ABSTRACT
The competitive displacement of Aphytis lingnanensis Compere by A. melinus DeBach was studied in Southern California in an attempt to explain why biological control works on California red scale, Aonidiella aurantii (Mask.), in southern California but is ineffective against this scale in the San Joaquin Valley. Our results indicated that A. melinus displaced A. lingnanensis because the former used a smaller scale size than the latter for the production of female progeny. Climatic effects on the age structure of California red scale created bottlenecks in the availability of suitable scale stages and hence a resource shortage ensued. Climate effects on the scale's age structure in the San Joaquin Valley created periods during which suitable scale stages for Aphytis were scarce or absent. These lengthy periods of resource scarcity alternating with periods of resource abundance prevented Aphytis from economically controlling the scale.
