The blue mud dauber (Chalybion californicum) is a metallic blue species of mud dauber wasp that is the primary predator of black widow spiders. It does not build its own nests, but uses nests abandoned by other mud dauber wasps. It is not normally aggressive. It is similar in shape and colour to the steel-blue cricket hunter (Chlorion aerarium). Like other types of wasps, males do not have an ovipositor, therefore cannot sting.