I found a reference to the tarnished plant bug (Lygus lineolaris) feeding on flowers and buds of astilbe:
http://homeguides.sfgate.com/astilbe-saxifragaceae-plant-75214.html

When I found a photo, it certainly looked like it might be your bug.

duplicate

This post was edited by nhbabs on Fri, Jun 21, 13 at 7:28

Thanks, nhbabs. I don't think that's it--it looks sort of similar, but mine is more like a tiny ladybug. Hard shell but I could see those hidden little wings sticking out from underneath like a ladybug has.

Probably an Oriental Beetle. See link below.

You can control the grubs with beneficial nematodes. For adults try neem or pyrethrins.

Steve

Here is a link that might be useful: Bug Guide

Thanks, Steve. I think oriental beetles are much larger, though? This is tiny, like the size of an aphid.

Steve--I realize now that you meant it was an oriental beetle doing the flower-eating, not that the bug pictured is an oriental beetle. You're right--when I went out with a flashlight tonight, I caught some asiatic garden beetles in action eating my astilbe. Still not sure what the tiny beetle is, but they seem to be harmless. thank you!