Toxicities of Selected Chemicals and Combinations Thereof to Pyrethroid Resistant Horn Flies, 1986

Five chemicals were tested singly and in combination against known resistant field-collected horn flies to evaluate their potential efficacy against resistant horn flies. The treated filter paper technique of Sheppard and Hinkle was used in which the chemicals were dissolved in acetone and diluted to produce the desired ranges of concentrations when pipetted onto filter paper. Flies were subsequently exposed to the dried residues while confined in plastic petri dishes and mortality was assessed at 2 h. Ca. 30-40 flies were used per rate per replication. Dosage-mortality regression lines were calculated using Daum’s program for probit analysis and resultant LC50 values were ranked within replicates and weighted. The combined weighted LC30 values were then used to rank the chemicals and combinations for efficacy. As horn fly populations with varying levels of resistance were used for the various replications, it would be inappropriate to compute mean LC30 values.