"Alla Bezroutchko" wrote
Is there any way to prevent mosquitoes from breeding / laying eggs /
whatever it is that they do in my rainwater butt? There seems to be a
lot of them in there, at least when the butt is partly empty.

I don't want to increase the mosquito population, particularly close
to the house where I live.

Try adding a few drops of cooking oil, just enough to make a film over
the surface, and use a close fitting lid to prevent them getting in
there in the first place.

--
Sue