dextrose is used so you don't have so much fermentation (might get your flies drunk)
A very very very very easy medium to make for flies is to simply add enough water to bakers yeast to make a thick gooey paste... sprinkle a bit of dry yeast on the top to stop the adults from sticking to it.
- A few drops of copper sulphate solution or the MHB mentioned above will stop the fungi
I have always used the recipe below and never added yeast to it. I add coloring to the medium so I can see the little larvae in the medium. I keep the unused amount in a container in the refrigerater for later use. I would half it to begin with. I raise my fruitflies in smaller Gatorade containers with thin cloth and a rubber band on top. I keep everything well sanitized and keep the containers in water to avoid mites and my fruitflies always seem to do just fine.