Effect of prey limitation on competitive interactions between a native lady beetle, Coccinella novemnotata, and an invasive lady beetle, Coccinella septempunctata (Coleoptera: Coccinellidae).

The size and geographic distribution of Coccinella novemnotata Herbst populations have been decreasing rapidly across North America closely following the establishment, spread, and population growth of the invasive seven-spotted lady beetle, Coccinella septempunctata L. To determine whether intraguild predation and competition for prey may be partially responsible for the decline, we paired first-instar larvae of two populations of C. novemnotata (eastern and western) with first-instar C. septempunctata at low or high aphid densities. Survival of both C. novemnotata populations was significantly lower when larvae were paired with C. septempunctata, and western C. novemnotata exhibited significantly lower survival compared with the eastern population. This relationship depended on aphid density with the greatest survival of both C. novemnotata populations occurring at the high aphid density. Both male and female C. novemnotata weighed less on the day of eclosion when paired with C. septempunctata as compared with pairings with conspecifics. In a second test, C. septempunctata and C. novemnotata instars were varied at the start of the trial and C. novemnotata survival to adulthood in the presence of C. septempunctata was dependent of the instar of C. novemnotata used to initiate the experiment. C. novemnotata exhibited higher rates of survival and weighed significantly more on the day of eclosion when C. novemnotata was older than its C. septempunctata partner. These results suggest that interspecific competition including intraguild predation by C. septempunctata may contribute to C. novemnotata population declines, but that the intensity of this impact may vary across C. novemnotata populations.