The alteration of natural cycles of light and dark by artificial light sources has deleterious impacts on animals and ecosystems. Many animals can also exploit a unique characteristic of light – its direction of polarization –as a source of information. We introduce the term “polarized light pollution” (PLP) to focus attention on the ecological consequences of light that has been polarized through interaction with human‐made objects. Unnatural polarized light sources can trigger maladaptive behaviors in polarization‐sensitive taxa and alter ecological interactions. PLP is an increasingly common byproduct of human technology, and mitigating its effects through selective use of building materials is a realistic solution. Our understanding of how most species use polarization vision is limited, but the capacity of PLP to drastically increase mortality and reproductive failure in animal populations suggests that PLP should become a focus for conservation biologists and resource managers alike.

The caddis flies Hydropsyche pellucidula emerge at dusk from the river Danube and swarm around trees and bushes on the river bank. We document here that these aquatic insects can also be attracted en masse to the vertical glass surfaces of buildings on the river bank. The individuals lured to dark, vertical glass panes land, copulate, and remain on the glass for hours. Many of them are trapped by the partly open, tiltable windows. In laboratory choice experiments, we showed that ovipositing H. pellucidula are attracted to highly and horizontally polarized light stimulating their ventral eye region and, thus, have positive polarotaxis. In the field, we documented that highly polarizing vertical black glass surfaces are significantly more attractive to both female and male H. pellucidula than weakly polarizing white ones. Using video polarimetry, we measured the reflection-polarization characteristics of vertical glass surfaces of buildings where caddis flies swarmed. We propose that after its emergence from the river, H. pellucidula is attracted to buildings by their dark silhouettes and the glass-reflected, horizontally polarized light. After sunset, this attraction may be strengthened by positive phototaxis elicited by the buildings' lights. The novelty of this visual-ecological phenomenon is that the attraction of caddis flies to vertical glass surfaces has not been expected because vertical glass panes do not resemble the horizontal surface of waters from which these insects emerge and to which they must return to oviposit.

We reveal here the visual ecological reasons for the phenomenon that aquatic insects often land on red, black and dark-coloured cars. Monitoring the numbers of aquatic beetles and bugs attracted to shiny black, white, red and yellow horizontal plastic sheets, we found that red and black reflectors are equally highly attractive to water insects, while yellow and white reflectors are unattractive. The reflection-polarization patterns of black, white, red and yellow cars were measured in the red, green and blue parts of the spectrum. In the blue and green, the degree of linear polarization p of light reflected from red and black cars is high and the direction of polarization of light reflected from red and black car roofs, bonnets and boots is nearly horizontal. Thus, the horizontal surfaces of red and black cars are highly attractive to red-blind polarotactic water insects. The p of light reflected from the horizontal surfaces of yellow and white cars is low and its direction of polarization is usually not horizontal. Consequently, yellow and white cars are unattractive to polarotactic water insects. The visual deception of aquatic insects by cars can be explained solely by the reflection-polarizational characteristics of the car paintwork.

1. We observed that the dragonfly species Sympetrum flaveolum, S. striolatum, S. sanguineum, S. meridionale and S. danae were attracted by polished black gravestones in a Hungarian cemetery. 2. The insects showed the same behaviour as at water: (i) they perched persistently in the immediate vicinity of the chosen gravestones and defended their perch against other dragonflies; (ii) flying individuals repeatedly touched the horizontal surface of the shiny black tombstones with the ventral side of their body; (iii) pairs in tandem position frequently circled above black gravestones. 3. Tombstones preferred by the dragonflies were in the open and had an area of at least 0.5 m 2 with an almost horizontal, polished, black surface and with at least one perch in their immediate vicinity. 4. Using imaging polarimetry, we found that the black gravestones, like smooth water surfaces, reflect highly and horizontally polarized light. 5. In double-choice field experiments with various test surfaces, we showed that the dragonflies attracted to shiny black tombstones display positive polarotaxis and, under natural conditions, detect water by means of the horizontally polarized reflected light. This, and the reflection-polarization characteristics of black gravestones, explain why these dragonflies are attracted to black tombstones. 6. If females attracted to the black gravestones oviposit on them, the latter constitute ecological traps for dragonflies that are not close to water.

Recently it was observed that the Hydropsyche pellucidula caddis flies swarm near sunset at the vertical glass surfaces of buildings standing on the bank of the Danube river in Budapest, Hungary. These aquatic insects emerge from the Danube and are lured to dark vertical panes of glass, where they swarm, land, copulate, and remain for hours. It was also shown that ovipositing H. pellucidula caddis flies are attracted to highly and horizontally polarized light stimulating their ventral eye region and thus have positive polarotaxis. The attraction of these aquatic insects to vertical reflectors is surprising, because after their aerial swarming, they must return to the horizontal surface of water bodies from which they emerge and at which they lay their eggs. Our aim is to answer the questions: Why are flying polarotactic caddis flies attracted to vertical glass surfaces? And why do these aquatic insects remain on vertical panes of glass after landing? We propose that both questions can be partly explained by the reflection-polarization characteristics of vertical glass surfaces and the positive polarotaxis of caddis flies. We measured the reflection-polarization patterns of shady and sunlit, black and white vertical glass surfaces from different directions of view under clear and overcast skies by imaging polarimetry in the red, green, and blue parts of the spectrum. Using these polarization patterns we determined which areas of the investigated glass surfaces are sensed as water by a hypothetical polarotactic insect facing and flying toward or landed on a vertical pane of glass. Our results strongly support the mentioned proposition. The main optical characteristics of "green," that is, environmentally friendly, buildings, considering the protection of polarotactic aquatic insects, are also discussed. Such "green" buildings possess features that attract only a small number of polarotactic aquatic insects when standing in the vicinity of fresh waters. Since vertical glass panes of buildings are abundant in the man-made optical environment, and polarotactic aquatic insects are spread worldwide, our results are of general interest in the visual and behavioral ecology of aquatic insects.