Persistence of aerially applied mosquito-pesticide, Naled, in fresh and marine waters

Naled, an organophosphate pesticide, received considerable attention during 2016 as it was applied aerially to control the first mosquito-borne Zika virus outbreak in the continental United States. Stakeholders living in affected areas raised concerns about its environmental impacts. One factor influencing environmental impacts is the persistence of the chemical applied. The objective of this study was to evaluate the persistence of naled - and its degradation bi-product, dichlorvos - in natural waters. Initial naled concentrations were measured at ground level after full-scale aerial spray activities. Laboratory experiments were designed to evaluate factors (fresh versus marine water chemistry, temperature, and sunlight) that may promote the degradation of naled and dichlorvos in the environment. Results show that natural fresh and marine water chemistry promoted naled degradation as experiments with de-ionized water resulted in half-lives greater than 6 days. The half-life in natural waters without light ranged from 5 to 20 h with lower half lives at higher temperatures. Under light exposure, degradation was accelerated and yielded more dichlorvos. Detectable levels (0.05 μM for naled and 0.10 μM for dichlorvos) were measured in water samples collected from the field during aerial spray events. Results can be used in risk assessments that consider both naled and dichlorvos to better understand ecological impacts and to develop improved public health recommendations.