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Happily mosquitoes are just an irritation, sometimes severe, to most of us in the UK but they’re a massive disease spreading problem in hotter parts of the world.
Other insects have similar lifestyles and biting midges are found where it’s damp or there are ponds and standing water. Even with fairly well fitted lids on the water butt, they often find their way in. One female can lay hundreds of eggs.
To avoid this happening float some olive oil on the top. Just enough to cover the surface with a very thin coating. About a tablespoon full should be enough.
After a month or two skim it off and replace with fresh oil. It may look a mess but it will not affect the water quality and I guarantee it will stop them from laying any eggs and kill any larvae.
Before you treat your water butt, remove anything floating on the top.
The only reason for using olive oil instead of the cheaper oils is that olive oil will not separate allowing minute particles to sink down into the water below. It also lasts longer without going rancid. Otherwise any oil does form a barrier and that’s what it’s all about.