Fitness effects of differentially toxic prey on members of the coccinellid community

Introduced and invasive species, such as the ladybeetle Harmonia axyridis, have been linked to declines of native species through mechanisms including intraguild predation and competitive superiority.  However, competitive differentials between natives and non-natives may be mitigated under certain circumstances.  For example, preliminary evidence suggests that some strains of Aphis craccivora are toxic to H. axyridis larvae, but less so to other coccinellids. The purpose of our study was to test the impact of A. craccivora on the fitness of multiple coccinellid species, to better understand how this aphid might structure natural enemy communities.  Using lab experiments, I monitored the fitness of four lady beetle species when placed in a no-choice environment with either toxic or non-toxic strains of A. craccivora.  Harmonia axyridis was severely susceptible to the toxic aphids at all life stages, with 100% larval mortality (mean survival time = 3.8±0.3 days), and adults experiencing 73% mortality over a two week assay.  In contrast, the native Coleomegilla maculata and the introduced Coccinella septempunctata were largely unaffected by the toxic aphid strain.  Finally, the native Hippodamia convergens was unable to develop on any strain of A. craccivora.  These results suggest that differentially toxic strains of A. craccivora may be beneficial for at least some native coccinellid species, potentially allowing them competitive release from the otherwise dominant H. axyridis.  Refuge food sources for subdominant competitors could aid in maintaining greater predator biodiversity within an ecosystem, which has been associated with improved biological control.