The final countdown: presence of an invasive mosquito extends time to predation for a native mosquito

Larvae of the predatory mosquito Toxorhynchites rutilus consume arthropods within container habitats, including native Aedes triseriatus and invasive Aedes japonicus mosquitoes. Previous studies, which did not account for common habitat attributes such as habitat structure and predation cues, conflict on whether Ae. triseriatus and Ae. japonicus differ in their vulnerability to predation. We conducted two laboratory experiments to assess how habitat attributes modulate Tx. rutilus predation on Ae. triseriatus and Ae. japonicus. In experiment one, we added fine particulate organic matter (FPOM) and assessed vulnerability for each species separately. Experiment two contained the following treatments: presence/absence of predation cues, presence/absence of habitat structure (FPOM and leaves) and three species combinations: Ae. triseriatus or Ae. japonicus alone, and both species together. We added one Tx. rutilus to feed in each microcosm for 24 h (experiment one and two) and until all prey were consumed (experiment two only). When reared alone, Ae. triseriatus had higher survival compared to Ae. japonicus in experiment one (71% vs. 52%) but there were no significant differences at 24 h in experiment two. When we followed the cohort to total predation, Ae. triseriatus had a lower daily survival rate compared to Ae. japonicus (hazard ratio 1.165) when the species were kept separately. When the species were mixed, however, Ae. japonicus was more vulnerable than Ae. triseriatus (hazard ratio 1.763), prolonging Ae. triseriatus time to total cohort predation. Both species were less likely to be consumed in the presence of predation cues. We detected no effect of habitat structure. These results demonstrate vulnerability is context dependent and the presence of an invasive congener can relax predation pressure on a native prey species when they co-occur in the same habitat.