Pyrethroid-synergist mixtures: toxicity, resistance and field efficacy toward pyrethroid-resistant horn flies (Diptera: Muscidae).

Abstract

Permethrin and cyhalothrin in combination with piperonyl butoxide (PB), DEF (S,S,S-tributyl phosphorotrithiolate), pirimiphos-ethyl and -methyl were evaluated in the laboratory and the field for activity against pyrethroid-resistant and susceptible horn flies, Haematobia irritants (L.). Horn fly adults from the pyrethroid-resistant population were resistant to permethrin (12.5-fold) but displayed a lower level of resistance to cyhalothrin and cyhalothrin-K (5.4- and 1.7-fold, respectively). Combinations of permethrin plus PB or DEF or both produced a low level of synergism (1.4 to 4.6-fold) but did not reduce the level of resistance present in the pyrethroid-resistant horn flies. The levels of synergism (2.0- to 6.6-fold) observed for combinations of PB, DEF, and PB + DEF with cyhalothrin were marginally higher than for permethrin. Unlike permethrin, the combination of cyhalothrin + PB + DEF resulted in a decreased level of resistance. Pirimiphos-ethyl and -methyl were highly toxic to the horn fly, with virtually no cross-resistance in the pyrethroid-resistant population. Mixtures of pirimiphos-ethyl plus either permethrin or cyhalothrin were highly effective against the resistant horn fly population, but no more so than pirimiphos-ethyl alone. Field studies in 1985 demonstrated that, although the combination of permethrin + PB + DEF was more effective than permethrin alone, control was still inadequate. In these same studies, cyhalothrin provided marginal control while cyhalothrin + PB + DEF produced good results. In 1986, the use of cyhalothrin-K and pirimiphos-methyl resulted in excellent control of the horn fly throughout the season, as did a higher concentration of cyhalothrin and cyhalothrin + pirimiphos-methyl.