Laura, the first image is showing an infestation of aphids. The white specks are discarded exoskeletons and the black goop is black sooty mold. The latter is a simple mold that feeds on the sugary excrement from the aphids. The other pictures illustrate the damage from leaf miners, I believe.

better link for you below .... sure looks like one of my cherry ... but i surely have no clue about CA plants .. frankly .. those look like someone drilled into the tree .... to perfectly the same to be an animal or bug ... what bug you might see may be there after the fact... a full pic of the tree might help see if the situation is salvageable.. i wonder if the prior owner tried to kill it by drilling holes and dumping poison into it???? ... if they did.. they failed because they didnt know where the cambian layer was.... ken Here is a link that might be useful: link

OK, I surrender :) The insecticide pyrethrins, under discussion, is contained in many products with different brand names. For example, you could check labels of "house and garden" aerosols that are sold in supermarkets. Aerosol might be convenient for you. The ingredients are pyrethrins + pepironyl butoxide. Be sure it is labeled for use on plants. This insecticide kills most bugs (good or bad) on contact, but has no residual effect. This means you must reach each insect with the spray. It is considered relatively safe for mammals. Another low-risk insecticide is spinosad. It is residual but has less effect on beneficial insects. Google the term for brand names. Fertilome, Monterrey, and Gardens Alive are some producers. You want the active ingredient spinosad. It will kill rose slugs. You need a tank sprayer to apply it. Use insecticides only on plants where there is active damage going on.

"Dragonflykiller is obviously a crackpot. I will not be cutting down the tree." Lol!! An extremely outdated thread but I couldn't resist commenting on the OP's last thoughts 13 years ago. The though of a greengage plum knocking a cat unconscious I find very amusing :-) Just how flipping big are these plums?? Now if we were talking about monkey puzzle cones, an argument could be made for an injury from falling objects! But a plum?? Seriously??