probably a caterpillar of a moth or similar. i don't know that slugs would climb like that to feed and i have not noticed any grasshoppers here yet.
look among the leaves for signs of caterpillars rolled up inside them, too. i don't know that much about insect species but there is always something chewing on most every plant at any given time - age old battle of gardeners and insects, ya know.
you could start a spraying routine(maybe something less toxic containing bt) if it appears that serious. when i find that sort of damage, i usually inspect the plants (very early morning is the best time) and try to pick them off manually. i am seeing similar on my 'omure yama' but seems to have stopped. a healthy local bird population never hurts, either. :-)

hi jeff,

thanks for the post. yes, i've scattered sluggo slug killer on the ground and sprayed neem oil on the leaves, and whatever's been eating at it seemed to have stopped. you are probably right, it's a caterpillar of some sort. been looking around for something every morning on the underside of the tree but darnit, pesky critter can hide!

hope this has done the trick. if not, i'll be back on this forum with more questions. thanks ;)

I agree with Jeff, it's most likely a catepillar/moth. Have you noticed any rolled up leaves, as well? We have a catepillar problem with our large (regular) maples. Every spring the catepillars (little green things) lay eggs in the leaves, and roll the leaves up. Plus, they eat the leaves (and they look just like your leaves look).

The catepillars also hang down from the maples, and latch onto leaves on plants below - like my sandcherry, and amelanchier - and continue their feeding frenzy on them. Lots of fun. I find with the shrubs, that I have access to (can't reach up into the maples...), if I spray the catepillars with soapy water, that will kill them. I have to get them early, though, or else they really eat up the leaves quickly. The funny thing is, I've never had a catepillar problem on any of my japanese maples (knock on wood).

Of course, 'your' pest could be something totally different... lol. Seeing as you never actually see the pest - catepillars don't move too quickly. :) Anyway, just a thought.