Reflected polarization guides chironomid females to oviposition sites

Chironomids (Diptera: Chironomidae; non-biting midges) are known to be carriers of the Vibrio cholerae bacterium, responsible for the fatal cholera disease in humans. It was recently discovered that chironomid females choose their oviposition site by a visual cue. In this study, we test the hypothesis that this visual cue is the linear polarization of light reflected from the water surface. We conducted two multiple choice field experiments using egg traps with different light intensities and polarizations. With controlled illumination, a higher number of eggs was found under both high intensity and high polarization. Under natural illumination, no eggs were found in the unpolarized traps, and the egg number increased with the percentage polarization regardless of the light intensity. Field measurements showed that at sunset, when chironomids are active, the intensity of light reflected from their natural ponds decreases by 96%, while the percentage polarization remains stable and high at 60%. Furthermore, the percentage polarization is positively correlated with the total organic carbon (TOC) concentration in the water. Orthogonal alignment of the microvilli found in ommatidia from the ventral part of the female eye may provide the anatomical basis for polarization sensitivity. We conclude that the percentage polarization of reflected light is the cue by which chironomid females choose their oviposition site. It is a stable cue and can provide information on the amount of food available to the larvae in the water. Based on our results, we suggest that manipulating the polarization of reflected light is a viable way to control chironomid populations and mitigate cholera dispersion.