If you found a little white worm in a cherry, it most likely is the larvae of the western fruit fly. Female flies make holes in cherries and deposit one egg in each one. The proceeding larvae then munch on the cherries, causing damage to the fruit. If not harvested, when the cherries drop, the larvae burrow in the ground for the winter, reappear as flies in the spring and mate. If the cherry came from your tree, act quickly to get rid of them before too much damage is done. Also, monitor the situation early the next growing season to stop them from depositing eggs in your cherries at all.
Set out several yellow sticky fly traps, available in garden centres and where pest products are sold. Place them in a sunny location, next to and up in the tree branches at eye-level. Set them out when daytime temperatures reach 23.9 degrees Celsius. This is when they usually emerge from the ground. If you already know you have western fruit flies, proceed to Step 3.
Examine any flies caught. Western cherry fruit flies are black with stripes on their wings. Once the first western cherry fruit fly is caught, plan to apply the first insecticide treatment in seven days.
Apply an insecticide to your tree seven days after the first fly is spotted or caught on the trap. Colorado Sate University recommends several different insecticides, including spinosad, which it states is the least toxic to mammals. Spray the insecticide according manufacturer directions, applying it over the fruits of your cherry tree.
Repeat applications as instructed on the label or by your local county extension office. Apply spinosad, for example, every seven days with the last application one week before harvest.
Lay a landscape fabric under the canopy of your tree to catch dropped cherries. Discard the dropped cherries just in case there are larvae inside of them. In the meantime, they can't get through the fabric to burrow underground. Let your neighbours with cherry trees know of the problem and encourage them to test for western fruit flies and spray also. Call your local county extension office to let them know you found the western fruit fly larvae in your cherry tree. This may help them monitor the situation in the community.