Interspecific versus intraspecific competition between a native and exotic ladybird beetle

Since the 1980s, the native ninespotted lady beetle, Coccinella novemnotata, has decreased in population density in the United States following the establishment of the non-native sevenspotted lady beetle, C. septempunctata. In this study, the larvae of C. novemnotata and C. septempunctata were reared both interspecifically and intraspecifically (C. novemnotata only) in pairs through adult eclosion at high and low aphid prey densities to determine if the former is affected differently when in the presence of the latter versus when in the presence of another conspecific.  Two populations of C. novemnotata were used, one from the Eastern U.S. and one from the Western U.S., as data from previous studies suggest that the body size of C. novemnotata from western states is smaller than that from eastern states. Effects on survival, time-to-adult eclosion, and adult weight were measured daily. Results show that C. septempunctata negatively affected the survival of both eastern and western populations of C. novemnotata, regardless of aphid density. In the presence of C. septempunctata, C. novemnotata survival began decreasing more rapidly overtime compared to survival of C. novemnotata in the presence of a conspecific, regardless of aphid density. The eastern population of C. novemnotata had a higher adult weight in the presence of a conspecific at the high aphid density, but there were no significant differences in weight between other treatments. These results suggest that exotic species may negatively impact native species in the field.