One beetle's prey is another beetle's poison: Differential fitness effects of toxic prey among coccinellid species

Introduced and invasive species, such as the lady beetle Harmonia axyridis, have been linked to the decline 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 the aphid Aphis craccivora are toxic to H. axyridis larvae, but are 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 coccinellid and 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, with 100% larval mortality (mean survival time = 3.8 ± 0.3 days). In contrast, the native Coleomegilla maculata and the nonnative Coccinella septempunctata were only slightly affected by the toxic aphid strains. Both species could survive, but experienced slower development to adulthood and decreased adult weight. Finally, the native Cycloneda munda was unaffected by the toxic strain of aphid. 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.