Effects of predation and commercial mosquito insecticides on the gray treefrog tadpoles

Background/Question/Methods

There is a pattern between declining amphibian populations and an increase in the use of commercial pesticides.  Many studies have looked into the effects of these pesticides, but few have done so in the context of other, natural stressors faced by amphibians.  We examine the impact of three commonly used mosquito insecticides in both the presence and absence of predators.   We tested two commercial formulations of Bacillus thuringiensis israelensis (BTi) products (Mosquito Dunks and Mosquito Bits), and one methoprene product (Mosquito Torpedoes).  For the experiment, we selected organisms representative of a simple food chain typical in most ponds, including a tadpole species (Gray treefrog), two snail species, and two species of dragonfly larvae.  The experiment had a fully factorial design consisting of two treatments: predators (with or without dragonfly larvae) and insecticides (no insecticides, Mosquito Dunks, Mosquito Bits, and Mosquito Torpedoes).  Each treatment was replicated three times in 1000L cattle tanks.  The tadpoles were collected, measured, and weighed upon metamorphosis.  We additionally measured various community and environmental parameters to examine other effects of the pesticides.

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Results/Conclusions

The presence of predators significantly reduced tadpole survival.  There was also lower survival in treatments with both pesticides and predators.  Although this trend only had marginal statistical significance, the survival differences between tanks with predators and no insecticides added and tanks with predators and pesticides was large enough to be biologically significant.  None of the treatments affected pH, temperature, primary productivity, snail abundance or dissolved oxygen.  However, there was an increase in chironomid abundance in the predator treatments and may be due to reduced tadpole competition caused by predation.  This is the first community-level test of commercial formulations of mosquito insecticides on amphibians which are advertised as being safe for all vertebrates.  While more research is needed, our results suggest that mosquito insecticides that are not directly lethal to non-target organisms can still be lethal when applied in conjunction with natural ecological factors, such as predation.