Fruit Flies

Many tropical fish in the wild eat a lot of insects. In the home aquarium they seldom get any. You won't find insects listed in the ingredients of flake fish food. About the only way to get insects into the diet of your fish is to raise fruit flies. For a long time fruit flies were something I had a lot of trouble raising.  I finally found a method that is not too much work and is reliable. I had had one continuous culture for 3 years, so I think I have finally figured them out.  I start with plastic bottles that soft drinks come in.  The best ones have wide openings, at least 1” in diameter. Wash them out and let them dry.

 You can buy commercial fruit fly medium. It is supposed to have mold inhibitors but I have found that it still often gets moldy. The other disadvantage with the commercial stuff is that you have to order a 20 year supply at a time to make it economical.    I prefer materials I can easily get locally. 

My culture medium starts with crushed Cheerios cereal. Cheerios seem to have some preservatives that helps keep them from getting moldy. I have used similar generic oat cereal but have had problems with mold. The critters you raise to feed your fish are not impressed with brand names and I usually buy generics to save money, but not here. Get the brand name in this case.  

I grind the Cheerios up in a blender. Don’t worry about getting it fine. Small chunks are fine, and probably better than finely ground. Mix equal parts of the crushed cereal with yellow corn meal.  I usually mix up a quart or so of it at time.

To start a culture put 4 rounded tablespoons of the dry mixture into the bottle.  Put in a couple of pinches of baker’s yeast. Shake it up for a bit to mix the yeast with the medium.  Add 3 tablespoons of warm water.  Finally, plug the top with a cotton ball or something similar to keep wild fruit flies and other insects out. 

It will look like you did not put in enough water, and it will take a while for the water to sink it. Don’t try to stir it. Just put it aside for a couple of days in a fairly warm spot to let the yeast to get started. I just set them on top of an aquarium.  

After a couple of days remove the plug and you will smell the yeast.  The next step is to place something for the flies to crawl on. I use strips of plastic canvas about 1” wide.  Plastic canvas is sold in craft shops.    

Put a funnel in the bottle and pour about a dozen flies from another culture into it. Remove the funnel and replace the plug.  Depending on the temperature, in a few days you will see the small maggots crawling around if you look closely.  Eventually they will crawl up the sides and form pupae. Then they will hatch as adults.  The first generation adults will then appear and can be shaken into the tank for feeding.

Although most bottles will be good for a couple of generations I try to start a new culture when the first new adults appear. The adults are reported to be most fertile shortly after emerging. At any given time I try to have 3-4 bottles at different stages. It is easy for a culture to get too old if you are not starting new ones to replace them.  I know one aquarist who marks the date on his cultures and starts another every 9 days.

There are a number of fruit fly strains that don’t fly.  Don’t mix different strains, and certainly don’t let any wild fruit flies into your cultures. If you do, then next generation will fly be able to fly and be a real pest if they get loose. That is what caused me to lose the 3 year old culture.

Fruit flies are great for top feeding killifish as well as other surface feeders like hatchet fish.  I find them as one of the best breeding conditioners for Procatopus and Epiplaytus species. Because the flies walk on the surface of the water, fish seem to need to learn how to eat them. Pour in some flies with a bit of dry flakes and the fish will quickly learn how to grab the flies.  

Fruit fly culture in a wide mouth soft drink bottle.  Small flies can be seen as spots in the bottle.