That behavior sounds like (the dreaded) whitefly, but I wouldn't call them MUCH smaller than gnats or fruit flies. Maybe they are, after all. They look like tiny, pure white moths (though they are not moths)that flit out from the under side of foliage only when disturbed.

Let's say that these insects are whiteflies. It's difficult for us to guess whether or not you can get rid of them without knowing how bad the infestation is. They are very persistent, once established. I'd try to get that plant isolated pronto or you WILL have 'tiny white bugs' on everything.

In my experience, a bad infestation of these insects warrants cutting the plant back to the ground and keeping the new growth pest free. At any given time, you will have whitefly eggs, several nymphal stages, and adults. Makes it very challenging! Sometimes the removal of the entire dang nursery is the only way to go.

Don't think the population is bad enough for that drastic measure? Try insecticidal soap applications, making sure that you direct the spray to the underside of the leaves where all of the activity is. I've also had good luck with Neem oil against whitefly, though I've not tried it on oregano. Neem doesn't have the greatest aroma (it stinks), so you might want to wait until you take the plant outside. Neem performs best when the temperatures warm up, anyway.

Hopefully, some others will have some good ideas about how to control these insects.

Hm, I don't know - these are so small that I can't really tell what they look like. However, they do behave just like you're describing, so perhaps you're right. I can't really say how bad the infestation is - I mean, they don't seem to be harming the plants, and they just sit there unless I disturb it. They've been there for a year and haven't shown any interest in the rosemary, but I do want to fill the blank spaces in that box with thyme at some point, so I'd like to get it taken care of.

I'll give the soap and/or Neem oil a try. BTW, if the insecticidal soap works, how do I keep them from coming back?

Well, whiteflies just aren't so small that you can't tell what they look like. At least, I don't think so. Can't imagine what else they would be. Anyone else have any ideas?

I cleaned off the top of my windowsill today and I found a few dead ones. Obviously, I was able to look at them more closely and they do sort of look moth-ish, so it's probably white fly. I moved the planter outside (we're in the 70s for the rest of the week!) and will try the soap spray later today. I also saw something online that recommends making infusing your water with garlic or onion and using that water to make the soap spray, because bugs don't like the scent - any luck with that? Thanks!

i have them as well. super tiny, they almost look like a bunch of white powder and they go down the stem and eat all the leaves and leave then stem alone...i think we may be talking about the same pest, as they are definitely not white flies or gnats...WAY SMALLER. I plant to try neem on the plant...good luck.