Disentangling the visual cues of an evolutionary trap for aquatic insects

Abstract Aquatic insects use polarized light as a reliable visual cue for locating water surfaces given their need to locate sites for oviposition. However, many man-made surfaces polarize light more strongly than natural waterbodies creating an evolutionary trap in which many species preferentially lay their eggs on these polarizing artificial surfaces. Previous work has shown that the attractiveness of artificial surfaces to aquatic insects is diminished by adding non-polarizing gridlines to these surfaces. However, it is unknown how this mitigation affects aquatic insect preferences. We tested two alternative hypotheses about how aquatic insects judge the quality of potential oviposition sites. The visual averaging hypothesis states that insects judge the quality of a surface based on the percent area of the surface that is polarizing. An alternative hypothesis is that the quality of a polarizing surface is judged by the degree to which it is fragmented by non-polarizing elements. This experiment was conducted using oil tray traps as artificial polarizers whose percentage of polarizing area and the presence/absence of fragmentation was manipulated. Only Diptera were captured in sufficient numbers to test the hypotheses. Our findings for the dominant family in our captures, Dolichopodidae, were consistent with the visual averaging hypothesis – increasing the percent area that was non-polarizing significantly decreased captures, but the fragmentation of a polarizing surface had no significant effect on the number captured. For the other families of aquatic Diptera combined, however, there was a complex interactive effect of percent area of a surface that was polarizing and its fragmentation by non-polarizing gridlines. For the conservation of aquatic insects, these findings support the effectiveness of reducing the attractiveness of artificial polarizing surfaces such as solar panels by adding non-polarizing elements, but also show that for some aquatic insects, it is important to consider if the non-polarizing elements fragment the surface.