EEEK! It's a bug!

Fire ants?

Not sure what characteristics you are looking for exactly - roaches hang out together, but don't have colonies with only a single female producing eggs, like bees and ants. Temperate insects differ quite a bit from tropical insects.

That said, you might use any number of colloquial names for insects that are not very species-specific - midges, gnats, no-see-ums, sand flies, black flies, mosquitoes. They all bite, and all bites can leave welts in sensitive folks. If you want something that itches but isn't caused by a bite, you might want to go with poison ivy (or oak, depending on what part of the country you're in).

Black flies: yuck. Their bites hurt. How far south do they go? Thay have them at the Jersey Shore sometimes.

I know out in Alberta in the summer we get horse flies, those buggers are big, but really sneaky, and when they bite you it hurts.

ninja edit:
Though I'm not sure they live in groups, or form colonies, so they might not work in your story - but man are they annoying!

Bedbugs.
Their bites itch like crazy, but the bugs don't carry human diseases.
And they're very hard to kill.
They're currently undergoing a population boom.

Fleas. They aren't colony insects, but they hang out in areas where animals have been. Like if you move into a carpeted apartment where flea-ridden dogs or cats were living, they could have left behind a viable population of fleas just waiting for another good meal.