Blue dirt dauber

Description:

Dirt daubers don't build nests, they add mud to nests abandoned by other wasps. It gives the finished product a lumpy appearance. Also known as spider wasps ... been observed to alight on a web and jiggle the strands, luring spiders into their grasp. The spider is stung, paralyzed and then placed into a cell with an egg, becoming food for the young.

Habitat:

Under a shallow overhang on a brick pillar
Widespread throughout North America

Notes:

Also called Blue mud dauber, Blue mud wasp
Behavior trait: Twitchy wings

From Animal Diversity Web:

During the summer, female blue mud wasps build nests by bringing water to abandoned mud nests made by other species of wasps. They form new mud chambers, stock them with paralyzed spiders and a single egg, then seal the chambers with more mud. Their offspring stay in the chamber, feeding on the spiders, and then pupating in a thin silk cocoon. They spend the winter in the nest, emerging the following spring as adults.

Blue mud wasps are not known to be aggressive and will not usually sting unless provoked to do so. They are typically a solitary species using their stingers only to paralyze prey spiders and other insects they might encounter.

Adults of this species feed on flower nectar, and possibly pollen. Individual wasps get most of their nutrition while they are larvae, feeding on spiders provided by their mother. Adult females capture orb-weavers (family Araneidae) and comb-footed spiders (family Theridiidae), often including black widow spiders (genus Latrodectus). These wasps capture their prey by paralyzing them with a sting. Some have been observed landing on orb webs and luring the spider out of its retreat, capturing and paralyzing the spider without getting caught in its web and becoming prey itself.

This species may help to control the population of black widow spiders. They sometimes nest around buildings, and may thus be a small nuisance, but its inoffensive habits and use of spiders as prey generally prevent it from being a pest.