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.
Susan
3 comments:
How interesting that you have the same problem. In the spring, all along the Mississippi River which runs north/south through the USA, mayflies also can cover the roadway and make it very slippery.
In 1987 in mid-August around 18th/19th there was an invasion of mayflies in the Cher. We had some in Epeigné but there was a mass in Montrichard and Saint-Aignan, so many that the following they were scooped up by a mechanical digger. Never since seen a similar invasion in the Cher although have seen lesser invasion on the Ardour in Gascony.
Thanks both for the info. I've read about this in the US, but didn't realise it occurred in Europe too sometimes.
I've had an email from John Bratton (who wrote A Review of the Scarcer Ephemeroptera and Plecoptera of Great Britain). He wonders if they swarm in such numbers that they obscure the view. He says "I bet it is for the drivers' benefit, not the mayfly's. Caenis is the most likely to form huge clouds, I think."
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