Urban sports fields as an emerging evolutionary trap for polarotactic dragonflies

We surveyed outdoor sports fields in Haikou City, Hainan Province, south China, from September to December 2024. For each field, we recorded its geographic coordinates and the composition of its surface colors. We observed whether dragonflies were laying eggs on the field surfaces, either with eyes or using binoculars. When egg-laying behavior was observed, we recorded the number of dragonflies ovipositing on each surface color. From a human visual perspective, the surface colors of the fields were easily distinguishable. We broadly classified them into four categories: red (R), green (G), blue (B), and yellow (Y). If a particular color was present at a site and used for oviposition, it was recorded as 1; if the color was present but not used, it was recorded as 0. Finally, we investigated the number of dragonflies that spawned at 37 sites (Table S1), along with the presence or absence of the four surface colors and whether they were used for oviposition (Table S2).To quantify the visual effect of the pitch surface from both human and dragonfly perspectives, we measured the reflectance spectrum of different colors using a spectrometer (AvaSpec-ULS2048, Beijing, China). We selected five of the twelve sites where dragonfly spawning was observed to measure the reflectance spectrum of different surface colors. These five sites included all four color types and were representative of two or more colors at each site. Reflectance spectra were measured following a standardized procedure (Yang et al. 2010). Each color was measured six times at each site.By investigating the attractiveness of different colored parts of the field to dragonflies, we found that dragonflies preferred to lay eggs on green and blue areas and rarely did so on red areas. Reflectance spectrum analysis showed that red was most distinct from the other colors, while green, blue, and yellow were less distinguishable or even indistinguishable, suggesting spawning preferences by dragonflies may stem from similarities between these colors and natural water features. Although sports fields create an evolutionary trap through polarized light as perceived by dragonflies, our study demonstrates that color vision could also provide a potential means of escape. By optimizing the color design of sports fields, the deceptive effects of polarized light can be mitigated, thereby contributing to the conservation of insect biodiversity within urban ecosystems.