An invasive, exotic coccinellid, Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae), was recently identified as a potential hazard to immature monarch butterflies, Danaus plexippus L. (Lepidoptera: Nymphalidae). To further evaluate the risk of H. axyridis impacting D. plexippus, we evaluated the influence of an alternate prey, Aphis nerii Boyer de Fonscolombe (Homoptera: Aphididae), on the predation rate of H. axyridis on D. plexippus larvae. When first-instar D. plexippus were placed in petri dish arenas with third-instar H. axyridis and varying densities of A. nerii, D. plexippus survival after 24 h increased significantly from 0% with 0 A. nerii present to 73% with 30 A. nerii present. In predation studies conducted in field cages, survival of D. plexippus larvae decreased with increasing densities of H. axyridis larvae and tended to increase with increasing A. nerii densities. These studies indicate that H. axyridis will feed on D. plexippus larvae in the presence of A. nerii, but that the presence of A. nerii lessens the severity of predation on D. plexippus.
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