Single vs. multiple introduction in biological control: the roles of parasitoid efficiency, antagonism and niche overlap.
Theoretical studies have presented conflicting conclusions about the value of introducing multiple parasitoid species to control insect pests, depending on the presence and form of niche separation in the model. We investigated the impacts of multiple species introduction on control using a modelling approach. We started with a discrete-time model of a pest, a primary parasitoid and a parasitoid that can interact antagonistically with the primary parasitoid as well as the pest. Implicit niche separation between parasitoids occurs via aggregated encounters with pests. We subsequently modified the simple model to include explicit niche separation and a refuge from parasitism, allowing the assumption of implicit niche separation to be relaxed. Multiple introduction is deemed beneficial if equilibrium pest densities are lower after the release of the interactive parasitoid. The outcome of biological control under the simple model depends largely on the search efficiency of the primary parasitoid. The primary parasitoid can co-exist with an antagonistic interactive parasitoid, and multiple introduction is beneficial provided that the level of antagonism by the interactive parasitoid is not too high. In the more complex model with explicit niche separation, the ratio of primary search efficiencies of the two parasitoids, niche occupancy and degree of aggregation are shown to be important predictors of the outcome of multiple introduction. We demonstrate the importance of refuge breaking by an additional parasitoid as a compelling reason for multiple introduction. In addition, antagonistic interactions between parasitoids can be mediated by overall efficiency and explicit niche separation. For both implicit and explicit niche separation scenarios, the interactive parasitoid was excluded at high levels of niche overlap. Minimum pest densities with a three-species equilibrium were similar between the two scenarios. Synthesis and applications. After consideration of explicit niche separation in a two parasitoid-one pest system, we conclude that multiple introduction is often a sound strategy in biological control, despite potential antagonistic interactions from competition, cleptoparasitism or facultative hyperparasitism. In selecting parasitoids for introduction, practitioners should evaluate the potential for niche separation between parasitoids and the overall efficiency of each parasitoid (primary search efficiency and aggregation of search) rather than dismiss a biological control candidate because of moderate antagonistic interactions.