Lamp-Lit Bridges as Dual Light-Traps for the Night-Swarming Mayfly, Ephoron virgo: Interaction of Polarized and Unpolarized Light Pollution

Abstract

Ecological photopollution created by artificial night lighting can alter animal behavior and lead to population declines and biodiversity loss. Polarized light pollution is a second type of photopollution that triggers water-seeking insects to ovisposit on smooth and dark man-made objects, because they simulate the polarization signatures of natural water bodies. We document a case study of the interaction of these two forms of photopollution by conducting observations and experiments near a lamp-lit bridge over the river Danube that attracts mass swarms of the mayfly Ephoron virgo away from the river to oviposit on the asphalt road of the bridge. Millions of mayflies swarmed near bridge-lights for two weeks. We found these swarms to be composed of 99% adult females performing their upstream compensatory flight and were attracted upward toward unpolarized bridge-lamp light, and away from the horizontally polarized light trail of the river. Imaging polarimetry confirmed that the asphalt surface of the bridge was strongly and horizontally polarized, providing a supernormal ovipositional cue to Ephoron virgo, while other parts of the bridge were poor polarizers of lamplight. Collectively, we confirm that Ephoron virgo is independently attracted to both unpolarized and polarized light sources, that both types of photopollution are being produced at the bridge, and that spatial patterns of swarming and oviposition are consistent with evolved behaviors being triggered maladaptively by these two types of light pollution. We suggest solutions to bridge and lighting design that should prevent or mitigate the impacts of such scenarios in the future. The detrimental impacts of such scenarios may extend beyond Ephoron virgo.

Mayflies are least attracted to vertical polarization: A polarotactic reaction helping to avoid unsuitable habitats

“…Because of the same stacked structure of the filter in both cases, the intensity and spectrum of the filter-transmitted torchlight were the same for the unpolarized and totally polarized state. This filter method was the same as used in our earlier field experiments with E. virgo [26].…”

“…The altogether 966 photographs were evaluated with our customdeveloped software (AlgoNet, http://www.estrato.hu/algonet) with an algorithm described in Száz et al [26] to count the mayflies attracted by the beam of torchlight of a given state of polarization. For counting the torchlit E. virgo mayflies in front of the dark night background, we first applied a Gaussian filter on the original image with a 31 × 31 pixels window and a 20 × 20 pixels width at half maximum to filter video noise.…”

“…In the case of mayfly species swarming immediately above the water surface, such as Ephoron virgo [Olivier 1791] ( Fig. 1) and P. longicauda [Olivier 1791], their positive polarotaxis is partly responsible for keeping them above water during their whole flying activity [9,21,26], while other mayflies may leave the water bodies up to a distance of 1 km [5]. In the latter case, positive polarotaxis guides the females back to water to oviposit.…”

“…The larvae of E. virgo develop only in rivers [14], while the larvae of C. robusta occur in slow-flowing streams, still waters and rivers [2,16,20]. There are similarities between their behaviours: They start to swarm after sunset [2,26] and do not leave the vicinity of the water surface [5]. At the beginning of swarming, the male subimagos of E. virgo emerging from exuviae land on the riverbank, moult to imagos, and then fly back to the river surface [13].…”

“…The male imagos fly rapidly in a straight line at a height of 2.5-5.0 cm directly above the water surface and mate with females. A typical event in the swarming behaviour of these mayflies occurs when they are approaching the bank, and before reaching it they suddenly reverse their direction of flight and fly back to the river mid-line, in order to keep their position above the water surface during swarming [26]. At the beginning, females fly above the water surface together with males ( Fig.…”