The provision of resource subsidies, such as flowers, may enhance the local abundance or performance of natural enemies and thereby improve “top-down” control of pests. A potential dilemma arises when both natural enemies and pests forage on the provided resource subsidies. Thus, the provision of selective resource subsidies may have major consequences for the population dynamics of predator-prey and parasitoid-host systems. However, there is a hierarchy of levels at which flowers may selectively favour the natural enemy. Plant availability only to natural enemy may be achieved through: (1) floral morphology, (2) nectar and pollen quality, (3) floral attractiveness and (4) insect morphology. A relative advantage to the natural enemy may be provided by: (5) there is some pest fitness improvement but greater for the natural enemy, (6) the natural enemy benefits from improved-quality prey/host more than prey/host does, (7) third trophic level benefits more than the fourth and (8) sex-ratio changes. In addition, resource subsidies may have selective effects (9) within guilds, (10) within species and (11) between fecundity/longevity improvements. In general, conservation biological control approaches have to consider this hierarchy of ways in which the provision of resource subsidies might affect the population dynamics of pest/natural-enemy systems in order to favour natural enemies more than pests.