TRANSITION FROM BISEXUAL TO UNISEXUAL CULTURES IN ENCARSIA-PERNICIOSI (HYMENOPTERA, APHELINIDAE) - NEW DATA AND A REINTERPRETATION

Three forms of Encarsia perniciosi Tower have been recognized based on their mode of reproduction and host choice: (1) a thelytokous form parasitizing California red scale, Aonidiella aurantii (Maskell), (2) a thelytokous, and (3) an arrhenotokous form parasitizing San Jose scale, Quadraspidiotus perniciosus (Comstock). The arrhenotokous E. perniciosi is a heteronomous hyperparasitoid (males develop as secondary parasitoids, either on their own species, usually a female, or that of some other species). Several studies have suggested that the arrhenotokous form will become thelytokous in one generation when cultured under constant temperatures in the laboratory. We tested this possibility by starting four arrhenotokous E. perniciosi cultures from different geographic localities and exposing them to constant temperatures. All four cultures remained arrhenotokous after 2, 3, 11, and 19 generations, respectively, under constant temperatures. A diagnostic allozyme variant of the enzyme system, PGM, allowed us to assign our cultures (four arrhenotokous, four thelytokous on San Jose scale, and one thelytokous on California red scale) unambiguously to form. Crossing tests indicated that thelytokous females did not mate with arrhenotokous males. We concluded that it is unlikely that the thelytokous forms were recently derived from the arrhenotokous forms in our sample. Therefore, the reports in the literature about cultures of E. perniciosi changing their mode of reproduction are more likely explained by the competitive displacement of the arrhenotokous by the thelytokous form in mixed cultures under laboratory conditions, or, alternatively, by inadequate attention to the fact that females of the arrhenotokous form emerge from 6 to 13 d before the males.