LAUSR

Insecticides are broadly applied in many urban areas of the western United States to control mosquito populations and reduce the prevalence of diseases such as West Nile and Zika Virus. We assessed the extent to which incidental exposure to the insecticide Permethrin affected drifting and benthic invertebrates in Spring Creek, Laramie, WY, USA. We collected drift samples before, immediately after (~ 3–6 h), and 1 day after (~ 28 h) insecticide application. We collected benthic invertebrates with a Hess sampler twice before and twice after spraying began. We measured an 11 × increase in the density of drifting aquatic insects immediately after pesticide application. Additionally, we observed an increase in drifting aquatic invertebrates 2.25 km downstream from treatment. A decrease in the biomass of benthic invertebrates and an increase in taxa richness at the end of the summer suggested that invertebrates are colonizing the stream but may be unable to persist. Fewer aquatic invertebrates in the stream may have cascading effects on the ecosystem by altering resources available to fish and riparian insectivores (e.g., bats, spiders, and amphibians). Understanding the unintended ecological impacts of current pest control practices could lead to more effective and less detrimental management strategies.