consume in the future. However, such prudent predators (“Milkers”) are vulnerable to cheaters that do not disperse as long as there is prey (“Killers”). Milker strategies are not evolutionarily stable if local populations are well connected by dispersal. This is because predators with a Milker strategy have a longer interaction period with their prey, which leads to longer exposure to subsequent predator invasions in their local patch. Such invaders may exploit the patch in their own interest as Killers, which in turn selects against Milkers. However, more isolated sites favour the evolution of Milkers, because of a low probability of co-invasion of Milkers and Killers in the same local prey patch. Here, we present experiments using native populations of P. persimilis collected in Turkey and Sicily in 2013 and 2014, respectively. We measured aerial dispersal and population dynamics in a system of detached rose leaves, spider mites and P. persimilis in small wind tunnels. Our experiments show that there is variation in the way predators exploit their food source and in the way that they disperse. We discuss these results in light of the theory.