Pyrethroid resistance distribution in Triatoma infestans and environmental association along the Argentine endemic zone.

Insecticide resistance is considered a barrier to chemical control of Triatoma infestans, the main vector of Chagas disease in the Southern Cone of South America. Although initiatives to reduce the incidence of the disease in the region have integrated different strategies, they have mainly relied on vector elimination using pyrethroid insecticides such as deltamethrin. Reports of pyrethroid resistance in connection with T. infestans control failures first emerged in northern Argentina and southern Bolivia. Recently, a mosaic pyrethroid-resistant focus has been described in the center of the Argentine Gran Chaco (Department of General Güemes, Chaco Province), characterized by the presence of susceptible and very highly resistant populations in the same area. The involvement of different resistance mechanisms has been proposed, together with the contribution of environmental variables that promote the toxicological heterogeneity described. In the endemic zone of Argentina, however, new questions arise: Are there any other clusters of resistance? Is there a relationship between the distribution of resistance and environmental variables (as has been observed at smaller scale)? We studied toxicological data from insects collected and analyzed at 224 localities between 2010 and 2020 as part of the resistance monitoring conducted by the Chagas National Program. The sites were classified according to the survival rate of insects exposed to a discriminant dose of deltamethrin: 0-0.19 were considered susceptible, 0.2-0.79 low-resistance, and 0.8-1 high-resistance. Localities were georeferenced to describe the spatial distribution of resistance and to identify environmental variables (demographics, land use, urbanization, connectivity, and climate) potentially associated with resistance. We used Generalized Linear Models (GLMs) to examine the association between resistance and environmental predictors, selecting error distributions based on the response variable definition. For the entire period, 197 susceptible localities were distributed across the endemic zone. Localities with different survival rates were found throughout the area; 9 high-resistance localities circled the two previously identified resistant foci, and 18 low-resistance in 6 provinces, highlighting their relevance for control planning. Precipitation variables were linked to resistance in all the GLMs evaluated. Presence/absence models were the most accurate, with precipitation, distance from the capital city, and land use contributing to the distribution of resistance. This information could be valuable for improving T. infestans control strategies in future scenarios characterized by unpredictable changes in land use and precipitation.

Morphometric study of the legs of the main Chagas vector, Triatoma infestans (Hemiptera: Reduviidae).

In triatomines, vectors of Chagas disease, active dispersal takes place by walking and flying. Flight has received more attention than walking although the last is the dispersal modality used by nymphs due to their lack of wings and also used by adults, which would facilitate the colonization and reinfestation of houses after vector control actions. The present work studied the morphometrical variation of Triatoma infestans legs, the main vector of Chagas disease the Southern Cone of South America. We described morphometric traits and the natural variation of each leg segment. Different linear, size and shape variables of each component of the three right legs of fifth instar nymphs of T. infestans were analyzed using morphometric tools. We analyzed differentiation, variation and correlation for each segment across the fore-, mid and hind legs using different statistical approaches such as general linear model, canonical variates analysis, test of equality of coefficient of variation and partial least square analysis. We also analyzed variation and correlation between segments within each leg with partial least square and morphometric disparity analyses. Our results showed that the segments differed between legs, as general trends, the dimensions (length, width and/or size) were greater in the hind legs, smaller in the forelegs and intermediate in the mid ones. The femur and tibia (length and/or width) showed differences in morphometric variation between legs and the femur and tibia showed the highest levels of correlation between legs. On the other hand, in the fore- and mid legs, the femur (length or width) showed similar variation with tibia and tarsus lengths, but in the hind legs, the femur showed similar variation with all segments and not with the tibia length, and there were strong correlations between linear measurement within each leg. Our results suggest that the femur and tibia could play a determining role in the coordination between the legs that determines the walking pattern. Considering that these segments would also be linked to the specific function that each leg has, this study suggests a preponderant role of the femur and tibia in the walking locomotion of T. infestans.

Can infection with Trypanosoma cruzi modify the toxicological response of Triatoma infestans susceptible and resistant to deltamethrin?

Chemical control plays a central role in interrupting the vector transmission of Chagas disease. In recent years, high levels of resistance to pyrethroids have been detected in the main vector Triatoma infestans, which were associated with less effectiveness in chemical control campaigns in different regions of Argentina and Bolivia. The presence of the parasite within its vector can modify a wide range of insect physiological processes, including toxicological susceptibility and the expression of resistance to insecticides. This study examined for the first time the possible effects of Trypanosoma cruzi infection on susceptibility and resistance to deltamethrin in T. infestans. Using WHO protocol resistance monitoring assays, we exposed resistant and susceptible strains of T. infestans, uninfected and infected with T. cruzi to different concentrations of deltamethrin in fourth-instar nymphs at days 10-20 post-emergence and monitored survival at 24, 48, and 72 h. Our findings suggest that the infection affected the toxicological susceptibility of the susceptible strain, showing higher mortality than uninfected susceptible insects when exposed to both deltamethrin and acetone. On the other hand, the infection did not affect the toxicological susceptibility of the resistant strain, infected and uninfected showed similar toxic responses and the resistance ratios was not modified. This is the first report of the effect of T. cruzi on the toxicological susceptibility of T. infestans and triatomines in general and, to our knowledge, one of the few on the effect of a parasite on the insecticide susceptibility of its insect vector.

Sublethal effects of a pyrethroid insecticide on cuticle thickness, wing size and shape in the main vector Triatoma infestans.

Exposure to sublethal doses of insecticide may affect biological traits in triatomines. We investigated the effects of toxicological phenotype (pyrethroid resistance status) and exposure to sublethal doses of deltamethrin on two traits of Triatoma infestans Klug (Heteroptera: Reduviidae) using a phenotypic plasticity experimental design. First-instar nymphs from 14 and 10 full-sib families from pyrethroid-susceptible and pyrethroid-resistant populations, respectively, were used. For the susceptible population, we treated first instars topically with acetone (control) or deltamethrin (treatment) once. For the resistant population, instars were treated once, twice and three times as first, third or fifth-instar nymphs, respectively. We measured cuticle thickness, wing size and wing shape of 484 emerging adults, and tested for treatment effects using mixed ANOVA and MANOVA models. Toxicological phenotype, exposure to deltamethrin and full-sib family exerted significant effects on cuticle thickness, wing size and wing shape. Adult triatomines previously treated with deltamethrin developed significantly thicker cuticles than control triatomines only in the resistant population and significantly bigger wings in both populations. Mean cuticle thickness and wing size increased with increasing exposures to deltamethrin. Exposure to sublethal doses of deltamethrin generated morphological modifications that may affect insect survival and flight dispersal, and hence may have evolutionary and epidemiological consequences.

Transcriptomic modulation in response to an intoxication with deltamethrin in a population of Triatoma infestans with low resistance to pyrethroids.

BACKGROUND: Triatoma infestans is the main vector of Chagas disease in the Southern Cone. The resistance to pyrethroid insecticides developed by populations of this species impairs the effectiveness of vector control campaigns in wide regions of Argentina. The study of the global transcriptomic response to pyrethroid insecticides is important to deepen the knowledge about detoxification in triatomines. METHODOLOGY AND FINDINGS: We used RNA-Seq to explore the early transcriptomic response after intoxication with deltamethrin in a population of T. infestans which presents low resistance to pyrethroids. We were able to assemble a complete transcriptome of this vector and found evidence of differentially expressed genes belonging to diverse families such as chemosensory and odorant-binding proteins, ABC transporters and heat-shock proteins. Moreover, genes related to transcription and translation, energetic metabolism and cuticle rearrangements were also modulated. Finally, we characterized the repertoire of previously uncharacterized detoxification-related gene families in T. infestans and Rhodnius prolixus. CONCLUSIONS AND SIGNIFICANCE: Our work contributes to the understanding of the detoxification response in vectors of Chagas disease. Given the absence of an annotated genome from T. infestans, the analysis presented here constitutes a resource for molecular and physiological studies in this species. The results increase the knowledge on detoxification processes in vectors of Chagas disease, and provide relevant information to explore undescribed potential insecticide resistance mechanisms in populations of these insects.

Phenotypic plasticity, canalisation and developmental stability of Triatoma infestans wings: effects of a sublethal application of a pyrethroid insecticide.

BACKGROUND: Triatomine control campaigns have traditionally consisted of spraying the inside of houses with pyrethroid insecticides. However, exposure to sublethal insecticide doses after the initial application is a common occurrence and may have phenotypic consequences for survivors. Here, using Triatoma infestans (the main vector of Chagas disease in the Southern Cone of South America) as a model species, we quantified the effects of exposure to a sublethal dose of pyrethroid insecticide on wing morphology. We tested if the treatment (i) induced a plastic effect (change in the character mean); (ii) altered environmental canalisation (higher individual variation within genotypes); (iii) altered genetic canalisation (higher variation among genotypes); and (iv) altered developmental stability (higher fluctuating asymmetry [FA]). METHODS: Each of 25 full-sib families known to be susceptible to pyrethroid insecticides were split in two groups: one to be treated with a sublethal dose of deltamethrin (insecticide-treated group) and the other to be treated with pure acetone (control group). Wings of the emerging adults were used in a landmark-based geometric morphometry analysis to extract size and shape measurements. Average differences among treatments were measured. Levels of variation among families, among individuals within families and among sides within individuals were computed and compared among treatments. RESULTS: Wing size and shape were affected by a sublethal dose of deltamethrin. The treated insects had larger wings and a more variable wing size and shape than control insects. For both wing size and shape, genetic variation was higher in treated individuals. Individual variations and variations in FA were also greater in deltamethrin-treated insects than in control ones for all full-sib families; however, the patterns of shape variation associated with genetic variation, individual variation and FA were different. CONCLUSIONS: Insects exposed to a sublethal dose of deltamethrin presented larger, less symmetrical and less canalised wings. The insecticide treatment jointly impaired developmental stability and genetic and environmental canalisation. The divergent patterns of shape variation suggest that the related developmental buffering processes differed at least partially. The morphological modifications induced by a single sublethal exposure to pyrethroids early in life may impinge on subsequent flight performance and consequently affect the dynamics of house invasion and reinfestation, and the effectiveness of triatomine control operations.

Genetic structure of deltamethrin-resistant populations of Triatoma infestans (Hemiptera: Reduviidae) in the Gran Chaco.

The genetic structure of natural populations offers insight into the complexities of their dynamics, information that can be relevant to vector control strategies. Microsatellites are useful neutral markers to investigate the genetic structure and gene flow in Triatoma infestans, one of the main vectors of Chagas disease in South America. Recently, a heterogeneous pyrethroid-resistant hotspot was found in the Argentine Gran Chaco, characterized by the highest levels of deltamethrin resistance found at the present time. We applied population genetics analyses to microsatellite and village data and search for associations between the genetic variability and the heterogeneous toxicological pattern previously found. We genotyped 10 microsatellite loci in 67 T. infestans from 6 villages with no, low, and high pyrethroid resistance. The most genetically diverse populations were those susceptible or with low values of resistance. In contrast, high-resistance populations had lower herozygosity and some monomorphic loci. A negative association was found between variability and resistant ratios. Global and pairwise FSTs indicated significant differentiation between populations. The only susceptible population was discriminated in all the performed studies. Low-resistance populations were also differentiated by a discriminant analysis of principal components (DAPC) and were composed mostly by the same two genetic clusters according to STRUCTURE Bayesian algorithm. Individuals from the high-resistance populations were overlapped in the DAPC and shared significant proportions of a genetic cluster. These observations suggest that the resistant populations might have a common origin, although more genetic markers and samples are required to test this hypothesis more rigorously.

Resistance to deltamethrin in Triatoma infestans: microgeographical distribution, validation of a rapid detection bioassay and evaluation of a fumigant canister as control alternative strategy.

Triatoma infestans (Klug) (Hemiptera: Reduviidae) is the main vector of Chagas disease in the Southern Cone of America and resistance to pyrethroid insecticides has been detected in several areas from its geographical distribution. Pyrethroid resistance presents a complex geographical pattern at different spatial scales. However, it is still unknown if the toxicological variability is a common feature within villages of the Gran Chaco were high resistance was descripted. The objectives of this study were to determine: (a) the microgeographical distribution of the deltamethrin-resistance in insects from Pampa Argentina village, (b) the performance of the insecticide impregnated paper bioassay to evaluate deltamethrin-resistance in field collected insects and (c) the lethal activity of the fumigant canister containing DDVP against insects resistant to deltamethrin. High survival of T. infestans exposed to discriminant dose was observed in the samples of all the evaluated dwellings, suggesting that the resistance to deltamethrin is homogeneous at the microgeographical level. Resistance determination by impregnated paper bioassay was similar to traditional topical determination, highlighting the use of this rapid methodology in field large-scale monitoring. The fumigant canister was not effective against resistant insects, remarking the need to develop suitable formulations that ensure minimal toxicological risk and high effectivity.

Behavioral response mediated by feces in Triatoma infestans (Hemiptera: Reduviidae: triatominae) susceptible and resistant to deltamethrin.

Chemical cues from feces promote aggregation behavior in Triatoma infestans nymphs and adults. Given the importance of T. infestans resistant to pyrethroids in several areas of Argentina and Bolivia, it would be important to know if there is an association with specific attraction and aggregation behaviors. These behaviors, to and surrounding refuges, play an important role in triatomine population dynamics, an important factor to consider and model for vector control strategies. The aim of the present study was to analyze the behavior of orientation to chemical signals emitted by feces from deltamethrin resistant (R) and susceptible (S) T. infestans. The behavioral assays were performed in a circular glass arena divided in two equal sectors. Fecal signals emitted by both S and R feces are attractants to fifth-instar nymphs of both S and R populations. Both toxicological phenotypes remained significantly longer on R feces, as compared to S feces. This is the first evidence in a triatomine, for the association of an aggregation behavior and insecticide resistance and may be the result of pleiotropic effects surrounding resistance genes.

Insecticide Resistance Mechanisms in Triatoma infestans (Reduviidae: Triatominae): The Putative Role of Enhanced Detoxification and Knockdown Resistance (kdr) Allele in a Resistant Hotspot From the Argentine Chaco.

Chagas disease affects around 6 million people in the world, and in Latin America, it is mainly transmitted by the kissing bug. Chemical control of the vector with pyrethroid insecticides has been the most frequently used tool to reduce the disease incidence. Failures of field control have been detected in areas of the Argentinian Gran Chaco that correlate with high levels of insecticide resistance. Here, we provide evidence of the mechanisms involved in the resistance to insecticides of field populations of T. infestans from General Güemes Department (Chaco Province, Argentina). The biochemical analysis suggests the increase in the activity of the degradative enzymes P450 oxidases and esterases as a minor contributive mechanism in low-resistance populations. The molecular study revealed high frequencies of the kdr L925I mutation at the voltage-gated sodium channel as responsible for the high resistance ratios detected. This knowledge contributes to the generation of comprehensive vector control strategies that reduce the incidence of the disease.

Interaction between two aggregation chemical signals in Triatoma infestans (Hemiptera: Reduviidae).

The nymphs and adults of Triatoma infestans spend much of their time aggregated among themselves within narrow and dark shelters. The search for a suitable shelter depends in part on the recognition of chemical signals coming from the feces and the cuticle of the other individuals who use the refuge. The aim of this study was determine the possible interaction between the chemical signals associated to the feces and to the cuticle of T. infestans. The results showed that the insects remained significantly more time on the feces that had contact with legs and the feces plus footprints than feces or footprints alone, demonstrating the interaction between evaluated signals. These results demonstrates also that feces extracted a chemical stimulus from the legs. Understanding the interaction feces-legs as an interaction feces-cuticle of legs, the results suggest that the feces could extract some cuticular compound with activity on the behavior of the insects. This is the first report of the interaction between the two aggregation signals recognized in T. infestans and of the increase in the behavioral response of insects exposed to feces that had contact with a cuticular structure.

Comparative and functional triatomine genomics reveals reductions and expansions in insecticide resistance-related gene families.

BACKGROUND: Triatomine insects are vectors of Trypanosoma cruzi, a protozoan parasite that is the causative agent of Chagas' disease. This is a neglected disease affecting approximately 8 million people in Latin America. The existence of diverse pyrethroid resistant populations of at least two species demonstrates the potential of triatomines to develop high levels of insecticide resistance. Therefore, the incorporation of strategies for resistance management is a main concern for vector control programs. Three enzymatic superfamilies are thought to mediate xenobiotic detoxification and resistance: Glutathione Transferases (GSTs), Cytochromes P450 (CYPs) and Carboxyl/Cholinesterases (CCEs). Improving our knowledge of key triatomine detoxification enzymes will strengthen our understanding of insecticide resistance processes in vectors of Chagas' disease. METHODS AND FINDINGS: The discovery and description of detoxification gene superfamilies in normalized transcriptomes of three triatomine species: Triatoma dimidiata, Triatoma infestans and Triatoma pallidipennis is presented. Furthermore, a comparative analysis of these superfamilies among the triatomine transcriptomes and the genome of Rhodnius prolixus, also a triatomine vector of Chagas' disease, and other well-studied insect genomes was performed. The expression pattern of detoxification genes in R. prolixus transcriptomes from key organs was analyzed. The comparisons reveal gene expansions in Sigma class GSTs, CYP3 in CYP superfamily and clade E in CCE superfamily. Moreover, several CYP families identified in these triatomines have not yet been described in other insects. Conversely, several groups of insecticide resistance related enzymes within each enzyme superfamily are reduced or lacking in triatomines. Furthermore, our qRT-PCR results showed an increase in the expression of a CYP4 gene in a T. infestans population resistant to pyrethroids. These results could point to an involvement of metabolic detoxification mechanisms on the high levels of pyrethroid resistance detected in triatomines from the Gran Chaco ecoregion. CONCLUSIONS AND SIGNIFICANCE: Our results help to elucidate the potential insecticide resistance mechanisms in vectors of Chagas' disease and provide new relevant information for this field. This study shows that metabolic resistance might be a contributing cause of the high pyrethroid resistance observed in wild T. infestans populations from the Gran Chaco ecoregion, area in which although subjected to intense pyrethroid treatments, vector control has failed. This study opens new avenues for further functional studies on triatomine detoxification mechanisms.

Modulation of the behavioral and electrical responses to the repellent DEET elicited by the pre-exposure to the same compound in Blattella germanica.

Insects under different stimuli from the environment modify behavioural responses due to changes in the sensitivity of neurons at the peripheral and/or at the central level of the nervous system. This phenomenon is called neuronal plasticity, and sensory adaptation is an example of it. An insect repellent is a chemical that produces oriented movements of the insects away from its source. In this work we studied the modulation of the behavioural and electrical response to the repellent N, N-diethyl-3-methylbenzamide (DEET) in males of the German cockroach B. germanica produced by previous exposure to the same repellent. Methods. We determined repellency using a circular arena, one half of which was treated with DEET. The time spent by insects in each half of the arena was measured, and a repellency coefficient (RC) was calculated. The RCs of pre-exposed and non-pre-exposed insects were compared. To determine a possible role of nitric oxide in the modulation of the response to DEET after pre-exposure, the nitric oxide donor S-nitroso-acetyl-cysteine (SNAC) was applied on cockroaches' antennae. The electrical activity of the cockroaches' antennae in response to DEET was recorded using electroantennogram (EAG) technique. The response to DEET was recorded also after a long stimulation with the same repellent, and after topical application of SNAC and dbcGMP (a cGMP analogue) on the antennae. Results. We found that previous exposure of B. germanica males to the repellent DEET produced an increase of the repellency at the behavioural level, measured as RC. A possible role of nitric oxide (NO) in the transduction pathway of this phenomenon is suggested, since treatment of the cockroaches with the NO donor SNAC also produced an increase of the repellency elicited by DEET. On the other hand, the response of the cockroaches' antennae exposed to DEET was determined electrophysiologically. The electrical activity in response to DEET decreased when the insects' antennae were stimulated with a long pulse of the repellent. The activity of the antennae was restored after 10 min. Treatment of the antennae either with SNAC or dbGMPc also produced a decrease in the response of the antennae to the repellent. Discussion.The previous exposure to a chemical stimulus can modify the behaviour associated to the same stimulus, increasing or decreasing the behavioural response. In the case of DEET we found that pre-exposure increased DEET repellency in male cockroaches. We also found NO involvement in a similar phenomenon. On the other hand, the test showed that DEET is perceived by insects' antennae as an odour. A long exposure of the antennae to DEET caused a transient decrease in the response of the antennae to the same compound. The same effect was achieved by treating the antennae with SNAC or dbcGMP, suggesting the involvement of the NO/cGMP system in the transduction pathway of the sensory adaptation phenomenon elicited by an odour in this species.

Identification of three cytochrome P450 genes in the Chagas' disease vector Triatoma infestans: Expression analysis in deltamethrin susceptible and resistant populations.

Cytochrome P450 monooxygenases play a predominant role in the metabolism of insecticides. Many insect P450 genes have frequently been associated with detoxification processes allowing the insect to become tolerant or resistant to insecticides. The increases of expression of P450 genes at transcriptional level are often consider responsible for increasing the metabolism of insecticides and seems to be a common phenomenon in the evolution of resistance development in insects. As pyrethroid resistance has been detected in Triatoma infestans, it was of interest to analyze genes associated with resistance to insecticides such as those encoding for cytochromes P450. With this purpose, the cDNA sequences of three cytochrome P450 genes (CYP4EM7, CYP3085B1, and CYP3092A6) were identified in this species. Primers and specific Taqman probes were designed from these sequences to determine their expression by quantitative PCR. The mRNA levels of the cytochrome P450 genes identified were determined from total RNA extracted from pools of fat body collected from individuals of different resistant and susceptible strains of T. infestans, and at different interval times after the topical application of the lethal doses 50% (LD50) of deltamethrin on the ventral abdomen of insects belonging to the different populations analyzed. It was detected overexpression of the CYP4EM7 gene in the most resistant strain of T. infestans and the expression of the three cytochrome P450 genes isolated was induced by deltamethrin in the susceptible and resistant populations included in this study. These results suggest that these genes would be involved in the detoxification of deltamethrin and support the hypothesis that considers to the cytochrome P450 genes of importance in the development of pyrethroid resistance.

Kdr mutations in Triatoma infestans from the Gran Chaco are distributed in two differentiated foci: Implications for pyrethroid resistance management.

Point mutations in the voltage-gated sodium channel, the primary target of pyrethroid insecticides, have been associated with the resistance in Triatoma infestans, an important vector of Chagas' disease. Hence, the sustainability of vector control programs requires the implementation of resistance management strategies. We determined the sensitivity of the molecular assays previously designed for early resistance detection to be used in pooled samples from a wide area of the endemic region, and validated them for their routine use in control campaigns for the monitoring of insecticide resistance in T. infestans. Consequently, we used these methods to examine the distribution of resistance-associated mutations in the sodium channel gene in populations of T. infestans from the Argentinean and Bolivian Gran Chaco. The PASA and REA assays tested proved sensitive enough to detect kdr SNPs in pooled samples, indicating these assays are suitable for routine screening in insecticide resistance surveillance. Two geographically differentiated foci were detected in T. infestans populations from the Argentinean and Bolivian Gran Chaco, with populations on the Bolivian-Argentinean border carrying L1014F mutation, and those from the Argentinean Chaco carrying L925I mutation. In all highly resistant populations analyzed, one of both kdr mutations was present, and toxicological assays determined that all pyrethroid resistant populations analyzed herein were sensitive to fenitrothion. The principal cause of pyrethroid resistance in T. infestans from the Gran Chaco ecoregion is kdr mutations in the sodium channel. Different levels of resistance occur in different populations carrying identical mutation, suggesting the existence of contributory mechanisms.

Insecticide resistance in vector Chagas disease: evolution, mechanisms and management.

Chagas disease is a chronic parasitic infection restricted to America. The disease is caused by the protozoa Trypanosoma cruzi, which is transmitted to human through the feces of infected triatomine insects. Because no treatment is available for the chronic forms of the disease, vector chemical control represents the best way to reduce the incidence of the disease. Chemical control has been based principally on spraying dwellings with insecticide formulations and led to the reduction of triatomine distribution and consequent interruption of disease transmission in several areas from endemic region. However, in the last decade it has been repeatedly reported the presence triatomnes, mainly Triatoma infestans, after spraying with pyrethroid insecticides, which was associated to evolution to insecticide resistance. In this paper the evolution of insecticide resistance in triatomines is reviewed. The insecticide resistance was detected in 1970s in Rhodnius prolixus and 1990s in R. prolixus and T. infestans, but not until the 2000s resistance to pyrthroids in T. infestans associated to control failures was described in Argentina and Bolivia. The main resistance mechanisms (i.e. enhanced metabolism, altered site of action and reduced penetration) were described in the T. infestans resistant to pyrethrods. Different resistant profiles were demonstrated suggesting independent origin of the different resistant foci of Argentina and Bolivia. The deltamethrin resistance in T. infestans was showed to be controlled by semi-dominant, autosomally inherited factors. Reproductive and developmental costs were also demonstrated for the resistant T. infestans. A discussion about resistance and tolerance concepts and the persistence of T. infestans in Gran Chaco region are presented. In addition, theoretical concepts related to toxicological, evolutionary and ecological aspects of insecticide resistance are discussed in order to understand the particular scenario of pyrethroid resistance in triatomines.

Lethal activity of individual and mixed monoterpenoids of geranium essential oil on Musca domestica.

Plant essential oils and its constituent molecules have been suggested as an alternative to control insect. The contribution of the constituents to the effect of the oil is determined by the interactions occurring between them. Synergistic interactions would improve the insecticide efficacy of the compounds due to the utilization of lower doses. We evaluated the insecticidal activity of geranium (Geranium maculatum L.) oil and its major constituents against Musca domestica L. and studied the toxic interactions in artificial mixtures of those constituents in the natural ratio. While synergistic interactions were determined in house fly in this study, these were of low intensity evidencing that the effect of each constituent was slightly modified by the other constituents present in the mixtures. The search for synergism between components is a strategy to improve the insecticide activity of natural compounds. The synergism helps to reduce the environmental and toxicological impact due to the reduction of the dose of use.

L925I mutation in the Para-type sodium channel is associated with pyrethroid resistance in Triatoma infestans from the Gran Chaco region.

BACKGROUND: Chagas' disease is an important public health concern in Latin America. Despite intensive vector control efforts using pyrethroid insecticides, the elimination of Triatoma infestans has failed in the Gran Chaco, an ecoregion that extends over Argentina, Paraguay, Bolivia and Brazil. The voltage-gated sodium channel is the target site of pyrethroid insecticides. Point mutations in domain II region of the channel have been implicated in pyrethroid resistance of several insect species. METHODS AND FINDINGS: In the present paper, we identify L925I, a new pyrethroid resistance-conferring mutation in T. infestans. This mutation has been found only in hemipterans. In T. infestans, L925I mutation occurs in a resistant population from the Gran Chaco region and is associated with inefficiency in the control campaigns. We also describe a method to detect L925I mutation in individuals from the field. CONCLUSIONS AND SIGNIFICANCE: The findings have important implications in the implementation of strategies for resistance management and in the rational design of campaigns for the control of Chagas' disease transmission.

Locomotor behaviour of Blattella germanica modified by DEET.

N,N-diethyl-3-methylbenzamide (DEET) is the active principle of most insect repellents used worldwide. However, its toxicity on insects has not been widely studied. The aim of this work is to study the effects of DEET on the locomotor activity of Blattella germanica. DEET has a dose-dependent repellent activity on B. germanica. Locomotor activity was significantly lower when insects were pre-exposed to 700 µg/cm(2) of DEET for 20 or 30 minutes, but it did not change when pre-exposure was shorter. Locomotor activity of insects that were pre-exposed to 2.000 µg/cm(2) of DEET for 10 minutes was significantly lower than the movement registered in controls. No differences were observed when insects were pre-exposed to lower concentrations of DEET. A 30-minute pre-exposure to 700 µg/cm(2) of DEET caused a significant decrease in locomotor activity. Movement was totally recovered 24 h later. The locomotor activity measured during the exposure to different concentrations of DEET remained unchanged. Insects with decreased locomotor activity were repelled to the same extent than control insects by the same concentration of DEET. We demonstrated that the repellency and modification of locomotor activity elicited by DEET are non-associated phenomena. We also suggested that the reduction in locomotor activity indicates toxicity of DEET, probably to insect nervous system.

Microgeographical study of insecticide resistance in Triatoma infestans from Argentina.

Chagas disease is a chronic parasitic infection restricted to America where it is currently estimated that 90 million people are at risk of acquiring the infection. Chemical control with pyrethroid insecticides has been effective to reduce disease transmission in several areas of the Southern Cone, although insecticide resistance has evolved and diminished the campaigns' results. Considering previous reports on the different levels of resistance between Triatoma infestans from different geographical areas, the objective of this work was to determine if T. infestans populations are toxicologically structured within localities. Response to the insecticide was measured and compared between houses of two Argentine localities. Different toxicity of deltamethrin was detected between dwellings of Chaco province, accounting for both susceptible and resistant houses within the same locality. However no difference was found among houses of Salta province. The results obtained in this work suggest that geographical structure is present not only at the between localities level, but also at the microgeograhical level.