While prevention should always be used to prevent fruit flies and fungus gnats they can come from unexpected sources and for this I keep a Bug Zapper in my basement near my bins on all the time. This is a low cost way to prevent them from spreading if they do take hold and a great way to know if you do have a problem before it becomes noticeable!

For those that have the worms in areas without proper electricity this will work great as you can use USB to charge it or a portable battery to run it for long periods of time. This can also be used to focus on a problem system and prevent bugs from spreading.

I know this says Mosquito right on the packaging but it works against other flying insects as well. What is really great is this is very safe to use and can kill the fungus gnat larvae before they start to fly and spread. This is also a more natural deterrent as it contains biological control instead of a chemical one. If you see on the package it even says for Organic Production so you can feel safe in using the castings in your yard later!

These sticky traps are fantastic at treating individual systems and I have found the worms don't tend to stick to them. I have left these in my bins for over a month and they are still sticky and don't drip into your system. These work really well at capturing flies as they move toward a high point to take flight and is another system you can use as a warning to ensure you don't have any flies or gnats in your system. Only drawback to these is once you pull off the plastic covering the sticky part you can't put these down they need to stay upright.

This powder when dry works as a great deterrent for any insect and I now use this exclusively in my house to stop ants from entering my house. The material when dry is very hard and breaks the exoskeleton of insects making it where they dry out. In a worm bin I have had mixed results in reigning in flies with this as bins are typically damp but it can be used effectively around the sides when the flies climb up. As a secondary benefit this can be used as grit to helps the worms grind up the food they are eating!

This is similar to Diatomaceous Earth and can act as a grit for the worms while adding trace minerals to help with cocoon production. Just as people worms should be given a varied diet to help them be healthy. This can be a good substitute for when ground up egg or oyster shells are not available.