Rachael Long, a University of California Cooperative Extension entomologist and crop adviser, recently told me a story about three wasps that people frequently encounter around their homes and often have misconceptions about.
Cooperative extension advisers get interesting questions from people about plants and insects found in homes, gardens and farms. This tale began when a woman came into the Woodland office and told Long, who represents Yolo, Solano and Sacramento counties, that she had messy wasps in her garage making mud nests and she just sprayed them with insecticide to get rid of them. She came into the Extension office to see what she could do about them. But she was a bit taken aback when Long told her that the wasps are called mud dauber wasps, are spider hunters, and can be beneficial to us.
Long explained that some wasps are specialized hunters and only focus on one type of prey to provision their nests with. The female mud dauber wasps only hunt spiders and individually make the small mud nests often seen in sheltered places like garages, sheds, ceilings, eaves, attics or walls in places where there are many spiders. They are solitary, not social, and as such are not aggressive, as they don’t defend nests. The adult mud dauber wasps primarily feed on nectar from flowers. Only the larva get their nourishment from spiders females have captured, paralyzed and provisioned their nests with.
Each mud nest may be composed of 15 to 20 cells. Each cell is packed with around 20 to 30 paralyzed spiders. This may add up to 500 spiders captured by each female wasp. Female wasps use special tactics to capture spiders. They are able to land on spider webs without being ensnared, and pluck the strands to mimic prey. The spiders then become a victim of the wasp’s paralyzing sting when they come out to look for victims. Please note that just a very few spiders have painful or poisonous bites. Many are important generalist predators in our gardens.
Long was intrigued by the wasps and broke open a number of nest cells in her garage to see what kind of spiders the wasps were capturing. Some nests had a variety of spiders and others had 100 percent black widow spiders. Did some wasps have preferences, or were there a lot of black widows in her garage? Puzzled, she went to the UC Davis Bohart Entomology Museum of Entomology (bohart.ucdavis.edu) to see if they could answer these questions. They told her that the mud nests provisioned with 100 percent black widows were the nests of the blue mud dauber wasps, a different type of wasp. These wasps reuse the mud dauber wasp nests, and focus on black widows as the primary prey to provision nests with in the same way the mud dauber wasps do. A good reason to leave some mud dauber nests intact!
Accessible mud helps the wasps build nests. In the course of doing research Long heard from someone who for many years used to put saucers of mud outside their house, garage and barn to encourage the mud dauber wasps to build nests for black widow spider control.
Many wasps (and bees) are mistaken for the aggressive, carnivorous and ground-nesting, social yellow jacket wasps. Mud dauber wasps are also mistaken for the social, non-native European paper wasps that live in colonies and construct hanging paper nests under eaves, and on bushes, trees, and arbors. These wasps are primarily caterpillar hunters and capture them to provision the nests for their young. The adults also mostly feed on nectar. As they are social, they defend their nests that can number up to about 200 wasps. In some gardens in our area they proliferate, and can become a problem.