TOXICITY OF FIPRONIL TO SUSCEPTIBLE AND RESISTANT STRAINS OF GERMAN COCKROACHES (DICTYOPTERA, BLATTELLIDAE) AND HOUSE-FLIES (DIPTERA, MUSCIDAE)

The toxicity of fipronil, a promising new insecticide, was evaluated a gainst several susceptible and resistant strains of German cockroach a nd house fly in the presence or absence of the P450 monooxygenase inhi bitor piperonyl butoxide (PBO). Fipronil was highly toxic to German co ckroaches based on 72-h LD50 values and PBO had little effect on the t oxicity. The Cld-R strain, which has >17,000-fold resistance to dieldr in, was 6.7- to 7.7-fold cross-resistant to fipronil compared with the susceptible strains. This cross-resistance was not effected by PBO. T he fipronil LD50 values in 6 other strains of German cockroaches were <2-fold different from the susceptible strains. Fipronil was also high ly 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 str ain. The OCR strain was 3,100-fold resistant to dieldrin and showed 31 -fold cross-resistance to fipronil. The AVER strain (abamectin resista nt) was 2-fold more sensitive to fipronil than the CS susceptible stra in. This increased sensitivity was not effected by PBO. The relatively low levels of cross-resistance we detected suggest that cross-resista nce may not be a limiting factor for the use of fipronil against Germa n cockroaches and house flies.