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U.S. Forest Service

The Blue Mud Wasp (Chalybion californium)

By Zoe Statman-Weil and Vicki Wojcik, Pollinator Partnership

The Black Widow Killer

If you are scared of spiders then you certainly are aware of the infamous black widow, which is poisonous and dangerous to humans. Fortunately, you will be relieved to hear that while we fear the black widow, this scary spider has a deadly enemy of its own. Chalybion californicum, the blue mud wasp, is most famous for its predation of black widows.

Blue Mud Wasp (Chalybion californium). Blue Mud Wasp (Chalybion californium). Photo by Johnny N. Dell,

The Chalybion californicum is an impressive and beautiful wasp, which can be identified by its blue and black sheen, a narrow petiole (“waist” between thorax and abdomen) and its length of 10 to 23 millimeters. Its range extends from northern Mexico to Southern Canada, although it has been introduced in Hawaii and Bermuda as well. It is most common in the Michigan area so if you are desperate for a sighting of this blue mud wasp and black widow killer head to the Great Lakes region!

Chalybion californicum feeds on nectar and is a pollinator of some common wildflowers. It has been reported to visit Berberis vulgaris, Daucus carota, and Zizia aurea to name a few. Like many other wasps, adult Chalybion californium needs floral nectar to provide the energy they need for flight. While the adults feed on flowers, they feed their developing larvae high protein foods like spiders (often the black widow, Latrodectus spp.). The Blue Mud Wasp captures these spiders by grabbing them from their webs or alternatively, luring them out of their place of protection. The wasp then uses its sting to immobilize the spider, preparing a meal for its developing young.

A unique quality of the Chalybion californicum is how it builds (or rather steals) its nest. It often reforms nests of the Sceliphron camentarium (black and yellow mud dauber wasp) by removing the larva of the unwanted species from the cells and replacing it with one of its own. The Chalybion californicum will use water to soften the nest of the S. camentarium and remove the spiders that are hosting the unwanted mud dauber larva. Then the Chalybion californicum will replace the cell with new spiders, lay an egg on the last spider, and close up the cell. The larvae grow from a white and silky cocoon to be up to 1-inch long, cream colored, legless, and with the appearance of a maggot. These nests can often be found in the shadow of buildings or bridges.

Chalybion californicum is a solitary wasp, and the females build and supply the nests individually. However, clusters of both sexes have been reported in low-light situations like at night or on a cloudy day in the shadow of a building or a rock. There can be several generations of wasps in a year.

Blue Mud Wasp (Chalybion californium). Blue Mud Wasp (Chalybion californium). Photo by Joseph Berger,

Blue Mud Wasp (Chalybion californium). A blue mud wasp crawling up a flower blossom. Copyright Lana Smith, BugGuide, 2008.

If the Chalybion californicum can feed treacherous black widow spiders to its larvae, then should we, too, be scared of it? Well the answer is no. While Chalybion californicum has the ability to sting us, it is rarely aggressive. Next time you are out in nature see if you can catch a glimpse of this bright blue wasp.

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