Estimating the consequences of apparent competition:: A method for host-parasitoid interactions

To evaluate the role of natural enemies for the coexistence of competing species, it is necessary to estimate quantitatively the way in which the selectivity of the natural enemies changes the strength of competition between the competitors. This paper presents one method for doing this in host-parasitoid systems with discrete generations. The method is illustrated by analyzing the interaction between variegated leafhoppers (Erythroneura variabilis) and grape leafhoppers (E. elegantula) feeding on domestic grapes. The analysis confirms earlier observations that the recent replacement of grape leafhoppers with variegated leafhoppers is likely to be a consequence of the shared egg parasitoid Anagrus epos. Finally, we argue that the method can provide-a tool to evaluate the potential of biological control measures involving alternative prey, e.g., in intercropping systems.