Persistence, Penetration, and Surface Availability of Chlorpyrifos, Its Oxon, and 3,5,6-Trichloro-2-pyridinol in Elm Bark

The persistence, penetration, and bark surface contact availability of chlorpyrifos and its main degradation products, the oxon and 3,5,6-trichloro-2-pyridinol, in elm bark were investigated after chlorpyrifos had been applied to the basal 1 m of elm tree trunks for control of overwintering elm bark beetles, the vector of Dutch elm disease, at different application times and sites. The initial residues of chlorpyrifos in elm bark were 388−444 μg/g. The initial chlorpyrifos residues dissipated quickly with half-lives of 1.1−2.9 days for the faster dissipation phase. The dissipation of chlorpyrifos during the later period became much slower, with the dissipation half-lives ranging from 205 to 228 days. No penetration of chlorpyrifos into the cambium layer and wood tissue was found during the experimental period of 791 days. In the contact test, the chlorpyrifos residues transferred from the treated bark during a 1-min contact were below the acceptable daily intake for humans immediately following drying of the treated bark and diminished over the period of the experiment.