Modification of spontaneous activity patterns in the malaria vector Anopheles gambiae sensu stricto when presented with host‐associated stimuli

Anopheles gambiae sensu stricto Giles (Diptera: Culicidae) is the main malaria vector in sub‐Saharan Africa. Mated females show a circadian rhythm of spontaneous activity under constant environmental conditions that extends across the scotophase (subjective night). The effect of host‐associated cues i.e. human foot odour supplemented with carbon dioxide (CO₂)] on this nocturnal activity pattern is studied in laboratory‐reared A. gambiae s.s. M molecular form. Sixteen mated females (5–10 days old) are held in individual chambers (diameter 3.5 cm, length 4.5 cm) in a wind‐tunnel with a continuous flow (8.0 cm s^?1) of clean air. At the onset of hours 3, 6 and 10 of the scotophase, their behaviour is recorded on video for 15 min in clean moving air and then for the next 15 min, with a specific treatment present in the clean airstream: (i) constant CO₂ (4.8%) plus human odour; (ii) pulsed CO₂ (5 s of every 30 s) plus human odour or (iii) control (clean air). Activities of individual mosquitoes are scored by direct observation of the video records for the incidents of ‘resting’, ‘walking’, ‘jumping’ and ‘flying’ in each of the 15‐min observation periods. There is a significant interaction between hour and treatment on mean changes in female activity level (P = 0.00004). Constant treatment increases the level of activity of A. gambiae females significantly, although only in hour 3 of the scotophase (P < 0.01), whereas pulsed and control treatments show no significant effect throughout the scotophase. Patterns of spontaneous activity in individual A. gambiae females thus appear to be modulated by host‐associated cues, resulting in a more active phase early in the night than might be expected from records of spontaneous activity under constant environmental conditions. Possible ecological and experimental implications of such an increase in activity are discussed in relation to host‐seeking behaviour.