Watch out for mayflies ?!! In October ?!
I first noticed this temporary warning sign on the bridge at Bonneuil-Matours on 10 October. I assume it is warning of a mass emergence from the River Vienne below of some late summer emerging species of mayfly like the Burrowing Mayfly Ephoron virgo (known as manne ie manna by French fishermen), which can emerge in vast numbers for a few days from big slow rivers like this. The mayflies are strongly attracted to the lights on bridges and end up coating the road surface, making it slippery from the body fat they store to get them through their short adult lives.
Many species of mayfly are extremely sensitive to modern agricultural chemicals, so it can be a sign that the water quality here is very good and unpolluted.