Interspecific interactions among Harmonia axyridis, Coleomegilla maculata, and Coccinella septempunctata (Coleoptera: Coccinellidae)

Concern over not-target effects of exotic coccinellid species on native coccinellids requires an assessment of the effect of intraguild predation and prey competition among species. We focused on two exotic coccinellid species and one native coccinellid species that can be found in alfalfa in Iowa. Larval nutrition is an important factor determining adult characteristics, which influence population dynamics and species interactions. In laboratory experiments, body size (adult weight, elytral length, elytral width) was affected by prey levels for three species of predatory beetles. Only Harmonia axyridis, completed development at each prey level and consistently increased body size as prey levels increased. When fed low prey levels, Coleomegilla maculata had the highest survival and fastest development time. Coccinella septempunctata larvae did not complete development when fed 2 aphids per day. Maternal effects were observed to influence larval and adult characteristics of each species. When larvae were fed high prey, survival decreased when caged with one or more potential competitors. Depending on prey level, C. septempunctata and Coleomegilla maculata had the lowest survival when paired with H axyridis. Harmonia axyridis survival was not influenced by the species with which it was caged and had the highest survival. Harmonia axyridis and C. septempunctata development time decreased when caged with one or more species regardless of aphid prey level. Coccinella septempunctata and H. axyridis adult weights were lower when caged with each other rather than when paired with C. maculata. During 2001, and 2002, coccinellid adults and aphids were sampled from four alfalfa fields in central Iowa. Aphis craccivora Koch, a new aphid species to Iowa, and an unidentified mite were collected both years. From 1987 to 1990, 77% of the coccinellids collected in alfalfa fields were native. In 2001 and 2002, native species accounted for approximately 58% of the adults collected in alfalfa. During both years, over 87% of the adults collected included C. septempunctata, Coleomegilla maculata and H. axyridis. Due to temporal occurrence in alfalfa, interactions between Coleomegilla maculata and H. axyridis were not likely, but C. septempunctata was present throughout the season and may interact with both species