I got an email the other day from Don, one of the senior nature guides at the arboretum (and my nature guide mentor, actually), in which he had forwarded a photo of a bug for me to try to identify. It was from a lady in nearby Buena Park, who had a bunch of "white worms" on her mulberry tree.
This is the photo she sent. Looks yucky, huh? Also, doesn't look like worms. Well, actually, it sort of does, but what kind of worms could be all over the branches like that? I called the lady, and she couldn't really describe them any better than her photo, but she was happy to have me come out and look at them. Don agreed to go with me, and we were like real Bug Detectives out on a case!
Once we were able to see them in person, it was clear that they were scale insects, but we didn't know exactly what kind.
In this photo you can see the oval shaped scale insects. The white stuff, which in the first photo looked like the white worms, are egg cases. If you look carefully, you can see some baby scale insects, tiny ovals called "crawlers", so named because they are much more mobile than the adults. I'm actually not sure if the larger, smoother ovals are the same species as the ones with the egg cases. Even our curator, Chris, thought there might be 2 different kinds of scale when he looked at it.
In this photo, the scale insects are on the backside of a leaf, and there are fewer of them, so it's a little easier to make out the individuals. Also, there were lots of ants all over these guys, as they collect the honeydew that the scales secrete.
When I got home, I googled white/scale/mulberry/pest, and the closest match I can find is the cottony camellia scale, Pulvinaria floccifera . There are pages about it here and here. The timing seems off, because they make their egg cases in May, but we've had such weird weather this year, who knows?
Chris says that horticultural oil is the best treatment for these scale insects.