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Résumé :
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Aphidophagous lady beetles feed on a variety of pre! in addition to preferred aphids. These alternative feuds ma!: serve only to maintain the predator but do not permit immature growth or adult reproduction. We tested whether larvae of the alfalfa weevil, Hypera postica (Gyllenhal), are alternative foods for adults of 2 species of lady beetles, Coccinella septempunctata L. and G. transversoguttata Brown, that breed in alfalfa while feeding on the larvae as well as aphids. When collected fi-om alfalfa fields and placed on diets of weevils only, adults of both species generally gained weight. However. in contrast to females fed pen aphids, Acyrthosiphon pisum (Harris),females fed weevils produced very few eggs, almost all of which were laid in the first few days after the predators were placed on the weevil diet. When dissected. females fed weevils li:td only very small (previtellogenic) eggs in their ovaries. Similarly, females produced very few eggs, and held only previtellogenic eggs of very small size in their ovaries, when provided only sucrose (dissolved in water in 15% solution). When provided both weevils and sucrose, however, females of both species laid eggs in modest numbers. These results indicate that weevil larvae serve as alternative foods for the lady beetles, and that reproduction nevertheless can occur in the absence of preferred aphid prey when weevil larvae are combined ill the diet with sucrose, another alternative food. Because nectar carl serve as a natural source of sucrose in or near, alfalfa fields, some reproduction by lady beetles may occur in fields infested with weevil larvae even in the absence or low abundance of aphids.
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