Depending on fluctuations in pest densities, a mixed diet can enhance thrips control by the generalist predator Amblyseius swirskii

Prey species can interact indirectly via a shared predator population, and these interactions can be either negative or positive, depending on the time scale and the type of dynamics. For example, theory predicts that strong population fluctuations can result in long-term positive effects between prey. Moreover, the dynamics and cycling of both prey populations may result in suboptimal diet mixtures for the predator, which can affect their reproduction and population dynamics. It is important to consider these indirect interactions for biological control of two pest species by a shared predator, because they can either disrupt or enhance control. We studied the dynamics of two major pest species on greenhouse cucumber: western flower thrips, Frankliniella occidentalis (Pergande), and greenhouse whitefly, Trialeurodes vaporariorum (Westwood), both of which can be controlled by the predatory mite species Amblyseius swirskii Athias-Henriot. We investigated how the presence and fluctuations in densities of whiteflies affected the biological control of thrips. In a first greenhouse experiment, strong population cycling was induced by releasing high densities of both pest species simultaneously. In a second experiment, this was prevented by releasing lower numbers of whiteflies over several weeks, resulting in a continuous presence of whitefly eggs as food for predatory mites. Population fluctuations resulted in a positive interaction; a strong increase in the second whitefly generation significantly delayed the suppression of thrips. In the second experiment, the opposite was found; repeated releases of whiteflies had a negative effect on thrips densities through a strong numerical response of the predators. Thus, this study demonstrates the potential for both positive and negative predator-mediated interactions between pests, which calls for caution in biological control of more than one pest species by generalist predators. The results of this study may help predict when and how alternative prey will affect pest-natural enemy dynamics.