Diel periodicity and visual cues guide oviposition behavior in Phlebotomus papatasi, vector of old-world cutaneous leishmaniasis

BackgroundPhlebotomine sand flies are vectors of human leishmaniases, important neglected tropical diseases. In this study, we investigated diel patterns of oviposition behavior, effects of visual cues on oviposition-site selection, and whether these affect the attraction of gravid Phlebotomus papatasi (Scopoli), the vector of old-world cutaneous leishmaniasis, to olfactory cues from oviposition sites.Methodology/principal findingsTo evaluate these questions, we conducted a series of experiments using attraction and oviposition assays within free-flight test chambers containing gravid females entrained under a 14:10 hrs light:dark photoperiod. By replacing sticky-screens or moist filter papers every three hours, we showed that oviposition site search occurs mainly in the latest part of the night whereas peak oviposition occurs during the early part of the night. Behavioral responses to olfactory oviposition cues are regulated by time-of-day and can be disrupted by transient exposure to a constant darkness photoperiod. Gravid females, but not any other stage, age, or sex, were attracted to dark, round oviposition jars, possibly resembling rodent burrow openings. This visual attraction disappeared in the absence of an illumination source. Egg deposition rate was not affected by jar color. Olfactory cues had the strongest effect when the visual cues were minimal.Conclusion and significanceOur study showed, for the first time, that visual cues in the form of oviposition-site color, lighting level, and photoperiod are important in guiding the oviposition behavior of phlebotomine sand flies. Furthermore, such visual cues could modify the flies’ sensitivity to olfactory oviposition cues. Our results suggest that chemosensory and visual cues are complementary, with visual cues used to orient gravid females towards oviposition sites, possibly at long- to medium-ranges during crepuscular periods, while olfactory cues are used to approach the burrow in darkness and assess its suitability at close-range. Implications to sand fly control are discussed.

研究背景
白蛉（Phlebotomine sand flies）是人类利什曼病的传播媒介，后者属于重要的被忽视热带病。本研究旨在探究白蛉产卵行为的日节律模式、视觉线索对产卵位点选择的影响，以及上述因素是否会影响旧世界皮肤利什曼病的传播媒介——巴氏白蛉（Phlebotomus papatasi (Scopoli)）孕雌个体对产卵位点嗅觉线索的趋性反应。

材料与方法/主要研究结果
为解答上述问题，本研究在光照周期为14:10小时的光暗循环条件下饲养孕雌白蛉，并于自由飞行测试舱内开展了一系列趋性与产卵行为实验。通过每三小时更换一次粘屏或湿润滤纸，我们发现白蛉的产卵位点搜寻行为主要发生在深夜时段，而产卵高峰期则出现在入夜初期。白蛉对产卵嗅觉线索的行为反应受昼夜时段调控，且可因短暂暴露于持续黑暗的光照周期而被干扰。仅孕雌白蛉（而非其他虫态、龄期或性别的个体）会被深色圆形产卵罐所吸引，此类容器可能模拟了啮齿动物洞穴的开口形态；在无照明光源的条件下，这种视觉趋性会消失。产卵率不受产卵罐颜色的影响，且当视觉线索最少时，嗅觉线索的作用效果最强。

结论与意义
本研究首次证实，产卵位点的颜色、光照强度与光照周期等视觉线索，对白蛉的产卵行为具有重要调控作用。此外，此类视觉线索可调节白蛉对产卵嗅觉线索的敏感性。研究结果表明，化学感应与视觉线索存在互补关系：视觉线索用于引导孕雌白蛉在晨昏时段的中长距离范围内定位产卵位点，而嗅觉线索则用于在黑暗环境中靠近洞穴并在近距离内评估其适宜性。本研究最后讨论了该发现对白蛉防控的启示意义。