Interactions between an introduced and indigenous coccinellid species at different prey densities

Coccinella septempunctata L. (Coleoptera: Coccinellidae), a Palearctic cocc inellid, has established and rapidly spread throughout the United States. T his quantitative examination of larval interactions between C. septempuncta ta and Coleomegilla maculata (DeCeer) (Coleoptera: Coccinellidae), a Nearct ic coccinellid, was conducted under controlled prey densities. Larvae of bo th coccinellid species are affected by a limited diet [one pea aphid per da y Acrythosiphon pisum (Harris) (Homoptera: Aphididae)] compared with an exc ess diet (> 20 aphids per day). Larval survival decreased from 86 to 63% in C. maculata and from 84 to 33% in C. septempunctata; mean preimaginal deve lopmental time increased from 20.6 to 26.7 days in C. maculata, and from 18 .1 to 32.0 days in C. septempunctata. Additionally, on one aphid per day, m ean adult weight was reduced from 12.39 to 9.79 mg in C. maculata, and from 39.57 to 14.44 mg in C. septempunctata. Interspecific interactions, favori ng C. septeumpunctata over C. maculata at a low prey density (one aphid per day), take the form of reduced survival of C. maculata compared with C. se ptempunctata (14 versus 66%). Reduced survival of C. maculata may be the re sult of competition for aphids or intraguild predation by C. septempunctata on C. maculata. No interspecific interactions (measured as effects on larv al survival, preimaginal developmental time, and adult weight) were observe d between larvae of these two species at a high prey density (> 20 aphids p er predator per day).