This is an interesting setup — this wasp is actually a parasite of a parasite. She’s walking over this tree gall, seeing if she can lay her eggs in any of the parasitic insects living inside the gall!
Galls (rounded protrusions of trunk or stem) form on plants when (usually, larval) insects burrow inside. The plant forms a hard shell around them, preventing them from moving further inside the plant but also protecting them from (most) predators while they pupate. That doesn’t work on this metaparasitic wasp, though!
The holes you see in the sides of the gall are from larvae of the original parasite, which have hatched and chewed their way out of the gall. The gall could hold many more larvae, though — the wasp certainly thinks it’s worth a look.
The kind people at bugguide.net have narrowed the wasp down to the family Eupelmidae, and, at 3mm, I suspect we’ll have to make do with that.