| Coexistence of resource specialists on a shared, limited, resource is important in both ecological theory and biological control. Ecological theory predicts that competition between two resource specialist species for a limited resource will result in extinction of one species, and if competitor species do coexist, resource density will usually be higher than with only the best resource exploiter. However, in biological control, natural enemies frequently coexist and maintain their shared resource, the pest, at low levels. In four chapters, I explore potential mechanisms of coexistence of Aphytis melinus and Encarsia perniciosi, two specialist parasitoids that have coexisted for ∼50 years and have maintained California red scale, a citrus pest, at levels ∼200 times lower than without parasitoids.; Ecological theory predicts that Encarsia and Aphytis could coexist if they aggregate their eggs, have different fitness among patches, or segregate their resource use. In field experiments, I found no evidence for aggregation or fitness differences at any spatial scale. Field observations suggested that Aphytis produces more progeny from scale on leaves than does Encarsia, but they produce similar numbers on stems. In field experiments, Aphytis preferred parasitizing on leaves at high scale abundance but showed no preference at low abundance. My laboratory experiments demonstrated that when both species were present in a scale together, Encarsia was killed and consumed by Aphytis. Aphytis' preference for scale on leaves only when scale are abundant is not sufficient to promote coexistence.; Theory also predicts that Encarsia and Aphytis could coexist via a non-spatial trade-off of advantages. To explore this, I added biological realism to a mathematical model and explored the effect on species coexistence. Although small changes in parameter values shifted the predicted competitive outcome, biological realism did not substantially change the coexistence region. Parameter estimates from the system suggest that Encarsia should prevail. This has not occurred, nor does it appear likely in the future.; My dissertation work has shown that the mechanism by which Aphytis and Encarsia coexist is likely to act at an extremely small spatial scale or be caused by a non-spatial trade-off of advantages. |
