IRS and vector behaviour — DTM&H MCQ

Correct answer: B — Endophilic (indoor resting) behaviour

The correct answer is B, endophilic (indoor resting) behaviour. Indoor residual spraying (IRS) works by depositing insecticide on walls and ceilings, so it only kills mosquitoes that rest on treated indoor surfaces after feeding. An. gambiae is classically highly endophilic and endophagic, which is precisely why IRS has historically been so effective against this vector in sub-Saharan Africa. If a population is or becomes exophilic (resting outdoors), it will simply avoid contact with the sprayed surfaces and IRS coverage fails regardless of insecticide potency. This behavioural trait, not biting time, host preference, larval site or flight range, is the direct mechanistic link to whether the insecticide ever reaches the mosquito. Why the other options are wrong: C. Larval habitat preference: this determines where larvicides or environmental management (source reduction) are targeted, not where an adult mosquito rests after a blood meal, so it is irrelevant to IRS contact. D. Crepuscular vs nocturnal biting: this affects the value of bite-prevention measures like bed nets and repellents timed to biting hours, but has no bearing on whether the insecticide-treated wall is contacted afterwards. A. Blood meal preference: anthropophily versus zoophily influences transmission efficiency and vectorial capacity, but a strongly anthropophilic exophilic mosquito would still evade IRS. E. Flight range from breeding sites: this informs the radius around breeding sites requiring larval control or the scale of spraying zones, not the surface-contact mechanism that IRS depends upon. Key point: IRS only kills mosquitoes that rest on sprayed indoor surfaces, so endophily (not biting time, host choice, larval habitat, or flight range) is the behavioural trait that determines its effectiveness.

Reference: Cochrane Database of Systematic Reviews, 'Indoor residual spraying for preventing malaria' (Pryce, Richardson, Lengeler, 2018): IRS is most effective against endophilic, endophagic Anopheles mosquitoes that rest on sprayed surfaces. https://pmc.ncbi.nlm.nih.gov/articles/PMC6441326/