I’m a big coffee drinker and fan of really great coffee so I’m always on the lookout for a use for coffee grounds. For example, I know worms treat them like candy and prefer them to most other foods (I have worm farms).
And I know that caffeine is a great slug killer but coffee grounds don’t contain enough left-over caffeine to really do anything to slugs except “maybe” deter them a bit. (it’s not the grounds the slugs don’t like, it’s the caffeine contrary to published Net blogs that say the grounds work)
What I didn’t know was that coffee grounds are pretty potent mosquito larva killers. Four heaping tablespoons or big soup spoons of coffee grounds – when mixed with 250 ml of water offer 100% control of larva. Kills ’em dead!
From a practical point of view – making sure this amount of used coffee grounds in water to get the right concentration is a standing pool of water is going to take a ton of grounds so it might not be practical there.
It may work as a spray if you can isolate a section of long grass where mosquito larva are living.
Where it is going to be practical is in the garden itself. So when you have leafy plants (Southern gardeners take note!) where water sits in the leaf axils (where the leaf hits the stem) and mosquitoes breed there, dusting plants with coffee grounds will put enough grounds into the axils to control breeding mosquitoes and eliminate one source of these bloodsuckers.
I also note the longer the caffeine mix sits around – the more potent it becomes so don’t wash it out.
Day-old caffeine solutions took 20 days to kill 100% of the larva while a 25 day-old solution killed 100% in 1 day.
So you can feed them to the worms in compost bins or dust plants in the garden.