Trials with chlorpyrifos (Dursban) as a systemic insecticide against the cattle louse

Data for louse control are presented chiefly on chlorpyrifos (Dursban) 0, 0-diethyl 0-(3, 5, 6-trichloro-2-pyridyl) phosphorothioate and fenchlorphos in one trial and chlorpyrifos and famphur in three trials. These animal systemics were tested on 168 dairy calves in four herds located in three regions of the North Island, New Zealand. Louse control, following single backline, dermal applications, showed 80%, 87% and 100% with dosages of chlorpyrifos at 5 mg, 13 mg and 20 to 200 mg per kg, respectively, and 100% and 93% with dosages of famphur at 20 mg per kg, respectively. Poor louse control (24 to 58%) with fenchlorphos was expected since this compound requires two applications 14 days apart. Ovicidal effect was demonstrated with chlorpyrifos and famphur. Minor scurfing and hair loss occurred on some calves with all compounds, but hair coats were normal 28 days after treatment. Calves given 100 mg to 200 mg per kg chlorpyrifos showed signs of organophosphate toxicity from 5 mins to 90 mins post-treatment but were normal thereafter.