Coccinella septempunctata L. (Coleoptera: Coccinellidae) has been introduced to North America in recent decades, raising concerns of adverse impacts on native lady beetles, including the congeneric C. transversoguttata richardsoni (Brown). The central focus of my dissertation is to understand the importance of reproduction, in particular, in promoting invasion of C. septempunctata and its replacement of native lady beetles in alfalfa fields of western North America. Studies were conducted to compare reproductive tactics of the invasive C. septempunctata and the native C. transversoguttata, by addressing: 1) maximum rate of reproduction of overwintered lady beetles, 2) population dynamics of the invasive and native lady beetles and their aphid prey, and seasonal reproductive patterns, over a 3-year period (2004'2006) in alfalfa fields, 3) reproductive tactics and plasticity in response to various prey availabilities, and 4) ovarian dynamics and observation of oosorption in response to prey removal. The invasive C. septempunctata gained a reproductive advantage over native, North American lady beetles from its larger body size when feeding on abundant prey. The invasive species gained additional advantage by its allocation of prey to larger numbers of relatively small eggs. In alfalfa fields, females of C. septempunctata reproduced more readily and laid more eggs than females of C. transversoguttata even at low prey density. C. septempunctata females collected from the field were also more successful in approaching their maximum body weights and reproduction, as observed under ideal conditions, than were females of native C. transversoguttata. In the laboratory, C. septempunctata females produced larger numbers of relatively small eggs, and they maintained their body weights even as they were producing eggs at low rates when aphids were provided in limited numbers. More immediate adjustment of reproductive effort with prey removal, and higher recovery of reproductive rate when prey again became available, were observed in C. septempunctata, compared with C. transcersoguttata. These reproductive abilities of C. septempucntata may contribute to its invasion success and dominance in alfalfa fields. Further studies are needed to determine why females of C. septempunctata are in better physiological condition than are females of C. transversoguttata in spring alfalfa fields.