Anopheles farauti is a homogeneous population that blood feeds early and outdoors in the Solomon Islands.

Tanya L Russell,Robert D Cooper,Hugo Bugoro,Thomas R Burkot,Allan Apairamo,Nigel W Beebe,Frank H Collins,Neil F Lobo

doi:10.1186/s12936-016-1194-9

Abstract

BackgroundIn the 1970s, Anopheles farauti in the Solomon Island responded to indoor residual spraying with DDT by increasingly feeding more outdoors and earlier in the evening. Although long-lasting insecticidal nets (LLINs) are now the primary malaria vector control intervention in the Solomon Islands, only a small proportion of An. farauti still seek blood meals indoors and late at night where they are vulnerable to being killed by contract with the insecticides in LLINs. The effectiveness of LLINs and indoor residual spraying (IRS) in controlling malaria transmission where the vectors are exophagic and early biting will depend on whether the predominant outdoor or early biting phenotypes are associated with a subpopulation of the vectors present.MethodsMark-release-recapture experiments were conducted in the Solomon Islands to determine if individual An. farauti repeat the same behaviours over successive feeding cycles. The two behavioural phenotypes examined were those on which the WHO recommended malaria vector control strategies, LLINs and IRS, depend: indoor and late night biting.ResultsEvidence was found for An. farauti being a single population regarding time (early evening or late night) and location (indoor or outdoor) of blood feeding. Individual An. farauti did not consistently repeat behavioural phenotypes expressed for blood feeding (e.g., while most mosquitoes that fed early and outdoors, and would repeat those behaviours, some fed late at night or indoors in the next feeding cycle).ConclusionsThe finding that An. farauti is a homogeneous population is significant, because during the multiple feeding cycles required to complete the extrinsic incubation period, many individual female anophelines will enter houses late at night and be exposed to the insecticides used in LLINs or IRS. This explains, in part, the control that LLINs and IRS have exerted against a predominantly outdoor feeding vector, such as An. farauti. These findings may be relevant to many of the outdoor feeding vectors that dominate transmission in much of the malaria endemic world and justifies continued use of LLINs. However, the population-level tendency of mosquitoes to feed outdoors and early in the evening does require complementary interventions to accelerate malaria control towards elimination.

Highlights

In the 1970s, Anopheles farauti in the Solomon Island responded to indoor residual spraying with DDT by increasingly feeding more outdoors and earlier in the evening
Environmental conditions outside of that normally experienced by a population are closely associated with phenotypic change in that population [1, 2], and one common example of a changed environment is prolonged exposure of insects to insecticides
The response of an insect population to insecticide pressure is governed by interacting ecological phenomena and depends on numerous factors including the timing and magnitude of pressure as well as the life-cycle stage that is targeted

Summary

Introduction

In the 1970s, Anopheles farauti in the Solomon Island responded to indoor residual spraying with DDT by increasingly feeding more outdoors and earlier in the evening. Anopheline mosquito populations were under intensive pressure from extensive DDT applications during the Global Malaria Eradication Program (GMEP), launched in 1955, and one of the technical reasons given for the failure to eliminate malaria was behavioural adaptations by the mosquito vectors [5,6,7]. Those mosquito populations that responded by changing their behaviour to avoid DDT by feeding and/or resting outdoors had a selective advantage. One possible explanation is that specific karyotype inversions associated with feeding or resting outdoors became assimilated in the population through selection

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