Stigmaeid-phytoseiid interactions and the impact of natural enemy complexes on plant-inhabiting mites

Interactions within the natural enemy complex of plant-inhabiting mites are of practical interest. There are frequently several predatory mite species present or available for introduction and it is difficult to tell whether a single species or combination of species should provide the most effective means of biological control. We used a simulation model to examine the interactions between predatory mites of two families, the Stigmaeidae and Phytoseiidae, representatives of which feed on phytophagous apple mites. To identify key differences in predatory behavior and life history, various stigmaeid parameters were substituted with those of phytoseiids, and changes in the efficacy of either stigmaeids alone or in combination with phytoseiids were evaluated graphically. At low prey densities stigmaeids held an advantage over phytoseiids in terms of efficacy because of their higher preference for prey eggs, higher oviposition relative to prey consumption and the ability to consume their own eggs, while at high prey densities the higher maximum predation rate of phytoseiids gave them a higher efficacy. Thus the weaker predator (the stigmaeid) is the more effective competitor at low prey densities. The model did not account for dispersal, but since stigmaeids are the weaker dispersers, the possibility of developing stability in this system is much greater than if phytoseiids were the better competitors. A combination of stigmaeids and phytoseiids was shown to have greater efficacy than either predator alone over a wide range of prey densities.