The small worm-like creatures on mulberries are most likely fruit fly larvae. Fruit flies frequently infest thin-skinned fruit such as mulberries, blackberries, grapes, raspberries and blueberries. They are particularly drawn to over-ripe, decaying fruit. Adult female fruit flies lay eggs inside fruit. The eggs hatch within three days, and the larvae will mature in the fruit within a matter of days. The larvae can live inside the fruit for as much as 13 days, ingesting and softening the fruit as it grows.
The fruit fly undergoes many transformations in its brief existence before it evolves into a brownish-colored adult, with red eyes and transparent, fly-like wings. It starts as an oval, white egg, approximately 0.6 millimeters long. Once hatched, it forms legless, headless, white or transparent larvae. Larvae, which are as much 6 millimeters long, are readily observable by the naked eye. These worm-like creatures are commonly observed in and around mulberry fruit before they turn into brown, football-shaped pupa.
Female fruit flies deposit eggs in the late summer, shortly before harvest. The fly will make a hole the size of a pin prick. The larvae will begin to feed on the mulberry fruit from the inside as they hatch. This accelerates fruit softening and collapse. Besides contaminating fruit with their larvae, the small fruit fly egg entry point provides an opening for other insects and disease-causing organisms.
Some debate exists concerning whether it is good idea to eat mulberries infected by fruit flies. Freezing fruit or soaking it in cold water will kill the larvae, but their dead carcasses will remain inside the fruit. Soaking fruit in salt water draws out the larvae and brings them to the surface. Once the larvae are skimmed, the majority of gardening professionals seem to consider the fruit safe, sanitized and in every way edible. Some gardeners, on the other hand, will suggest throwing away the fruit because of possible secondary infections.
End the fruit fly cycle. Remove affected fruit from the tree, seal in bags and place in sunlight for a few weeks to suffocate immature fruit flies. Address adult population using protein bait and male annihilation bait. Some lures and insecticides are effective only at killing male fruit flies. These male annihilation baits will eliminate only males but will prevent future mating and egg production. Protein bait techniques use protein made from yeast as bait for spot-sprayed insecticide. For best results, combine treatments and use a high density of bait stations to lure the pests.