In North America, declines in several indigenous species of coccinellids may be related to the spread and increased densities of Harmonia axyridis Pallis (Coleoptera: Coccinellidae) and Coccinella septempunctata L. (Coleoptera: Coccinellidae). In this study, the response to pea aphid prey levels [2, 4, 10, and 20 Acrythosiphon pisum Harris (Homoptera: Aphididae)] was quantified and compared for H. axyridis, C. septempunctata, and for the native species, Coleomegilla maculata DeGeer (Coleoptera: Coccinellidae). Development time, survival, sex ratio, adult weight, length, width and shape of the adult elytra were compared within and among species, as indicators of potential competitiveness of each species. Development time of each species significantly increased as prey levels decreased (p<0.001). Harmonia axyridis and C. maculata had similar survivorship at low prey levels (70-85%), while C. septempunctata had lower survival (0-10%). Weight of H. axyridis adults significantly increased in response to increased prey levels. In contrast, the weight of C. maculata adults significantly increased only between 2, 4, and 10 aphids per day. Elytral shape of Harmonia axyridis was influenced by sex and the larval prey levels (P<0.0007). The elytra shape of C. maculata was not significantly influenced by the prey levels or sex. The ability to adapt to surrounding environmental conditions may be an indicator of increased competitiveness. Our results indicate that the responses of H. axyridis to varying prey levels are more adaptive than C. septempunctata or C. maculata as measured by the development time, elytra shape and adult weight.
Species 1: Coleoptera Coccinellidae Harmonia axyridis (Multi-colored Asian Lady Beetle)
Species 2: Coleoptera Coccinellidae Coccinella septempunctata (Seven spotted Lady Beetle)
Species 3: Coleoptera Coccinellidae Coleomegilla maculata (Twelve spotted Lady Beetle)
Keywords: Competition, adaptation
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