Host plant quality affect the population dynamics and biological control of spidermites by two predators

It is well established that plant quality affects the physiology and behavior of herbivores and their natural enemies (bottom-up control). Additionally, natural enemies directly and indirectly affect herbivores and plants (top-down control). However, less is known about how bottom-up and top-down forces interact to influence population dynamics. We used a system consisting of two host plants, tomato and jack bean, varying in quality for the polyphagous two-spotted spider mite (Tetranychus urticae), and two predatory mites, Neoseiulus californicus (McGregor) and Phytoseiulus macropilis (Banks). We performed a population-dynamical experiment using the two host plants, and both predatory mite species alone or in combination as treatments. We found that different host plant quality significantly affected the dynamics of spider mite and predatory mites. From a biological control perspective, P. macropilis alone successfully controlled T. urticae on tomato, and in a few replicates on bean, whereas N. californicus failed to do so on both. Also, the combined released of N. californicus and P. macropilis did not improve biological control efficacy compared to P. macropilis alone. The impact of one predatory mite species in the growth rate of the other also differed among host plants. Phytoseiulus macropilis negatively impacted N. californicus growth rate on tomato, but not on bean, whereas N. californicus did not have an effect on P. macropilis. Our results show that a better understanding of how host plant quality affects the targeted herbivores and the selected natural enemies can be very relevant for biological control programs.