Tony said:

This is the first year I've tried growing hardy hibiscus from seed. I've
got some plants in the ground and some in pots. Unfortunately the
caterpillars are devastating them. I'll go out and find entire plants with
their leaves "skeletonized." I'm not sure exactly what kind of caterpillar
is doing this. It resembles a hornworm in color/shape, but I haven't seen
any really large ones to know for sure. (The largest I've seen on plants is
about .75" long and about 1mm in diameter.
snip
Can anyone offer any recommendations? So far the bugs only seem to be going
for the hardy hibiscus (and possibly a nearby mexican petunia, but I'm not
for sure on it).

These plants have been sprayed at least 6 times with BT since late May. I
think I need to move on to something else.

Sawflies. Unlike lepidoptera caterpillars, Bt won't do them in. If the situation
is beyond hand-picking, try an ultra-refined horticultural oil (safe for use in
summer) or a pyrethrum-based insecticide.

Has links to pictures:

http://www.hgic.umd.edu/diagn/flow/sawflies.html

--
Pat in Plymouth MI

Any technology distinguishable from magic is insufficiently advanced.
(attributed to Don Marti)