@johnlx , what stinker says about the bacteria. There are products containing Bt which is meant for caterpillars on leaves of many vegetables but will not work on insect larvae in water or below the soil surface. Look for the products that contain Bt
i which is a related bacteria but will work on the larvae in the soil or in water.
The product can be a small object that will float on the water while it is releasing the bacteria over several days. Or it can be loose particles that have been impregnated with the Bt
i and can be put in water or sprinkled on the surface of the soil.
I have seen both versions in gardening stores and in many hardware stores. Most likely it will be available in your country and easy to find once you get used to seeing what it looks like.