Why do highly polarizing black burnt-up stubble-Welds not attract aquatic insects? An exception proving the rule

Horizontal black surfaces are usually very attractive to polarotactic aquatic insects, since they generally reXect highly and horizontally polarized light, and thus imitate water surfaces. We monitored highly polarizing black burnt-up stubble-Welds, but surprisingly never found aquatic insects or their carcasses in the ash, although Xying polarotactic insects were abundant in the area, which we showed by attracting them to horizontal black plastic sheets. To explain this observation, we measured the reXection–polarization characteristics of burnt-up stubble-Welds in the red (650 nm), green (550 nm) and blue (450 nm) parts of the spectrum at three directions of view relative to the solar meridian. We established that (i) the degree of linear polarization p of light reXected from the black ash is high; (ii) p is the higher, the darker the ash; (iii) the direction of polarization of reXected light is nearly horizontal only towards the solar and antisolar meridians, and it is tilted in other directions of view; (iv) the standard deviation of both the degree and the direction of polarization of reXected light is large. The latter two characteristics explain why burnt-up stubble-Welds are unattractive to aquatic insects. These results may be important in the study of the wider environmental eVects of post-harvest burning. © 2006 Elsevier Ltd. All rights reserved.