Spiders Living at Wasp Nesting Sites: What Constrains Predation by Mud-Daubers?

The nests of mud-daubing wasps (Hymenoptera: Sphecidae) are excellent sources of spiders (Peckham and Peckham, 1898; Rau, 1935; Muma and Jeffers, 1945; Dorris, 1970). Females of these solitary wasp species construct mud nests during the late spring and summer. They provision each brood cell with a number of spiders which they capture and paralyze by stinging. The wasp lays an egg on one of these spiders and, upon hatching, the larva consumes all the spiders within the brood cell. When development is complete, the new adult wasp chews a hole in its brood cell and emerges. A cell in the nest of mud-daubers such as Sceliphron caementarium or Chalybion californicum may contain in excess of 25 spiders. It seems likely then that mud-dauber predation may be a significant factor influencing population dynamics and evolution of those spider genera taken as prey (see also Eberhard, 1970). But this view of wasp and spider interactions is incomplete. The same sites at which mud-daubers nest are also used by both wandering and webbuilding spiders for capturing prey and tending eggs. Mud-dauber nests themselves are often used by spiders for these activities. In fact, among the group of spiders active at mud-dauber nesting sites are species that are regularly taken as prey by those same spiderhunting wasps. Intrigued by this fact, I initiated field studies that addressed the following questions: 1. What groups of spiders are found living at nesting sites of mud-daubers? 2. What is the nature of the interactions between wasps and spiders at these sites? 3. If wasps do not hunt spiders at nesting sites, what factors constrain them from doing so?