Pesticide Resistance: Strategies and Tactics for Management

Insecticides have played and still fulfil a very important role in sustainable production of food, animal feed and also as protection against disease vectors. They act to suppress insect populations and, as a consequence of their use, insecticide resistance has evolved. An overview of insecticide resistance mechanisms in insects is given, focusing on the metabolic systems involved in xenobiotic metabolism in the class Insecta. Several enzyme families (e.g., esterases, mixed function oxidases, glutathione S-transferases) are involved in insecticide detoxification, sequestration and excretion and have differing relative importance within the various taxonomic groups. A brief discussion of their impact on control strategies is given.

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An overview of the main pathways of metabolic resistance in insects

We conducted standard insecticide susceptibility testing across western Kenya and found that the Anopheles gambiae mosquito has acquired high resistance to pyrethroids and DDT, patchy resistance to carbamates, but no resistance to organophosphates. Use of non-pyrethroid-based vector control tools may be preferable for malaria prevention in this region.

/pdf/pyrethroid-and-ddt-resistance-and-organophosphate-b5vpi84acy.pdf

Pyrethroid and DDT Resistance and Organophosphate Susceptibility among Anopheles spp. Mosquitoes, Western Kenya.

https://espace.library.uq.edu.au/view/UQ:637917/UQ637917_OA.pdf

The pharmacology of voltage-gated sodium channel activators.

Deforestation has been linked to a rise in malaria prevalence. In this paper, we studied longitudinally 20 spots, including forested and deforested portions of a temporary river in a malarigenous frontier zone. Larval habitat parameters influencing distribution of Anopheles darlingi (Diptera: Culicidae) larvae were studied. We observed that larvae were clustered in forested-deforested transitions. For the first time in the literature, it was verified that parameters determining larval distribution varied from deforested to forested areas. The proximity to human dwellings was also a significant factor determining distribution, but larvae was most importantly associated with a previously undescribed parameter, the presence of small obstructions to river flow, such as tree trunks within the river channel, which caused pooling of water during the dry season (‘microdams’). In deforested areas, the most important factor determining distribution of larvae was shade (reduced luminance). Larvae were absent in the entire studied area during the wet season and present in most sites during the dry season. During the wet-dry transition, larvae were found sooner in areas with microdams, than in other areas, suggesting that flow obstruction prolongs the breeding season of An. darlingi. Adult mosquito densities and malaria incidence were higher during the dry season. Our data correlate well with the published literature, including the distribution of malaria cases near the forest fringes, and has permitted the creation of a model of An. darlingi breeding, where preference for sites with reduced luminance, human presence and microdams would interact to determine larval distribution.

Spatial clustering and longitudinal variation of Anopheles darlingi (Diptera: Culicidae) larvae in a river of the Amazon: the importance of the forest fringe and of obstructions to flow in frontier malaria.

Constant and extensive use of chemical insecticides has created a selection pressure and favored resistance development in many insect species worldwide. One of the most important pyrethroid resistance mechanisms is classified as target site insensitivity, due to conformational changes in the target site that impair a proper binding of the insecticide molecule. The voltage-gated sodium channel (NaV) is the target of pyrethroids and DDT insecticides, used to control insects of medical, agricultural and veterinary importance, such as anophelines. It has been reported that the presence of a few non-silent point mutations in the NaV gene are associated with pyrethroid resistance, termed as ‘kdr’ (knockdown resistance) for preventing the knockdown effect of these insecticides. The presence of these mutations, as well as their effects, has been thoroughly studied in Anopheles mosquitoes. So far, kdr mutations have already been detected in at least 13 species (Anopheles gambiae, Anopheles arabiensis, Anopheles sinensis, Anopheles stephensi, Anopheles subpictus, Anopheles sacharovi, Anopheles culicifacies, Anopheles sundaicus, Anopheles aconitus, Anopheles vagus, Anopheles paraliae, Anopheles peditaeniatus and Anopheles albimanus) from populations of African, Asian and, more recently, American continents. Seven mutational variants (L1014F, L1014S, L1014C, L1014W, N1013S, N1575Y and V1010L) were described, with the highest prevalence of L1014F, which occurs at the 1014 site in NaV IIS6 domain. The increase of frequency and distribution of kdr mutations clearly shows the importance of this mechanism in the process of pyrethroid resistance. In this sense, several species-specific and highly sensitive methods have been designed in order to genotype individual mosquitoes for kdr in large scale, which may serve as important tolls for monitoring the dynamics of pyrethroid resistance in natural populations. We also briefly discuss investigations concerning the course of Plasmodium infection in kdr individuals. Considering the limitation of insecticides available for employment in public health campaigns and the absence of a vaccine able to brake the life cycle of the malaria parasites, the use of pyrethroids is likely to remain as the main strategy against mosquitoes by either indoor residual spraying (IR) and insecticide treated nets (ITN). Therefore, monitoring insecticide resistance programs is a crucial need in malaria endemic countries.

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Mutations in the voltage-gated sodium channel gene of anophelines and their association with resistance to pyrethroids – a review

Two natural populations of Anopheles darlingi , collected in the intra, peri and extra domicile of two townships in the State of Amazonas, Coari and Manaus, were assayed as to their hematophagic behaviour-related genetic variability patterns. Findings revealed a large number of polymorphic fragments as well as high genetic variability in these populations. Polymorphic loci rate (P) and heterozygosity (He) in the Coari population varied between 77.63% - 84.86% and 0.2851 – 0.3069, respectively, with the highest and lowest genetic variability being detected in the intra- and peri-domicile sub-populations, respectively. The Manaus population showed genetic variability and heterozygosity similar to those in Coari (P= 75% - 78.94% and He= 0.2732 – 0.2741), where higher genetic variability was detected in the intra-domicile as well. Chi-square data (x 2 = 695.89; GL= 304; P < 0.001) and FST (F ST = 0.0775 ± 0.0072) were significant, indicating micro-geographic structuring resulting from some decreased gene flow. These findings point out the selective pressure response to the residual insecticide remaining on household walls after the entire neighbourhood had been sprayed with it during malaria control programs.

/pdf/variabilidade-genetica-em-populacoes-de-anopheles-darlingi-58fekikzyc.pdf

Variabilidade genética em populações de Anopheles darlingi (Diptera: Culicidae) e relação ao comportamento da atividade de picar, analisada por RAPD

Malaria remains one of the most serious public health problems in the world. The most effective strategic tool to reducing the transmission of the disease is the elimination of the mosquito vectors, through applying chemical insecticides against the adult form. However, the overuse of
such compounds has been causing a strong selection pressure on vector populations, resulting in the emergence of resistance. In this study, we aimed to adapt a method for assessing susceptibility status to pyrethroid insecticides on larvae of Anopheles darlingi and Anopheles marajoara. This method was previously named as Knockdown Simplified Bioassay. The samples were collected in Manaus, Sao Gabriel da Cachoeira and Iranduba in the state of Amazonas, in Macapa, Amapa, and Rio Branco, Acre. Bioassays were performed using 4th instar larvae. Each larva was placed in cups containing 20 mL of the insecticide solution and the time of knockdown was observed for 30 minutes. Based on natural population most susceptible (Sao Gabriel da Cachoeira), diagnostic doses obtained were 0.1 and 0.4 ppm. Loss of susceptibility was observed in populations of Manaus and Iranduba, possibly due to selection effect of insecticides on the specific resistance alleles.

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Adaptação de um bioensaio simplificado para avaliação do status de susceptibilidade em larvas de Anopheles darlingi e Anopheles marajoara ao piretroide deltametrina

Ana Paula Barbosa da Silva

Papers

Mutations in the voltage-gated sodium channel gene of anophelines and their association with resistance to pyrethroids – a review

Variabilidade genética em populações de Anopheles darlingi (Diptera: Culicidae) e relação ao comportamento da atividade de picar, analisada por RAPD

Adaptação de um bioensaio simplificado para avaliação do status de susceptibilidade em larvas de Anopheles darlingi e Anopheles marajoara ao piretroide deltametrina