Answer: Those worms are the larvae of the western cherry fruit fly. In the spring, adult flies lay eggs under the skin of the cherries, and in a larval form, the maggots (or worms) feed on the inner meat of the cherry. As the cherries ripen, the larvae drop to the soil and remain in a dormant state until the next spring where they emerge as mature flies, to lay eggs in both sweet and tart cherries.
It is very difficult to tell if there are fruit fly maggots in maturing cherries. The fly's eggs and the puncture wound left from where the female fruit fly inserted them are very small and not readily detectable, but a puncture wound may show up as a divot on the surface of the cherries when ripened. When untreated cherries are ripe, the cherries that have a small hole in them may be safer to eat than the ones without, as that is the exit hole of the maturing larvae.
The best way to control western cherry fruit flies is by not allowing them to get into the fruit, making it necessary to treat the cherries periodically with a pesticide. Effective insecticides spinosad (Success or Entrust), permethrin, carbaryl (Sevin), methoxychlor, malathion, pyrethrum (Pyganic), endosulfan (Thiodan), and azadirachtin (Neem, Azatin). Apply according to label directions
For helpful cultural control, place plastic landscape fabric or another barrier on the ground under the canopy of cherry trees to prevent larvae in dropped fruit from burrowing into the soil where they will pupate for the winter. Landscape fabric placed in the spring will also prevent adults from emerging from the soil. Keep the fabric in place year-round and prevent a buildup of soil and debris on top that would provide pupation sites for the fruit fly.
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