Carrier solvent effects on the toxicity and penetration rate of fluralaner in the house fly, Musca domestica L. (Diptera: Muscidae)

Abstract

Penetration of an insecticide through the external insect cuticle is a critical determinant of spray contact efficacy and one of the many challenges in formulation efforts. Careful consideration of carrier solvent properties has been historically viewed as a way to improve penetration characteristics of insecticides through favorable physicochemical properties such as logP, but few studies exist on how modern carrier solvents impact cuticular penetration. Further, several models on cuticular penetration have been proposed in the past but few tests have ever been conducted to confirm them. The present study looked at how five commonly used solvents, butyl lactate, polar carbonate, propylene carbonate, and triethyl citrate, impact toxicological endpoints and penetration rates through the cuticles of house flies, a species often targeted with spray products. A two-compartment model of diffusion was also tested. The insecticide tested was the isoxazoline fluralaner, a chemical that has gained recent attention for potential in house fly control products. All solvents were found to have an antagonistic effect on mortality at 24 and 48 h in diagnostic dosing assays, with the exception of butyl lactate, which was statistically equivalent to acetone alone. LogP did not explain trends in mortality seen with any of the tested carrier solvents. Additionally, neither butyl lactate nor triethyl citrate altered penetration rates of fluralaner up to 8 h post treatment, compared to acetone. A simple two-compartment model similarly did not sufficiently explain penetration rates of fluralaner, belying first-order rate kinetics as a model for this process.