Relative Contribution of Monooxygenase and Esterase to Pyrethroid Resistance in Triatoma infestans (Hemiptera: Reduviidae) from Argentina and Bolivia

Abstract

Recently, high resistance to pyrethroid insecticides has been associated with ineffective Þeld 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 proÞles to nonpyrethroid insecticides. The Salta population showed high resistance ratios (RRs) to deltamethrin and only slight differences in the susceptibility to fenitrothion and Þpronil compared with the reference strain. Otherwise, the La Rioja population showed a lower RR to deltamethrin and no resistance to fenitrothion or Þpronil. Finally, the Yacuiba population had high a RR to deltamethrin, but it was susceptibility to fenitrothion and Þpronil. 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 signiÞcant. Activity of speciÞc 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 resis-tance 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. infestans. KEY WORDS Triatoma infestans, insecticide resistance, resistance ratios, monooxygenase, pyrethroid esterase Triatoma infestans (Klug) (Hemiptera: Reduviidae) is a blood-sucking bug and the main vector of ChagasÕ disease in Argentina. National control campaigns against this vector have been based on pyrethroid insecticides for Ͼ20 yr. However, resistance to pyre-throids has been detected in certain areas of Argentina since 1997 (Vassena and Picollo 2003, González Au-dino et al. 2004). In 2002, high resistance to pyrethroid insecticides was associated with ineffective Þeld treatments against T. infestans in northern Argentina (Picollo et al. 2005). Resistance to pyrethroids in a variety of insects has been found to be associated with elevated monooxygenases and esterases (Wilkinson 1983, Op-penoorth 1984). These enzymes contribute to resis-tance by rapid hydrolysis of insecticides to their non-toxic forms (Karunaratne 1998, Hemingway et al. 2004). Resistance associated with increased metabo-lism in T. infestans was found in a deltamethrin-resis-tant Þeld population from Salta, Argentina (González Audino et al. 2004). This study demonstrated a con-sistent correlation between elevated monooxygenase activity as determined on individual abdomens through ethoxycoumarin-O-deethylase (ECOD) ac-tivity, and deltamethrin resistance. Moreover, the study showed a signiÞcant difference in nonspeciÞc esterase activity measured as phenyl-thioacetate and ␣-naphtyl acetate activities between the susceptible and Salta populations. Later, a new ßuorescent sub-strate for examining pyrethroid-cleaving esterases was reported by Santo Orihuela et al. (2006). The high sensitivity of this ßuorescent technique allowed the