Pyrethroid insecticides are the front line vector control tools used in bed nets to reduce malaria transmission and its burden. However, resistance in major vectors such as Anopheles arabiensis is posing a serious challenge to the success of malaria control.Herein, we elucidated the molecular and biochemical basis of pyrethroid resistance in a knockdown resistance-free Anopheles arabiensis population from Chad, Central Africa. Using heterologous expression of P450s in Escherichia coli coupled with metabolism assays we established that the over-expressed P450 CYP6P4, located in the major pyrethroid resistance (rp1) quantitative trait locus (QTL), is responsible for resistance to Type I and Type II pyrethroid insecticides, with the exception of deltamethrin, in correlation with field resistance profile. However, CYP6P4 exhibited no metabolic activity towards non-pyrethroid insecticides, including DDT, bendiocarb, propoxur and malathion. Combining fluorescent probes inhibition assays with molecular docking simulation, we established that CYP6P4 can bind deltamethrin but cannot metabolise it. This is possibly due to steric hindrance because of the large vdW radius of bromine atoms of the dihalovinyl group of deltamethrin which docks into the heme catalytic centre.The establishment of CYP6P4 as a partial pyrethroid resistance gene explained the observed field resistance to permethrin, and its inability to metabolise deltamethrin probably explained the high mortality from deltamethrin exposure in the field populations of this Sudano-Sahelian An. arabiensis. These findings describe the heterogeneity in resistance towards insecticides, even from the same class, highlighting the need to thoroughly understand the molecular basis of resistance before implementing resistance management/control tools.
In 2016, northern Nigeria experienced a devastating infestation by the tomato leaf miner, leading to soaring in prices of tomatoes across the country. Unfortunately, information on the resistance status of this pest is lacking in northern Nigeria, hampering appropriate control measures. Here, we identified to species level and, using bioassays, characterised insecticide susceptibility profile of a field population of a tomato leaf miner from northern Nigeria. Highest resistance was observed with λ-cyhalothrin (a Type II pyrethroid) with a low mortality (18.52% at 56 h) and LD50 of 7461.474 ppm. Resistance was also established toward propoxur and chlorpyrifos-methyl with average mortalities each of 56% and LD50s of 1023.51 ppm and 106.351 ppm, respectively. Highest susceptibility was observed from abamectin with mortality of 86% and LD50 of 0.034 ppm. Pre-exposure to the synergist piperonylbutoxide significantly recovered λ–cyhalothrin susceptibility ((mortality~90%, χ2 = 98.35, p < 0.0001) and LD50 = 0.92 ppm) implicating P450 monoxygenases. No significant changes were observed on pre-exposure to diethyl maleate and triphenylphosphate-inhibitors of glutathione S-transferases and carboxylesterases, respectively. Sequencing of domain II of the voltage-gated sodium channel established 1014F kdr mutation 100% fixed in both λ-cyhalothrin-alive and dead larvae. These findings highlight the challenges for control of this invasive agricultural pest in northern Nigeria.
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