Flies in the tunnel

[Issy]

Wondering if anyone else has experienced this - we just returned from a weekend away - went to inspect the polytunnel and found loads of flies in it - not sure what to do ! Nothing seems to have been affected - have cucumbers, strawberries and lettuce all growing well.

[JMCKG]

Not quite the same thing, but similar problem. Down the plot on Monday returned today to find our shed invested with flies. Luckily had a old tin of fly killer, so got most if not all. Looked around could not see anything dead or food waste etc. that may have attracted them. First time this has happened, any ideas why?

[Jungle Jane]

They could be cluster flies looking to hibernate I was reading that they let off some pheromone to let other flies know,I haven't posted the link because it was an advert.