General Visual and Contingent Thermal Cues Interact to Elicit Attraction in Female Aedes aegypti Mosquitoes.

Female Aedes aegypti mosquitoes use multiple sensory modalities to hunt human hosts and obtain a blood meal for egg production. Attractive cues include carbon dioxide (CO2), a major component of exhaled breath [1, 2]; heat elevated above ambient temperature, signifying warm-blooded skin [3, 4]; and dark visual contrast [5, 6], proposed to bridge long-range olfactory and short-range thermal cues [7]. Any of these sensory cues in isolation is an incomplete signal of a human host, and so a mosquito must integrate multimodal sensory information before committing to approaching and biting a person [8]. Here, we study the interaction of visual cues, heat, and CO2 to investigate the contributions of human-associated stimuli to host-seeking decisions. We show that tethered flying mosquitoes strongly orient toward dark visual contrast, regardless of CO2 stimulation and internal host-seeking status. This suggests that attraction to visual contrast is general and not contingent on other host cues. In free-flight experiments with CO2, adding a dark contrasting visual cue to a warmed surface enhanced attraction. Moderate warmth became more attractive to mosquitoes, and mosquitoes aggregated on the cue at all non-noxious temperatures. Gr3 mutants, unable to detect CO2, were lured to the visual cue at ambient temperatures but fled and did not return when the surface was warmed to host-like temperatures. This suggests that attraction to thermal cues is contingent on the presence of the additional sensory cue CO2. Our results illustrate that mosquitoes integrate general attractive visual stimuli with context-dependent thermal stimuli to seek promising sites for blood feeding.