Smallest Anopheles farauti occur during the peak transmission season in the Solomon Islands

Kimberley Mclaughlin,Hugo Bugoro,Tanya L Russell,Thomas R Burkot,Robert D Cooper,Nigel W Beebe,Jance Oscar,Scott A Ritchie,Allan Apairamo

doi:10.1186/s12936-019-2847-2

Abstract

BackgroundMalaria transmission varies in intensity amongst Solomon Island villages where Anopheles farauti is the only vector. This variation in transmission intensity might be explained by density-dependent processes during An. farauti larval development, as density dependence can impact adult size with associated fitness costs and daily survivorship.MethodsAdult anophelines were sampled from six villages in Western and Central Provinces, Solomon Islands between March 2014 and February 2017. The size of females was estimated by measuring wing lengths, and then analysed for associations with biting densities and rainfall.ResultsIn the Solomon Islands, three anopheline species, An. farauti, Anopheles hinesorum and Anopheles lungae, differed in size. The primary malaria vector, An. farauti, varied significantly in size among villages. Greater rainfall was directly associated with higher densities of An. farauti biting rates, but inversely associated with body size with the smallest mean sized mosquitoes present during the peak transmission period. A measurable association between body size and survivorship was not found.ConclusionsDensity dependent effects are likely impacting the size of adult An. farauti emerging from a range of larval habitats. The data suggest that rainfall increases An. farauti numbers and that these more abundant mosquitoes are significantly smaller in size, but without any reduced survivorship being associated with smaller size. The higher malaria transmission rate in a high malaria focus village appears to be determined more by vector numbers than size or survivorship of the vectors.

Highlights

Malaria transmission varies in intensity amongst Solomon Island villages where Anopheles farauti is the only vector
The size of adult mosquitoes is governed by competition during the immature aquatic stages; with the body size of emerging adults being directly associated with larval densities
Anopheles farauti was the dominant species in Jack Harbour, Haleta, Tuguivili and New Mala, with 100% of anophelines captured in Jack Harbour and Haleta being An. farauti

Summary

Introduction

Malaria transmission varies in intensity amongst Solomon Island villages where Anopheles farauti is the only vector. This variation in transmission intensity might be explained by density-dependent processes during An. farauti larval development, as density dependence can impact adult size with associated fitness costs and daily survivorship. Vector control with indoor residual spraying (IRS) and insecticide-treated nets (ITNs) is responsible for 80% of the reduction in Plasmodium falciparum cases in Africa between 2000 and 2015 [1]. The fitness of adult anopheline mosquitoes (adult survival and fecundity) can be influenced by interacting environmental and density-dependent factors, with fitness directly associated with adult body size [16,17,18]. Smaller adults can be less successful in mating, have reduced fecundity [16, 19, 20] and lower survival rates [21,22,23] and, have a lower potential for transmitting malaria [15, 24]

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