George W. Uetz Professor Ph.D., University of Illinois Animal Behavior, Behavioral Ecology, & Arachnology

Spider Defenses Against Wasps
the “Early Warning Effect”

Predatory wasp Poeciliopompilus mixtus Photo by Linda Rayor

Colonial web-building spiders are vulnerable to predation from several species of predators (wasps) and parasitoids (flies and wasps).  Selection pressure from these natural enemies has resulted in evolution of various anti-predator behaviors. 

Predatory wasps are an important source of mortality for M. incrassata. Encounters between colonial spiders and wasps increase with colony size, but individual risk varies as a consequence of several mechanisms (Rayor and Uetz 1990, 1993; Uetz and Hieber 1997).  As colonies increase in size, colonies are easier for wasps to locate, but individual risk decreases because of a "dilution effect" (safety in numbers).  Moreover, spiders in the central core of the colony are less likely to be attacked by wasps than those on the peripheral edge, and larger females seek cover there ("selfish herd effect"). However, we also found that when wasps attack colonies, they move from spider to spider in a "trapline" fashion, but their capture success is less in larger colonies and declines with each successive attack.  We have demonstrated that colonial webs provide an “early warning effect” whereby spiders are alerted to danger from approaching predator attacks by vibrations transmitted through the communal webbing (Uetz et al. 2002).  In the photo, Jay Boyle is testing this hypothesis with a simulated predator vibration stimulus (SPVS).  Spiders in larger colonies responded to the vibration source at greater distances and in greater numbers, and spiders "attacked" later in the sequence responded sooner.  

M. incrassata colony	Jay Boyle with Simulated Predator Vibration Stimulus (SPVS)	side view of Metepeira web showing interconnections

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