According to: http://www.enerzine.com/3/13776+decouvr ... ctes+.html
"We realized quite recently that the fouling of the leading edges of the blades of these turbines by insects on a wind turbine installed in the countryside when it is running on a beautiful summer evening will pick up a considerable number of insects So we could say to ourselves that the impact is only aesthetic, at a pinch, but, in fact, not at all. We realized that the impact in terms of production loss was around 20%, 30% or even 40% in the worst case. This means that the economic and industrial stakes are considerable in controlling the impact of roughness and the surface condition of these machines. "
Presentation of research by MINES ParisTech on the subject: http://www.dailymotion.com/video/xpvopn ... riale_tech