Efficacy of freshly mixed coumaphos suspensions adjusted to various pH levels for treatment of cattle infested with Boophilus annulatus (Say) (Acari: Ixodidae)

Abstract

Reductive dechlorination of coumaphos results in the formation of a highly toxic material, potasan, but lower pH (≤ 5.5) of the liquid containing coumaphos may eliminate this degradation process. However, the effects of lower pH on the rapid settling of coumaphos particles and on the efficacy of the pesticide are not known. Three suspensions were made from a freshly mixed stock solution of a flowable formulation of coumaphos at 0.125% active ingredient at pH values of 6.7, 5.5, and 4.5. Coumaphos particles were observed to remain well suspended in all pH treatments before, during, and for up to 1 h after applications were made on tick infested cattle, suggesting that there was no break in any of the coumaphos suspensions. Likewise, observations on the behavior of each of the animals (both treated and untreated) appeared to be similar and uniform across all groups, suggesting that the lowering of the pH would not cause any noticeable adverse reaction in the treated animals. Data obtained from the coumaphos suspensions at each of the three pH treatments (pH 6.7, 5.5, and 4.5) against Boophilus annulatus (Say) resulted in control of the index of reproduction (IR) that ranged from 94.4 to 96.5%, indicating that pH had little effect on the efficacy of the acaricide. Results also indicated that the pH of coumaphos suspensions had little effect on engorgement weights (range 270–280 mg), egg mass weights (range 88–100 mg), or percent hatch of eggs (range 57–71%) of females that survived the treatments. Therefore, we concluded that coumaphos suspensions with a pH as low as 4.5 would have no observable adverse effect on the rapid settling of coumaphos particles in suspension, cause no noticeable adverse reactions in treated animals, and remain highly efficacious against B. annulatus.