Neighbor Recognition and Context‐dependent Aggression in a Solitary Wasp, Sphecius speciosus (Hymenoptera: Sphecidae)

Female cicada‐killer wasps ( Sphecius speciosus ) nest underground and provision one brood chamber at a time with cicadas upon which they lay a single egg. Females commonly nest in dense aggregations of several dozen burrows. Although there is usually only one female per nest, occasionally two females encounter each other in the same burrow. Burrow owners exhibit variable tolerance of intruders (i. e. conspecific females who enter their nest), ranging from apparent indifference to intense aggression (biting and stinging). Field observations and experimental manipulations show that a nest owner's behavior toward intruders is influenced by 1) the presence of an exposed cicada in the owner's burrow, 2) whether or not the intruder is nesting near the nest owner, and 3) the intruder's size. A nest owner was most aggressive when an exposed cicada was present in its burrow and the intruder was a large nonneighbor (i.e. a female who was not nesting within 1 m of the nest owner in the same nest aggregation). Owners were markedly less aggressive when there was no exposed cicada and the intruder was a large nonneighbor or a small neighbor (i.e. a female who was nesting within 1 m of the nest owner in the same nest aggregation). Owners displayed little or no aggression when there was no exposed cicada and the intruder was a large neighbor. Owners apparently modulate their aggression according to their susceptibility to usurpation and/or perhaps cleptoparasitism. The ability of S. speciosus females to vary their aggression in different contexts suggests that selection can favor plasticity in social responses even in solitary Hymenoptera. Such flexibility may have been a forerunner of the elaborate social assessment mechanisms of eusocial insects.