I have a generally healthy Ceanothus (California Lilac) along the back fence (species forgotten, but I think it's a popular garden selection, now 12' tall and doing a fine job of hiding the neighbor). I've given the garden a lot of neglect in the recent months but found some time to do general cleaning and tidying the other day. I noticed that there were many ants crawling into the Ceanothus and further investigation showed that one of the branches was covered in scale! The Argentinian ants will farm this parasite, much as they farm aphids.
I've had scale problems before, but usually with exotic plants, not natives. However, here was undeniable proof that the natives aren't completely resistant to garden pests. A bit closer look at the shrub in question showed that only this one branch was affected. Why?
Apparently this one branch had grown onto the top of the fence where it rubbed in the wind and chafed the bark, partially girdling this branch. The Ceanothus then couldn't maintain normal defenses against scale and other infestations (pictures for the courageous below). I cut the branch off, chopped it up, threw it in the green waste bin, and considered my horticultural detective work done for the day.
The damage that lead to the infestation: