Competitive Exclusion of Aphytis lingnanensis by A. melinus: Potential Role of Host Size

Aphytis melinus, an ectoparasitoid of California red scale, Aonidiella aurantii, replaced A. lingnanensis, another ectoparasitoid, in certain California citrus areas during the 1960's following the former's introduction in 1956-1957. We show that, although both species attack a similar range of host sizes, A. melinus utilizes a smaller threshold scale size for the production of daughters. Size differences of scales within a citrus tree (largest scales occur on fruits, smallest on wood, and inter? mediate on leaves), seasonal variation in scale age structure, multivoltinism ofthe scale and parasitoid populations, seasonal availability of citrus fruits (the substrate that supports the largest scales), dif? ferences in the temperature tolerances ofthe two parasitoid species (A. lingnanensis is less tolerant), and differences in their vagility (A. melinus is more vagile), probably all favor A. melinus in competition with A. lingnanensis. A. lingnanensis does not realize its fecundity advantage in female progeny when these species compete because A. melinus preempts California red scale before it grows into the size range preferred by A. lingnanensis. Collection records through 1972 show that A. melinus continued to extend its geographical range coastally in southern California through 1972.