A prospective risk assessment was conducted for adult butterflies potentially exposed to the mosquito control
insecticide naled. Published acute mortality data, exposure data collected during field studies, and morphometric data (total surface area and fresh
body weight) for adult butterflies were combined in a probabilistic estimate of the likelihood that adult butterfly exposure to
naled following aerial applications would exceed levels associated with acute mortality. Adult butterfly exposure was estimated based on the product of (1)
naled residues on samplers and (2) an exposure metric that normalized total surface area for adult butterflies to their fresh weight. The likelihood that the 10th percentile refined effect estimate for adult butterflies exposed to
naled would be exceeded following aerial
naled applications was 67 to 80%. The greatest risk would be for butterflies in the family Lycaenidae, and the lowest risk would be for those in the family Hesperidae, assuming equivalent sensitivity to
naled. A range of potential guideline
naled deposition levels is presented that, if not exceeded, would reduce the risk of adult butterfly mortality. The results for this risk assessment were compared with other risk estimates for butterflies, and the implications for adult butterflies in areas targeted by aerial
naled applications are discussed.