Caterpillar on clethra alnifolia wiping it out!

Hi all,
Need some help with an issue with clethra alnifolia (aka sweet pepperbush, aka summersweet). I keep the plants extremely well-watered (they are next to a rain barrel) and they are in full sun...thought I had found the perfect spot for them until this happened.
Small 1/4" long black caterpillar wiping out the leaves of two new shrubs I purchased 6 weeks ago. The caterpillars looks very similar to the virburnum leaf beetle larvae that have decimated the highbush cranberries around here (this is zone 5 microclimate in northwest NJ). However, they are black instead of the whitish color of the VLB larva.
The c. alnifolia plants were fresh and vibrant when I got them from Edge of the Woods Nursery in Allentown, PA. The caterpillars have wiped out the new bright-green growth to the stems (similar to virburnum leaf beetle damage) and are now attacking the older dark-green growth. Fresh growth is sprouting, but the caterpillars are quickly attacking and destroying it.
All I can find on google is, "this is a pest-free shrub". When I look up "clethra alnifolia caterpillar" or "clethra alnifolia host plant" all the sites say, "great nectar plant" but nothing about being a host plant for caterpillars. So I really need help with a caterpillar/pest that doesn't seem to exist!
Thanks very much, Dan
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I wonder if they might be alder leaf beetle (Agelastica alni) larvae
http://tinyurl.com/28knbn3
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In article

Hmm, wrong photo, similar species. Try this
http://tinyurl.com/33fw89k
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Dan said:

Could be sawfly larvae. Sawflies, scale and spidermites are the only pests I recall being mentioned for Clethra.
A gallary of sawfly larvae photos: http://www.pbase.com/tmurray74/sawfly_larvae
--
Pat in Plymouth MI

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