Factors affecting egg production and oviposition in populations of Colias philodice eurytheme Boisduval (Lepidoptera: Pieridae)

Abstract

THE ALFALFA CATERPILLAR, Colias philodice eurytheme^Boisduvëil, is a native insect described from California about four years before the introduction of alfalfa (Boisduval, 1852; Hendry, 1925).4 Originally, its distribution was restricted to the western portion of North America, but with removal of the forest and grass cover and the planting of alfalfa in other areas, the geo­ graphical distribution was greatly extended (Hovanitz, 1944, 1945 ; Gerould, 1946; Smith and Allen, 1954). At present, Colias is found from the Atlantic to the Pacific oceans and from southern Canada to southern Mexico (Hovanitz, 1944,1950). Before the introduction of alfalfa into California, the alfalfa caterpillar probably occurred in low numbers over much of the state on a variety of native legumes. Such common species as Lupinus succulent us Douglas, Lupinus bicolor Lindley, Trifolium tridentatum Lindley, and Lotus sub- pinnatus Lagasca were undoubtedly important hosts and, to a lesser extent, other species in these genera and in Astragalus, Vicia, Lathyrus, and Psoralea contributed to the food supply. The greatest numbers of caterpillars probably developed on a seasonal succession of legumes during wet springs. In the hot, dry summers, the quantity of suitable host plants was reduced, but a few caterpillars developed in favorable spots in moist river bottoms. With the cultivation of irrigated alfalfa, a new and abundant source of food was provided for both larvae and adults. As a result, not only was the geographical distribution of the species extended, but the average density was also raised. Furthermore, in contrast to the earlier conditions, the peak population levels now occur during the hot, dry summer (Michelbacher and Smith, 1943). In other words, introduction of alfalfa and irrigation has modified the ecosystem of Colias philodice eurytheme, resulting in a higher general equilibrium position. The California climate, the inherent character­ istics of Colias, and the mortality factors of the environment have not changed appreciably in quality. The changes have been brought about by alterations in quality and quantity of food supply, and modification of local climates and microenvironments produced by cultivation of alfalfa. From the viewpoint of local population dynamics (the viewpoint of the alfalfa grower), the factors causing variations from season to season and site to site are of primary interest. Such local variations occurred before the introduction of alfalfa, and now occur from one alfalfa field to another as well as within a single field. The level of these local infestations depends on the number of eggs laid on the alfalfa and the subsequent development and mortality of the eggs and larvae. The mortality factors, which are variable, have been discussed elsewhere (Michelbacher and Smith, 1943; Smith and Allen, 1954 ; Allen, 1958 ; Allen and Smith, 1958 ; Thompson and Steinhaus, 1950). The primary objective of the present study, conducted from 1949 to 1952, was to determine some of the factors influencing egg production and oviposi- tion of Colias philodice eurytheme, and their significance in population fluctuation. The study was approached from three different aspects. First, those factors such as the morphology of the reproductive system, mating habits, and oogénesis were considered since they are a prelude to oviposition. Second, the characteristics of the environment, especially the physical factors, were investigated since they are important in influencing the oviposition rates. Finally, population phenomena apart from the activities of the individual and the effects of the environment were considered in relation to oviposition.