The decline of Coccinella novemnotata Herbst, the ninespotted lady beetle, across North America has been attributed to the introduction of Coccinella septempunctata L. It has been suggested that C. septempunctata negatively impacted C. novemnotata through a combination of mechanisms. We investigated the effects of scramble competition and intraguild predation between groups of C. septempunctata and C. novemnotata. A novel aspect of these experiments for this species combination was that we provided beetles the option to cannibalize conspecifics or predate on heterospecifics (i.e. intraguild predation); thus, we were able to compare interspecific versus intraspecific competition. Increasing prey density resulted in significantly lower rates of intraguild predation on C. novemnotata by C. septempunctata. Percentage survival of C. novemnotata grouped with C. septempunctata at low and high aphid densities was 6 and 61%, respectively. For our second study, we increased the spatial complexity and volume of the assay system, and provided prey ad libitum. C. novemnotata survival from first-instar to adult was significantly lower than C. septempunctata survival when grouped heterospecifically (43 vs 61% survival, respectively). Finally, we conducted a study to determine if hungry larvae discriminate conspecific versus heterospecific larvae by testing whether they predated selectively on the basis of species, which they did not appear to do. We conclude that C. novemnotata larvae suffer greater rates of intraguild predation from C. septempunctata compared with cannibalism, that this difference appears to be due to size asymmetry between the two species, and that local conditions impact the severity of intraguild predation by C. septempunctata.
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