Effectiveness of the elimination efforts against Triatoma infestans (Klug) in South America through residual application of pyrethroid insecticides has been highly variable in the Gran Chaco region. We investigated apparent vector control failures after a standard community-wide spraying with deltamethrin SC in a rural area of northeastern Argentina encompassing 353 houses. Insecticide spraying reduced house infestation less than expected: from 49.5% at baseline to 12.3 and 6.7% at 4 and 8 mo postspraying, respectively. Persistent infestations were detected in 28.4% of houses, and numerous colonies with late-stage bugs were recorded after the interventions. Laboratory bioassays showed reduced susceptibility to pyrethroids in the local bug populations. Eleven of 14 bug populations showed reduced mortality in diagnostic dose assays (range, 35 ± 5% to 97 ± 8%) whereas the remainder had 100% mortality. A fully enclosed residual bug population in a large chicken coop survived four pyrethroid sprays, including two double-dose applications, and was finally suppressed with malathion. The estimated resistance ratio of this bug population was 7.17 (range, 4.47–11.50). Our field data combined with laboratory bioassays and a residual foci experiment demonstrate that the initial failure to suppress T. infestans was mainly because of the unexpected occurrence of reduced susceptibility to deltamethrin in an area last treated with pyrethroid insecticides 12 yr earlier. Our results underline the need for close monitoring of the impact of insecticide spraying to provide early warning of possible problems due to enhanced resistance or tolerance and determine appropriate responses.

Permethrin-resistant colonies of Pediculus capitis (De Geer) from Buenos Aires were used to establish a resistance profile and to examine resistance mechanisms. All permethrin-resistant head lice (resistance ratio from 52.8 to > 88.7) were also resistant to d-phenothrin (resistance ratio from 40.86 to > 48.39) and deltamethrin (resistance ratio from 16.24 to 38.06). No cross-resistance to carbaryl was found in any of the pyrethroid-resistant P. capitis tested. Otherwise, all resistant colonies showed low to high levels of resistance to beta-cypermethrin. This pyrethroid had never been applied as a pediculicide in Argentina; however, the high level of resistance found in these permethrin-resistant colonies (resistance ratio from 9.74 to 50.97) demonstrated that pyrethroid cross-resistance occurred to this novel insecticide. Treatment with piperonyl butoxide (PBO) or triphenylphosphate (TPP) significantly decreased the toxicity of permethrin in the four colonies tested. The esterase inhibitor TPP produced lower enhancement of toxicity than the multifunction oxidase inhibitor PBO in the colonies having the highest resistance levels. Results presented here concerning the cross-resistance profile and synergism by enzyme inhibitors in permethrin-resistant head lice demonstrated that enhanced metabolism was involved in the pyrethroid resistance. However, the substantial degree of resistance that remained after synergism suggested the presence of another resistance mechanism. Cross-resistance to pyrethroid and susceptibility to the carbamate carbaryl suggested a common action mechanism.

Chagas disease is a major health concern in Latin America, and Triatoma infestans (Klug, 1834) is responsible for the majority of cases of Chagas disease in the continent. After the discovery of deltamethrin highly resistant populations in the neighboring areas to Yacuiba (Bolivia) and Salvador Mazza (Argentina), we studied T. infestans populations north and southward, with the aim of describing the range of the resistant area. In addition, tests were conducted to describe the susceptibility to fipronil in deltamethrin-resistant populations. Tarija populations were highly resistant to deltamethrin, showing that the resistant area is greater than previously reported. Argentinean and Paraguayan populations were susceptible or presented moderate to low levels of deltamethrin resistance. Resistance to fipronil was found in Bolivian populations, although this insecticide was effective against Argentinean populations. This study shows that the Argentinean and Paraguayan populations are currently under successful control with deltamethrin. However, continuous resistance monitoring is necessary. We found that fipronil is a viable option for Argentina, whereas the resistance found in Bolivia implies that new formulations are needed to control T. infestans in Bolivia. Further research is required to find new alternatives of control in those areas that are currently suffering from high infestation rates.

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Insecticide resistance in Pediculus humanus capitis De Geer 1778 (Phthiraptera: Pediculidae) from different countries has been well documented in the last years. Otherwise, little research has been reported about insecticide resistance in insect embryos and none in human louse embryos. In this work, we studied insecticide resistance in eggs of three head lice populations whose pyrethroid resistance was shown in adults and nymphs compared with a susceptible laboratory body louse strain. All head louse populations showed high permethrin resistance in eggs. Levels of permethrin resistance (LCRs) assessed in eggs by immersion technique were higher than those previously reported for the corresponding populations of adults by topical application. Comparison of LCR values for different populations showed that there was a direct relationship between the resistance levels assessed in eggs and those in adults. All permethrin-resistant eggs showed high resistance to d-phenothrin and dichloro-diphenyl-trichloroethane (DDT) and low resistance to carbaryl, which is in common with the resistance profile established for adults and nymphs. The results concerning the high resistance to pyrethroid (permethrin and d-phenothrin) and the cross-resistance to DDT and carbaryl detected in head louse eggs and adults suggested similar resistance mechanisms in eggs and adults of head louse populations from adults.

Rhodnius prolixus Stahl, 1859 and Triatoma infestans (Klug, 1834) (Hemiptera: Reduviidae) are the major vectors of Chagas disease in Latin America. As no vaccine or suitable drugs are available, the control of this disease relies on the vector control by insecticide application in the infested houses. The use of these compounds requires caution to avoid toxicity to man and other non-target organisms. Moreover the development of insecticide resistance in field populations was already demonstrated by Vassena et al. (2000). Studies about safe compounds with insecticidal properties acting on novel targets are necessary for better alternative control against triatomine vectors.Aliphatic alcohols (from C2 to C18) showed ovicide and larvicide effect against the mosquitoes Aedes aegypti Linneo and Aedes scutellaris Walker. In particular, 1-decanol, 1-undecanol and 1-dodecanol proved to be highly effective on eggs, larvae and pupae of both species (Sinniah 1983). Mougabure Cueto et al. (2002) demonstrated insecticide activity of aliphatic alcohols against susceptible and permethrin-resistant Pediculus humanus capitis. The highest activity was found for the 1-dodecanol and the lowest for 1-octanol. Toxicity to head lice increased with the increase in carbon atoms in the alcohol and was not correlated with resistance to permethrin. Another work showed that the addition of 1-dodecanol to d-phenothrin lotions, increased the pyrethroid insecticide activity to head lice (Mougabure Cueto . The insecticidal activity of aliphatic alcohols was attributed to some effect on the cuticle structure. For head lice, it was concluded that the aliphatic alcohol must partitioned between the water of the lotion and the epicuticular waxed layer of the insect, and derivatives of greater hydrophobic should move readily into the head lice wax (Mougabure Cueto et al. 2002). The lack of correlation between permethrin resistance and alcohol pediculicide activity was attributed to a mode of action different to pyrethroid. Enhancement of pediculicidal activity produced by the incorporation of 1-dodecanol to experimental lotions, was attributed to a higher penetration of the insecticide due to the cuticle damage produced by the alcohol (Mougabure Cueto et al. 2000).The aim of this study was to investigate the biological effect of 1-dodecanol against R. prolixus and T. infestans, and the relationship between the triatomicide activity and the cuticle development. MATERIALS AND METHODSInsects -R. prolixus and T. infestans susceptible strains (CIPEIN strain) reared in the laboratory at 28 ± 1°C, 50-60% r.h. and 12:12 h light-dark period. Teneral first nymphs and post-teneral first nymphs were selected for topical application. Teneral first nymphs aged from 1 to 3 h old, and post-teneral first nymphs aged from 24 to 36 h old. Teneral adults and post-teneral adults of R. prolixus were selected for the injection bioassay. Teneral adults aged from 1 to 4 h old, and post-teneral adults aged from 24 to 48 h old.Chemicals -1-dodecanol (98%, Aldrich, Milwakee, WI...