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

Abstract

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.

Figure 1. Mayflies attracted en masse to shiny black cars due to the highly and horizontally polarized light reflected from the car-body.

(A, B) Mass-swarming Ephemerella hendrickson. (C) Egg-laying Ephemera danica. (D) Thousands of mass-swarming female Ephoron virgo mayflies landed on a windscreen, onto which they laid their yellow egg batches. Photos A and B were taken by Dr. Rebecca Allen (Michigan State University, USA), while photos C and D originate from Dr. György Kriska (Eötvös University, Budapest, Hungary).

Figure 2. Photograph, patterns of the degree of linear polarization d and the angle of polarization α (clockwise from the vertical), and areas detected as water by polarotactic insects (for which the reflected light has the following characteristics: d>15%, 80^o<α<100^o) of a matt black car measured with imaging polarimetry from five different directions of view in the blue (450 nm) part of the spectrum.

The angle of elevation of the optical axis of the polarimeter was −20^o from the horizontal. The number-plate of the car is screened by a white rectangle.

Figure 3. Degree of linear polarization d 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.

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).

Figure 4. Total numbers of mayflies landed on the shiny black, matt black and matt grey horizontal test surfaces in experiment 1.

The inset is a photograph of a mayfly landed on the shiny black test surface. The number of repetition is 6 (see Materials and methods, and Discussion).

Figure 5. Total numbers of dolichopodids landed on the shiny black, matt black and matt grey horizontal test surfaces in experiment 1.

The inset is a photograph of a dolichopodid fly landed on the matt black test surface. The number of repetition is 6 (see Materials and methods, and Discussion).

Figure 6. 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.

The inset is a photograph of a tabanid fly landed on the matt grey test surface. The number of repetition is 20 (see Materials and methods, and Discussion).