PERSISTENCE, PENETRATION, AND SURFACE AVAILABILITY OF CHLORPYRIFOS, ITS OXON, AND 5,5,6-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-tr ichloro-2-pyridinol, in elm bark were investigated after chlorpyrifos had been applied to the basal 1 m of elm tree trunks for control of ov erwintering elm bark beetles, the vector of Dutch elm disease, at diff erent application times and sites. The initial residues of chlorpyrifo s in elm bark were 388-444 mu g/g. The initial chlorpyrifos residues d issipated quickly with half-lives of 1.1-2.9 days for the faster dissi pation phase. The dissipation of chlorpyrifos during the later period became much slower, with the dissipation half-lives ranging from 205 t o 228 days. No penetration of chlorpyrifos into the cambium layer and wood tissue was found during the experimental period of 791 days. In t he contact test, the chlorpyrifos residues transferred from the treate d bark during a 1-min contact were below the acceptable daily intake f or humans immediately following drying of the treated bark and diminis hed over the period of the experiment.