possibly azalea caterpillar as pictured in the link below.
i have these to show up in certain years and they will quickly defoliate portions of the plant, eating leaves but leaving any stems or woody parts of the plant.
i sprayed them with an insecticide but you could use bt for a less "chemical" approach to control. the plants were o.k. afterwards but skimpy looking, of course.
seems to me like they retreat into the foliage or mulch at night(?)

Here is a link that might be useful: azalea caterpillar

i have the same problem. the worm hunches up and will eat the whole leaf but leave the ribbs of the leaf the exbury azaleas seem to get the worst of it.i use sevin as much as i hate to use poisions but it works.the worm has a tan head,& starts eating when the blooms trart opening & can defoliate a plant in a matter of days so it takes close watching

Entire leaves being eaten by caterpillars is an indication of an infestation of caterpillars such as the Red-headed Azalea Caterpillar, Datana major. These caterpillars are black with rows of white or pale yellow spots, reddish brown legs, head and neck area and are 2 1/2 inches when mature. Preferred host plants are azaleas, but they may also attack witch hazel, sumac, apple, red oak and andromeda. The caterpillars feed together when young and disperse as they mature. Branches or entire plants may be defoliated. Damage occurs in late summer and fall. Look for caterpillars when defoliation damage occurs, and if only a few caterpillars are present, pick them off by hand. If needed, spray shrubs with B.t. (Bacillus thuringiensis ), a microbial insecticide that is specific for caterpillars. It is sold under various trade names including Caterpillar Attack, Thuricide, Dipel, etc. Apply sprays when caterpillars are numerous and less than 3/4 of an inch long. Larger caterpillars must be sprayed with a registered residual insecticide such as carbaryl (Sevin 50WP) or cyfluthrin (Bayer Advanced Garden Multi-Insect Killer).

Hi, I live in SW Wisconsin and have had something eating my azaleas in late spring, too. A couple seasons ago whatever- it-is (I suspect it is a caterpillar but I've never found one--they seem to hide during the day)started eating one azalea growing in the shade--last year another azalea was hit. This year by the time I first noticed the damage most of my dozen azaleas were at almost completely defoliated and all of them showed extensive damage. I sprinkled Sevin on the foliage of them all--the white powder looked like crap but did the trick--they put out new foliage. Next year I am going to treat the plants at the first sign of damage on any of them. The insects referred to by the other posters and indeed all the internet sources I've found refer to insects that are confined to the SE US and/or damage Azaleas late in the season, not in the spring just after the leaves emerge. It's unfortunate to have to treat a plant to be able to grow it at all, but azaleas are worth it. If the pest is actually a caterpillar the Bt bacillus is likely to work, too. If you see one of the critters next spring, please post a description.

Steve, it's always a good idea to identify the target pest before treating for it. You can easily find what has been damaging your azaleas if you will go out in the evening, after dark, with a flashlight and inspect.

I also have the problem with the little green worms/caterpillars. I have picked off about 50 on one Azalea plant. The next day I find more. I too would like to know what they are and how to get rid of them. I have looked at several pictures and I still have not seen a picture of them. They are very small, the biggest one is about an inch. They start at the edge of the leave, only leaving the vein.

Interestingly, inch long green caterpillars are called "inch worms." They may be coming from another tree or shrub. When this small size, they have voracious appetites.

The best and safest spray for caterpillars is B.t. (Bacillus thuringiensis ), a microbial insecticide that is specific for caterpillars. It is sold under various trade names including Caterpillar Attack, Thuricide, Dipel, etc. Apply sprays when caterpillars are numerous and less than 3/4 of an inch long. Larger caterpillars must be sprayed with a registered residual insecticide such as carbaryl (Sevin 50WP) or cyfluthrin (Bayer Advanced Garden Multi-Insect Killer).

I had these same small, green caterpillars defoliate my exbury azalea earlier this spring. I sprayed BT and they disappeared....the rest of the plant seems to be doing fine now. I'm wondering if there is something I can do now....or early next spring to PREVENT them before they start feasting. Any ideas?

It would probably cause more harm than good to start preemptive spraying. Many sprays kill beneficial bugs that prevent lace bug and other rhododendron problems. Many rhododendron problems are prevent by spiders and birds. Unfortunately many sprays destroy them. BT is about the safest spray there is since it only affects caterpillars. It doesn't bother birds that eat the caterpillars.

The best practice is to spray the caterpillars, when they or their damage is visible, with BT and hope that you broke the cycle and there won't be many young ones next year.