Can citric acid be used as fungicide in banana based fly (Drosophila melanogaster) food?
I have no data on the fungicidal and insecticidal properties of citric acid at various concentrations (and you may be hard pressed to find a good study on this), but I have worked in some fly labs and I understand the composition of fly food only peripherally. The most commonly and most abundantly used fungicide in a lot of fly labs is methylparaben (trade names Tegosept or Nipagin in Europe). I imagine food recipe-makers have arrived at this solution through trial and error over the years; this is as much an art as it is a science, in balancing the ingredients with humidity, temperature, species and genotype with desired generation time. Methylparaben is used with and without propionic acid for Drosophila melanogaster. Do you have methylparaben in your fly food? Also, I think citric acid is not commonly used, again I imagine because technicians probably found it inferior to its alternatives.
The National Drosophila Species Stock Center at Cornell uses methylparaben with its banana food recipe.
Alternatively, please take a look at the variety of recipes Bloomington Drosophila Stock Center uses. To my knowledge they supply and stock the largest collection using the most recipes.

Fly foods
Alternative recipes