Combined Effects of Virus, Pesticide, and Predator Cue on the Larval Tiger Salamander (Ambystoma tigrinum)

Emerging diseases and environmental contamination are two of the leading hypotheses for global amphibian declines. Yet few studies have examined the influence of contaminants on disease susceptibility, and even fewer have incorporated the role of natural stressors such as predation. We performed a factorial study investigating the interaction of the insecticide carbaryl, dragonfly predator cue, and the emerging pathogen Ambystoma tigrinum virus (ATV) on fitness correlates and disease susceptibility in tiger salamander larvae. Four week old larvae were exposed for 22 days in a 2 (0, 500 μg/l carbaryl) × 2 (control, predator cue water) × 2 (0, 1 × 10⁴ pfu ATV) factorial designed laboratory study. Results show significant impacts to survival of larvae for both virus and predator cue treatments, as well as an interactive effect between the two, in which predator cue strongly exacerbated disease-driven mortality. There was a clear pattern of reduced survival with the addition of stressors, with those where all three stressors were present exhibiting the worst effects (a decrease in survival from 93 to 60%). On those that survived, we also detected several sub-lethal impacts in mass, SVL, and development. Predator cue and pesticide treatments significantly reduced both SVL and mass. Virus and predator treatments significantly slowed development. Stressors also exhibited opposing effects on activity. Predator cue caused a significant reduction in activity, whereas virus caused a significant increase in activity over time. These results highlight the importance of examining combined natural and introduced stressors to understand potential impacts on amphibian species. Such stressors may contribute to the emergence of ATV in particular regions, raising concerns about the influence of pesticides on disease emergence in general.