Here is the photo that went with my SF Chronicle (and sfgate.com) column last Sunday. I'll bet it looks fine on the website, but in the paper it was in B&W and was printed so low contrast that you couldn't really see the damage on the plums. So for those who couldn't make it out, here it is. Those scars were caused by a leafroller, a caterpillar that is the larva of a moth.
What was interesting to me is that when you see photos of the damage this caterpilar causes, you always see either the caterpilar or the damage it does to leaves, rather than fruit damage. It is the sleuth symptom, never considered important enough to show, so always a mystery to most of us.
I have seen damage like this for a long time, on various fruits, and nary a photo anywhere that was captioned to explain what did it. My dad called them "bee stings", though we both knew that wasn't right. Thank you to Anita Crotty, who took this photo and asked what caused the damage. And thank you to Dr. Paul Vossen, who filled me in on the cause of the scars.
Apparently the caterpillar feeds on young leaves, making a bit of webbing to roll a leaf around it to hide from predators. If the leaf is next to some very young fruit, the caterpillar nibbles a bit of that too. If ignored, the problem can get worse every year. If you see these scars one year, you can prevent the insect from overwintering on your tree by using a dormant oil spray in January or February, before the buds begin to open, or, failing that, using some Bt spray when you see tiny caterpillars, about when the tree is in flower.