Interactions between introduced and native predatory ladybirds (Coleoptera, Coccinellidae): factors influencing the success of species introductions

1. Why some exotic species invade successfully while others do not is poorly understood. This study focuses on the inability of the exotic predatory ladybird Adalia bipunctata (L.) (Coleoptera, Coccinellidae) to establish and spread in Japan. In particular, this study considers the interactions between A. bipunctata and three native Japanese ladybird species, Harmonia axyridis Pallas, Coccinella septempunctata Linn., and Propylea japonica Mulsant. 2. The number of aphids occurring on the host plants declined dramatically over the course of larval development in the presence of H. axyridis or C. septempunctata. In contrast, many aphids remained at the end of the experiment when only A. bipunctata or P. japonica was present. 3. The survivorship of A. bipunctata from second instar to adult was significantly reduced in the presence of H. axyridis, but not in the presence of C. septempunctata or P. japonica. Most deaths were the result of intra-guild predation when H. axyridis or C. septempunctata larvae were present. Larvae of H. axyridis and C. septempunctata suffered very high mortality in this experiment, but survived better when co-occurring with A. bipunctata than when occurring only with conspecifics. In contrast, P. japonica survived better when occurring with conspecifics than with A. bipunctata. 4. The finding that larvae of the exotic species A. bipunctata died at high rates from intra-guild predation by the native Japanese ladybirds H. axyridis and C. septempunctata may bear on the question of why A. bipunctata has not established more successfully in Japan whereas H. axyridis and C. septempunctata have been very successful in establishing themselves in North America.