Collect and destroy affected fruits. Fruit flies are fewest in hot weather and peak in the rainy season, so adjust the sowing time. Plough to expose the pupae. Set cucurbit fruit-fly traps at 4 per acre. Neem oil 3% is also listed. Do not use copper or sulphur dust on melons — they damage the plants.
Pending expert review
Source
Crop Production Guide – Horticulture Crops 2020 (Directorate of Horticulture and Plantation Crops, Tamil Nadu & Tamil Nadu Agricultural University (TNAU)), Watermelon – Plant protection (fruit fly), p. 102
