As adults, many predatory insects must adjust to a constantly changing prey environment while balancing between survival and reproduction. Two laboratory experiments were conducted to compare reproductive responses of females of two species of lady beetles, invasive Coccinella septempunctata L. and native C. transversoguttata richardsoni (Brown), in Utah alfalfa fields to varying availability of prey. When both lady beetles were placed immediately on experimental diets after being collected from the field (first experiment) and when they were provided excess prey for 14 d before being placed on experimental diets (second experiment), C. septempunctata produced more but individually smaller eggs than C. transversoguttata. Overall, however, in both experiments, C. septempunctata and C. transversoguttata responded similarly when they consumed pea aphids in varying amounts, by laying fewer and less viable eggs when fewer prey were consumed. In particular, the experiments provided no evidence that C. septempunctata converts pea aphids into eggs at a relatively higher rate than C. transversoguttata under limited prey availability. However, C. septempunctata had greater ability than C. transversoguttata to maintain body weight, even as they were producing eggs at low rates. This suggests that low aphid availability is less stressful for C. septempunctata, perhaps because it has more physiological ability than C. transversoguttata to assimilate pea aphid nutrients at low aphid availability. Such ability might contribute to the numerical dominance of the introduced C. septempunctata in alfalfa fields, which have supported low numbers of aphids in recent years.
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