Microbial contamination of artificial insect food media can jeopardise the viability, productivity and survival of many insect cultures, including Drosophila melanogaster. Here we investigated and improved upon control methods for one common contaminant, Penicillium roqueforti. We found that the combined effect of methyl p-hydroxybenzoate (23.7 mM), propionic acid (67.5 mM) and sorbic acid (8.9 mM) (PSNPS treatment) was the most effective of the four candidate treatments, at inhibiting the growth of P. roqueforti. PSNPS treatment inhibited 100% of visible P. roqueforti growth for 21 days (a complete D. melanogaster life cycle) and thus reduced the risk of transmitting infection to the next generation. Although the PSNPS treatment negatively affected the two D. melanogaster fitness components, survivorship (number of adults) and biomass (live weight), it did not prevent successful reproduction and is suitable for short-term treatment of P. roqueforti infections.