Immigration and the Dynamics of a Predator-Prey Interaction in Biological Control

1. A field experiment was conducted to determine if a predator-prey interaction in biological control could be best described by single-population or metapopulation dynamics. 2. A case of successful biological control on apple trees, the phytophagous mite, Panonychus ulmi, and its predator, Typhlodromus pyri was selected, and two factors were manipulated, the number of potentially interacting populations (spatial arrangement of trees) and the initial density of the prey. 3. Spatial arrangement of trees significantly affected immigration, with higher immigration and higher turnover rates where there were more potentially interacting populations. 4. Densities were higher and persistence was greater where immigration rates were higher. 5. Immigration slowed the tendency for natural and augmented density populations to converge to similar densities. 6. The patterns observed were consistent with the hypothesis that T. pyri can sometimes drive P. ulmi populations to extinction on individual small apple trees. 7. Increasing the number of interacting populations and thus immigration rates slowed down the tendency to extinction, but it is not clear whether at the spatial scale of an orchard the ultimate outcome would still be extinction of P. ulmi or if persistence is the rule.