Currently, there is a reliance on indoor vector control with long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) to reduce malaria transmission. LLINs and IRS are most effective in killing mosquitoes that enter houses late at night seeking a blood meal. In response to widespread LLINs coverage, vectors, including Anopheles farauti in the Solomon Islands, have responded by displaying behavioural resistance by biting earlier and outdoors and thus avoiding contact with the insecticides in LLINs. This behavioural resistance is feared to be a threat to the effectiveness of LLINS.
However, LLINs appear to be maintaining effective control. This conundrum of effective control of an outdoor early biting vector by an intervention (LLINs) that acts on indoor late-night biting was investigated by careful quantitative measurements of key vector parameters. Effective malaria control by LLINs in the Solomon Islands is explained by the following 4 facts. There is limited resistance to pyrethroids due to modest selective pressure, since mosquitoes avoid contract with insecticides. Anopheles farauti consists of single populations of mosquitoes who mostly feed outdoors and early within a single feeding cycle. During each feeding cycle, some individual An. farauti bite indoors late at night, exposing a small proportion of the largely susceptible vector population to the insecticides in LLINs. Finally the short 2-day feeding cycle of An. farauti means that sporozoite-infected An farauti will have to take 5-6 blood meals; hence, there will be a cumulative impact on the age structure of the vector population with most mosquitoes having a high probability of having entered a house late at night before they survive the length of the extrinsic incubation period. Nonetheless, elimination will likely require vector control tools that target other bionomic vulnerabilities to suppress transmission outdoors.