Feeding and survival of Culicoides sonorensis on cattle treated with permethrin or pirimiphos-methyl

The persistence of permethrin (5% a.i.) and pirimiphos-methyl (27% a.i.), a pplied to the dorsum of calves in the field against Culicoides sonorensis W irth and Jones (Diptera: Ceratopogonidae), was estimated using a hair-blood -feeding bioassay in the laboratory. Hair clippings were taken before treat ment and 3, 7, 14, 21, 28, 42 and 56 days after treatment from the dorsum, side and belly of treated and control calves. Laboratory-reared insects wer e allowed to feed through thin hair layers and a parafilm membrane on sheep blood warmed using a water-jacketed feeder. Some intoxication after exposu re to hair was noted up to 28 days after treatment with permethrin and up t o 14 days after treatment with pirimiphos-methyl. Hair from the dorsum caus ed more intoxication for a longer period than hair from other body regions. Permethrin and pirimiphos-methyl applied to the back did not significantly reduce overall engorgement (body regions pooled) after treatment. Permethr in residues on hair remained far higher on the back than other body regions and were related to insect intoxication and reduction in engorgement in th e laboratory. Residues on belly hair never exceeded 12 p.p.m. and did not r esult in significantly reduced feeding at any time. Engorged insects that e xhibited sublethal intoxication from feeding through permethrin-treated hai r did recover and matured numbers of eggs comparable to controls. Field tri als using treated and control calves and enclosure nets showed that dorsal applications of 5% permethrin were not effective in reducing engorgement, d espite some intoxication. Vacuum samples from a calf showed that C. sonoren sis fed primarily on the belly. A 0.2% permethrin application on the belly (250 ml) did result in > 80% reduction of C. sonorensis in the enclosure ne ts at 3 and 7 days after treatment, but activity had subsided by 10 days af ter treatment. The utility of insecticidal treatments for suppression of th is vector is discussed.