Too many flies can yield a poor plum harvest. They not only eat the fruit, but some flies, such as sawflies and their young larvae, eat the leaves and can cause a plum tree to lose its leaves. Sometimes, the damage is irreversible. The best time to get rid of the flies is when they are in their larval stage, which is often while plum trees are flowering. If you keep the larvae under control, the fewer flies you will have to contend with.
Clean up around and under your plum tree to avoid attracting flies. Discard dropped and rotting fruit, as well as any other debris, such as the dropped flowers. Clean up nearby garbage as well and keep trashcan lids on tight. Clean up any pet feces near your plum tree immediately.
Pick off the larvae when they appear on the leaves, usually in spring and early summer (when the tree is in blossom) and then again in late summer. Wear gloves if desired, and pull the larvae off the leaves and dump them in a bucket or can of soapy water.
Spray leaves with a hard stream setting on your garden hose sprayer. Spray the larvae that were too stubborn to pick off. The water will dislodge the larvae from the leaves and much of them will not return or they may die. Unfortunately, some may return, so keep an eye out for stubborn fly larvae.
Rub the leaves with the larvae with an insecticidal soap or horticultural oil that is labeled to control flies. That works well for small larvae populations and as an alternative to manual picking or spraying with water.
Hang up yellow sticky flytraps to catch full grown flies. That will help control the number of eggs that are going to be laid. Hang them on branches throughout the tree and replace as needed.
As a last resort, apply an insecticide labeled to kill the larvae. Examples include insecticides that contain the active ingredients acephate, carbaryl or permethrin. Try not to get any on the fruits if they have started to develop, and always follow the application and dosing instructions on the label. Wash the mature fruit thoroughly before eating.