Field Assessment of Naled and Its Primary Degradation Product (Dichlorvos) in Aquatic Ecosystems Following Aerial Ultra-low Volume Application for Mosquito Control

Naled, an organophosphate insecticide, is applied aerially at ultra-low volumes over aquatic ecosystems near Sacramento,
California, USA, during summer months for mosquito control. Two ecosystem types (rice fields and a flowing canal) were
sampled in 2020 and 2021. Naled and its primary degradation product (dichlorvos) were measured in water, biofilm, grazer
macroinvertebrates, and omnivore/predator macroinvertebrates (predominantly crayfish). Maximum naled and dichlorvos
concentrations detected in water samples one day after naled application were 287.3 and 5647.5 ng/L, respectively, which
were above the U.S. Environmental Protection Agency’s aquatic life benchmarks for invertebrates. Neither compound was detected in water more than one day after the application. Dichlorvos, but not naled, was detected in composite crayfish samples up to 10 days after the last aerial application. Detections in water from the canal showed that the compounds were transported downstream of the target application area. Factors such as vector control flight paths, dilution, and transport through air and water likely affected concentrations of naled and dichlorvos in water and organisms from these aquatic ecosystems.