I was photographing some orange aphids on milkweed, and noticed these tiny wasps (3mm) laying eggs on them. Maybe inside them? I couldn't tell. The wasp would approach the aphids with its abdomen and ovipositor pointed / folded underneath (It was the only time she walked slowly enough that I could follow her movement), and then rush forward while extending her abdomen even farther, and give an aphid a quick poke. The poke only takes a second or two, tops. She seems genuinely intent in staying away from the aphids, although the aphids didn't react to her presence at all, even when she poked them. None of the aphids had even exuded that orange wax from their cornicles, as I've seen most every time aphids are under attack by lady beetles or their larvae.

Anyway, it was fascinating and I watched at least three or four of these girls ganging up on this aphid colony. I might add, although there were ants aplenty on this milkweed, they paid no attention to these orange aphids. Does milkweed make the honeydew less palatable to ants?

I measured the aphids afterward: The smallest are 1mm and the larger ones go up to about 3.5mm.
More, larger images at Aphid Mummy Wasp