Parasitoid coexistence: Linking spatial field patterns with mechanism

Coexistence of resource specialists can be maintained through various spatial mechanisms, each potentially inducing different resource dynamics. We used observational data to suggest plausible spatial coexistence mechanisms for Aphytis melinus and Encarsia perniciosi, two parasitoids of California red scale. Aphytis consistently produced more progeny from scale insects on leaves, and Encarsia produced progeny equally from scale insects on leaves and stems. To investigate the mechanism inducing this pattern, we performed field experiments to test two possible explanations. In the absence of substrate choice and at a given abundance of scale, the number of progeny produced by each species was the same on stems as on leaves. When given a choice, Encarsia had no substrate preference, while Aphytis preferred scale insects on leaves, producing results qualitatively similar to the field observations. Ecological theory suggests that this form of habitat preference may facilitate coexistence. Although this mechanism of coexistence is clearly compatible with biological control by multiple natural enemies in the red scale system, theory dealing with this type of interaction has not thoroughly examined the predictions for resource dynamics.