Impact assessment of spray drift from silvicultural aerial applications of permethrin on aquatic invertebrates using mosquito bioassays

Abstract

Permethrin (Pounce®) was applied at 70 g active ingredient per hectare by fixed-wing aircraft to a 16-ha jack and red pine plantation under application and meteorological conditions that were highly conducive for insecticide drift. Mortalities of an indicator aquatic insect species, Aedes aegypti mosquito larvae, and corresponding permethrin residues in shallow, artificial aquatic habitats, along with droplet densities on ground Kromekote® cards, were measured along five lines downwind and crosswind of the spray block. At 30, 60, 120, 180, and 240 m downwind, larval mortalities averaged 40, 39, 51, 18, and 34%; permethrin residues averaged 1.4, 1.1, 1.2, 0.6, and 0.8 μg/L; and droplet densities averaged 25, 22, 17, 14, and 10 droplets per centimeter squared, respectively. At similar distances crosswind of the block, mortalities were 60, 53, 11, 1, and 0%; residues to 180 m were 1.6, 2.6, 1.1, and 0.3 μg/L; and droplet densities were 30, 22, 12, 2, and 0.1 droplets per centimeter squared, respectively. Mortalities and residues were not high for such shallow water (6.7 cm depth), particularly beyond 120 m, and were less than or similar to the values predicted from a swath-superposition model. Impact on sensitive aquatic arthropods and stream insects in deeper water (≥25 cm), more representative of natural habitats, is predicted to be insignificant at such distances. A buffer zone of 150 m around productive water bodies (depth ≥25 cm) is suggested for aerial permethrin applications to such plantations.