Natural History of Solitary Eumenine Wasps

Among solitary wasps, individual females build nests and tend their young alone without the cooperation of other members of their species. Many solitary eumenine wasps use preexisting cavities such as hollow twigs or vacant insect tunnels in dead wood as nest sites. Within the cavity, a female suspends an egg from the ceiling at the deepest recess of the hole and then leaves to hunt for caterpillars. When she finds a suitable caterpillar, she paralysis it with stings, flies back to her nest and stuffs it in with the egg. When enough caterpillars are placed in the nest for the complete growth of her offspring, she builds a partition from mud sealing in the egg with its provisions. She then repeats egg laying and hunting to produce a linear series of cells within the cavity. In nests that contain both sexes, mother wasps deposit fertilized eggs in the innermost cells to produce daughters, and they deposit unfertilized eggs in the outer cells to produce sons. Sons in the outer cells receive less food than daughters in inner cells, develop more quickly, and emerge as young adults several days ahead of their sisters. In some species, such as Euodynerus foraminatus, the emerging males compete among themselves, and the winner remains at the nest to mate with the later emerging females. Consequently, inbreeding occurs in some of the solitary eumenine wasps (see inbreeding by eumenine wasps).

Because many solitary wasps nest in preexisting cavities such as hollow twigs, female wasps readily move into artificial cavities such as sticks with drilled holes. These "bird houses for wasps" are often referred to as trap-nests. The figure above represents a completed nest of E. foraminatus containing nine cells separated by partitions that have been fashioned from mud. The inner four cells produced daughters, and the outer four cells produced sons. The egg deposited in the fifth cell failed to develop to adulthood. Based on the weight of caterpillars provided by the mother (224 mg) it is reasonable to infer that this cell was intended to produce a daughter.