Assessing potential collateral damage to tiger salamanders from insecticide applications for plague mitigation

Abstract/Summary

Ideal disease mitigation measures for wildlife are safe and benign for target species, non-target organisms, humans, and the environment. Identifying unintended collateral effects is a key consideration. Insecticides are commonly used to control flea (Siphonaptera) vectors of the plague bacterium Yersinia pestis to protect prairie dogs (Cynomys spp.) and their plague-susceptible obligate predators, endangered black-footed ferrets (Mustela nigripes). Currently, deltamethrin dust and fipronil baits are used in this context. A variety of animals use prairie dog burrows as nesting habitat or as refuge, which potentially exposes them to deltamethrin, and to fipronil (and metabolites) in prairie dog feces. Western tiger salamanders (Ambystoma mavortium) use prairie dog burrows and have permeable skin. We used western tiger salamanders to assess the effects of deltamethrin and fipronil, at realistic concentrations in “burrow” mesocosms, on salamander survival, physiology, and behavior. Treatments included (1) deltamethrin dust, (2) prairie dog feces containing fipronil and fipronil sulfone metabolite, and (3) appropriate controls. All 29 salamanders survived the 63-d post-treatment period. Over 28 d post-treatment, salamander body mass increased and there were no differences in body mass changes among the experimental groups. We did not detect insecticide residues in tissues from 8 control salamanders and only detected fipronil sulfone in tissues from 3 of 10 salamanders tested in mesocosms with fipronil feces. Deltamethrin was detected in tissues from 9 of 11 salamanders tested in deltamethrin mesocosms. Salamanders were detected outside of burrows, via photographs taken at 15 min intervals, more frequently after (than before) treatments, particularly in the deltamethrin mesocosms. For the deltamethrin group, photograph detection rates were highest 1-day post-treatment (suggesting the salamanders may have attempted to avoid deltamethrin by spending more time outside burrows) and declined by about 1% every 10 d post-treatment, down to pre-treatment patterns. Deltamethrin concentrations in whole body samples correlated positively with the amount of time the salamanders were undetected in photographs (a proxy for time spent in burrows). Experimentation is recommended to evaluate the potential effects of deltamethrin, in particular, on herpetofauna occupying prairie dog colonies under natural conditions.