Unexpected attraction of polarotactic water-leaving insects to matt black car surfaces: mattness of paintwork cannot eliminate the polarized light pollution of black cars.

The horizontally polarizing surface parts of shiny black cars (the reflection-polarization characteristics of which are similar to those of water surfaces) attract water-leaving polarotactic insects. Thus, shiny black cars are typical sources of polarized light pollution endangering water-leaving insects. A new fashion fad is to make car-bodies matt black or grey. Since rough (matt) surfaces depolarize the reflected light, one of the ways of reducing polarized light pollution is to make matt the concerned surface. Consequently, matt black/grey cars may not induce polarized light pollution, which would be an advantageous feature for environmental protection. To test this idea, we performed field experiments with horizontal shiny and matt black car-body surfaces laid on the ground. Using imaging polarimetry, in multiple-choice field experiments we investigated the attractiveness of these test surfaces to various water-leaving polarotactic insects and obtained the following results: (i) The attractiveness of black car-bodies to polarotactic insects depends in complex manner on the surface roughness (shiny, matt) and species (mayflies, dolichopodids, tabanids). (ii) Non-expectedly, the matt dark grey car finish is much more attractive to mayflies (being endangered and protected in many countries) than matt black finish. (iii) The polarized light pollution of shiny black cars usually cannot be reduced with the use of matt painting. On the basis of these, our two novel findings are that (a) matt car-paints are highly polarization reflecting, and (b) these matt paints are not suitable to repel polarotactic insects. Hence, the recent technology used to make matt the car-bodies cannot eliminate or even can enhance the attractiveness of black/grey cars to water-leaving insects. Thus, changing shiny black car painting to matt one is a disadvantageous fashion fad concerning the reduction of polarized light pollution of black vehicles

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

<div><p>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 <i>Ephoron virgo</i> 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 <i>Ephoron virgo</i>, while other parts of the bridge were poor polarizers of lamplight. Collectively, we confirm that <i>Ephoron virgo</i> 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 <i>Ephoron virgo</i>.</p></div

Estimated numbers of <i>Ephoron virgo</i> mayflies attracted to polarized and unpolarized light sources placed above the Danube river on four dates in 2012.

<p>Number (mean ± standard deviation) of mayflies attracted to horizontally polarized (continuous line) and unpolarized (dashed line) light as a function of time (= local summer time = GMT + 2 hours, where GMT = Greenwich Mean Time). Each estimate is based on 10 photographs (see subsection <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0121194#sec015" target="_blank">Experiments with linear polarizers</a> of the <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0121194#sec011" target="_blank">Materials and Methods</a>). Comparisons of total detections over the course of each test indicate that more individuals were attracted to polarized versus unpolarized light sources of the same intensity: (A) 23 August, N = 11582, U = 181, Z = 10.302, p < 0.00001; (B) 24 August, N = 22786, U = 674.5, Z = 11.388, p < 0.00001; (C) 27 August, N = 5425, U = 303.5, Z = 8.945, p < 0.00001; (D) 28 August, N = 93935, U = 8682, Z = 10.465, p < 0.00001, where N is the number of total mayfly detections, U is the parameter giving the sum of ranks used in the non-parametric method, Z is the standard deviation of data for a given p, and p is the level of significancy (p < 0.05 means significant).</p

As Fig. 3 for the same asphalt road illuminated by light from the clear sky after sunset and measured from two different directions of view, when the optical axis of the polarimeter pointed toward South (A) and East (B).

<p>(C) As A and B for the down-stream side of the bridge and the Danube river.</p

Nocturnal artificial sources of unpolarized and horizontally polarized light interact to attract polarotactic insects.

<p>Unpolarlized light sources (e.g., street-lamps) may attract perching or flying nocturnal polarotactic insects directly (<i>Ephoron virgo</i> mayfly illustrated). Alternatively, unpolarized light from the street-lamp can become horizontally polarized through reflection from smooth, dark surfaces like asphalt, simulating the appearance of a water body.</p

(A, B) Color photograph, patterns of the degree <i>d</i> and angle α (clockwise from the vertical) of linear polarization, and areas detected polarotactically as water (for which <i>d</i> > 15% and 80° < α < 100°) of the dry asphalt road on the bridge (above the river Danube at Tahitótfalu) illuminated by bridge-lamps at night during the mass congregation of <i>Ephoron virgo</i> mayflies.

<p>The patterns were measured by imaging polarimetry in the blue (450 nm) part of the spectrum from two different directions of view, when the optical axis of the polarimeter pointed toward North (A) and West (B). The angle of elevation of the optical axis of the polarimeter was 20° from the horizontal. In the α-pattern the local direction of polarization of asphalt-reflected light is shown by a double-headed arrow. The white spot composed of millions of <i>Ephoron virgo</i> carcasses on the asphalt road below the bridge-lamp is well visible on the photographs as well as in the patterns of the degree of polarization <i>d</i> and the area detected as water.</p

Degree of linear polarization <i>d</i> of the shiny black (sb), matt black (mb) and matt grey (mg) horizontal test surfaces used in experiments 1 and 2 measured with imaging polarimetry in the red (650 nm), green (550 nm) and blue (450 nm) parts of the spectrum when sun- and skylight (A, Supplementary Fig. S4I) or canopylight originating from trees and bushes (B, Supplementary Fig. S4II) was reflected by the test surfaces.

<p>Columns: averages. Vertical bars: standard deviations. The average is calculated for the whole area of each test surface (corresponding to 2500×4000 = 10 000 000 pixels in the pictures and polarization patterns).</p

Statistical comparisons (Kruskal Wallis and Mann-Whitney U test) between the numbers of the three reactions (landing, touching, looping) of tabanids to the shiny black, matt black and matt grey horizontal test surfaces in experiment 2 (Fig. 6, Table S2).

<p>Statistical comparisons (Kruskal Wallis and Mann-Whitney U test) between the numbers of the three reactions (landing, touching, looping) of tabanids to the shiny black, matt black and matt grey horizontal test surfaces in experiment 2 (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0103339#pone-0103339-g006" target="_blank">Fig. 6</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0103339#pone.0103339.s007" target="_blank">Table S2</a>).</p

Total numbers of reactions (touching, landing and aerial looping) of tabanids to the shiny black (sb), matt black (mb) and matt grey (mg) horizontal test surfaces in experiment 2.

<p>The inset is a photograph of a tabanid fly landed on the matt grey test surface. The number of repetition is 20 (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0103339#s4" target="_blank">Materials and methods</a>, and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0103339#s3" target="_blank">Discussion</a>).</p