The Rose Slug gets busy every year in early summer and can skeletonize your rose leaves. Starting sometime in May. You see them every year. The leaves have hundreds of holes, brown spots and look just awful. When you look you often don't see anything.
The culprit is the Rose Slug, This is the larval stage of a Sawfly insect. The Rose slug larvae turns into a pupae and overwinters in the soil and emerge in spring as a Sawfly to lay eggs on your rose leaves. [I have no idea what the Sawfly really looks like since I have only seen pictures. They are dark brown and tiny. ....only the damage that comes afterwards.] The eggs hatch out into these little green larvae that look a bit like a caterpillar. They eat like crazy and then disappear. Most varieities have only have one cycle but they can do lots of damage in a short period of time. My customers talk about them being a problem all summer and the article I read said that some kind can have more than one cycle. If anyone has more information for southern California why not share it with us.
After the leaves are skeletonized and brown it does no good to spray. That's like locking the barn door after the horse is gone! Too late. All the Bayer Rose products with cyfluethrin work really well. On the labels of other products they list what that product is effective on. Look for Sawflies, larval insects etc. We have them most of them for sale.

You can just barely see the little green slug at the top of the second leaf from the left. The next picture is a close up. The dark part of the larvae is the leaf munchings going through the gut of the larvae...Gross!

The Organic way. You can choose to not use an insecticide and hand pick and spray off the little slugs worms with a strong spray of water.
Horticultural oil used when they are first attacking will work and is a very safe alternative. Trichograma beneficial wasps won't help and BT will not work. both of these are specific to caterpillars and this is a larvae not a caterpillar. Sorry!

The Bayer Rose products that have cyfluthrin is The Bayer rose and flower spray and that is one of the products that will kill them. Ortho Orthene Systemic Insect Control Concentrate and Ortho Orthene systemic Insect control with Acephate are all brands that will help. Spray just as soon as you begin to see damage. Fenitrothion is another chemical that is listed but is not available to the public. Chlorpyrifos is the chemical found in Sevin. this works but be careful not to get it on other flowers that the Bees like because it is toxic to bees.

Thank you to Hortipm for more information on the Rose slug larvae.

Sawflies and rose slugs Description:
These primitive wasps called sawflies because females of most species have a saw-like structure on the abdomen tip used to insert eggs into plant tissue. Larval stages are caterpillar-like, with a well-developed head capsule and 3 pairs of true legs behind the head; hairless body. Some sawfly larvae are slug-like, appearing slimy, unsegmented and translucent, greenish to black, while others appear wax-covered in some of their developmental stages. Adults vary from 3/4 to 2 inches long. Damage: Adults are rarely seen and do not sting. Most sawflies are somewhat host-specific. Larvae of some species are leaf rollers, web formers, leaf skeletonizers, leafminers, shoot borers, or cause plant galls. Adults can be found on flowers. Life cycle: Life cycles vary by species, but generally they overwinter as a pre-pupa in a cocoon in the ground or other protected place, pupating in the spring. In early summer, adults oviposit eggs in or on plant tissue. Larvae develop through several stages (up to 6 instars) before pupating, producing 1 generation per year. Some species have several generations annually.