Predatory lady beetles (or ladybirds; Coleoptera: Coccinellidae) have been introduced to North America in recent decades, raising concerns of adverse impact on native species, especially native lady beetles. In northern Utah, Coccinella septempunctata (Linnaeus) first appeared in 1991, and now it is predominant among predatory ladybirds in alfalfa. To evaluate the potential role of reproductive tactics in accounting for the displacement of the native lady beetle C. transversoguttata richardsoni (Brown) by the invasive C. septempunctata, current population dynamics of prey and lady beetles, and reproductive state of lady beetles at varying times of collection were examined in alfalfa hay fields farmed by Utah State University near Logan, in 2004 and 2005. Collected ladybirds were immediately brought to the lab and the females’ wet weight and number of eggs produced by females for forty-eight hours were measured. The populations of aphids in the alfalfa fields increased from late April through July. In both years, the wet weight of field-collected females of C. septempunctata was greater than that of C. transversoguttata richardsoni through the season. Furthermore, reproductive activity of C. septempunctata was more readily and greater than that of C. transversoguttata richardsoni early in the season, when aphid density was low in alfalfa fields. These results comparing reproductive tactics of the invader C. septempunctata with those of C. transversoguttata richardsoni may provide insight as to why C. septempunctata has established itself so successfully in North America.