Toxicity of Fipronil to Susceptible and Resistant Strains of German Cockroaches (Dictyoptera: Blattellidae) and House Flies (Diptera: Muscidae)

Abstract

The toxicity of fipronil, a promising new insecticide, was evaluated against several susceptible and resistant strains of German cockroach and house fly in the presence or absence of the P450 monooxygenase inhibitor piperonyl butoxide (PBO). Fipronil was highly toxic to German cockroaches based on 72-h LD50 values and PBO had little effect on the toxicity. The Cld-R strain, which has >17,000-fold resistance to dieldrin, was 6.7- to 7.7-fold cross-resistant to fipronil compared with the susceptible strains. This cross-resistance was not effected by PBO. The fipronil LD50 values in 6 other strains of German cockroaches were <2-fold different from the susceptible strains. Fipronil was also highly toxic to house flies and PBO further enhanced this toxicity 10-fold. The LPR strain was 15-fold cross-resistant to fipronil. PBO did not block expression of cross-resistance in LPR, suggesting monooxygenases are not a major mechanism of cross-resistance to fipronil in this strain. The OCR strain was 3, 100-fold resistant to dieldrin and showed 31-fold cross-resistance to fipronil. The AVER strain (abamectin resistant) was 2-fold more sensitive to fipronil than the CS susceptible strain. This increased sensitivity was not effected by PBO. The relatively low levels of cross-resistance we detected suggest that cross-resistance may not be a limiting factor for the use of fipronil against German cockroaches and house flies.