Discussion, historical data, and laboratory experiments are used to show that competitive displacement may occur even when there are no apparent shortages of requisites or direct aggression between ecological homologues. The final determinant is a relative difference in effective progeny production, R (as modified or even reversed by other environmental factors—such as natural enemies). These concepts are opposed to those of (Huffaker and Laing (1972)) who consider “escape from natural enemies” or the effects of other extrinsic factors as resources or requisites.

The decline of yellow scale, Aonidiella citrina, on citrus in southern California was documented with changes in distribution and abundance mapped from earliest records until its extinction sometime in the 1960s. Possible explanations—such as natural enemies, insecticides, and climate, were examined and found untenable. Extinction took place only where California red scale, A. aurantii, was present with yellow scale for a number of years. This led to the hypothesis that the California red scale was an ecological homologue of the yellow scale and that competitive displacement was responsible for extinction.

Results of laboratory tests all favored California red scale: (1) Gross fecundity, sex ratio, minimum and average time per generation were better, and more crawlers of California red scale than yellow scale survived to reproduce. (2) Computer calculation of the compound effective progeny production of a female scale and that part of her progeny which were themselves producing crawlers showed the “compound-interest” effect (California red