Relative contribution of monooxygenase and esterase to pyrethroid resistance in Triatoma infestans (Hemiptera: Reduviidae) from Argentina and Bolivia.

Recently, high resistance to pyrethroid insecticides has been associated with ineffective field treatments against Triatoma infestans (Klug) (Hemiptera: Reduviidae) in northern Argentina. Samples were collected from two areas in Argentina (Salta and La Rioja) and one are in Bolivia (Yacuiba), and they were subjected to toxicological and biochemical assays. All populations were resistant to deltamethrin, but they showed different profiles to nonpyrethroid insecticides. The Salta population showed high resistance ratios (RRs) to deltamethrin and only slight differences in the susceptibility to fenitrothion and fipronil compared with the reference strain. Otherwise, the La Rioja population showed a lower RR to deltamethrin and no resistance to fenitrothion or fipronil. Finally, the Yacuiba population had high a RR to deltamethrin, but it was susceptibility to fenitrothion and fipronil. In several cases, deltamethrin-resistant populations had higher susceptibility to bendiocarb than the reference strain. Measured activity of P450 monooxygenase in individual insects (based on ethoxycoumarine-O-deethylase), tended to be higher in the deltamethrin-resistant populations, but the differences were not statistically significant. Activity of specific esterases determined by the hydrolysis of 7-coumaryl permethrate demonstrated an increase in the percentage of insects with higher esterase activity in the Salta and La Rioja populations. Unexpectedly, the Yacuiba population showed lower pyrethroid esterase activity than the reference strain. The different pyrethroid resistance patterns found in T. infestans from three geographical regions within Argentina and in Bolivia suggests that enzyme-based pyrethroid resistance in this species has multiple origins. Nevertheless, because nerve insensitivity (related to the presence of the kdr gene) is also an important mechanism related to pyrethroid resistance, further studies on the kdr gene should be carried to clarify the relative contribution of each pyrethroid-associated mechanism in deltamethrin-resistant populations of T.

7-Coumaryl permethrate and its cis- and trans-isomers as new fluorescent substrates for examining pyrethroid- cleaving enzymes.

New esterase substrates were synthesized using cis-, trans- and cis-trans-permethrinic acid chloride and then used to measure pyrethroid-cleaving enzymes in insects. The new substrates, namely cis-, trans- and cis-trans-7-coumaryl permethrates (7-CP), show a structure very similar to permethrin insecticide and yield fluorescent products on hydrolysis. These substrates were hydrolyzed by a commercial porcine preparation that provided esterase-specific activity, and were stable at different pH values (5.2-7.8). Studies made with house fly, Musca domestica (L.), homogenates showed that these compounds are appropriate for determining pyrethroid hydrolysis activity on individual insects. The measured activity of house fly esterase was 870 relative fluorescence units (RFU) min(-1) with cis-7-CP as substrate, 1117 RFU min(-1) with trans-7-CP as substrate and 1423 RFU min(-1) with cis-trans-7-CP as substrate. The fluorescent substrates for pyrethroid-cleaving esterases described in this paper have advantages over methods already given in the literature. They are substrates with structures very similar to pyrethroids, the cleavage of which can be followed by an increase in fluorescence emission at 440 nm; it takes only about 5 min to measure the reaction, and moreover the high sensitivity of the fluorescence technique allows the quantification of esterase activity on individual insects.