Web Master:
Unfortunately, you photo did not come through. It is most likely a type of sawfly, maybe a plum sawfly, my first thought was a Plum and Cherry Slug but there bodies are black and shiny. The best time to control sawfly is in late spring, around November, before the insects have had a time to multiply. Non chemical prevention methods include the removal of insects by hand, clearing the area around the tree so insects that have fallen from the tree have nowhere to hide, increasing air circulation through the branches by pruning trees in winter. There various chemicals that can be used include Success Ultra, a Yates product, use as per instructions on label.