Many species of wasps construct their nests below the surface of the ground. This subterranean nesting strategy is a common reproductive and defensive mechanism across a wide range of wasp families. The decision to nest underground is influenced by factors like soil composition and the availability of existing cavities, and it protects their developing young.
Ground Nesting Wasps: Solitary Versus Social
The wasps that utilize the ground for nesting fall into two distinct categories based on their societal structure: solitary and social. This difference in lifestyle dictates the size and complexity of the resulting subterranean structure. Social wasps, such as various species of yellow jackets (Vespula and Dolichovespula), form large, communal colonies led by a single queen.
These social colonies have an extensive division of labor, with non-reproductive workers building and defending the nest and foraging for food for the larvae. They often house thousands of individuals at the peak of the summer season. Solitary wasps, in contrast, such as digger wasps and cicada killers, operate independently.
A solitary female wasp constructs an individual burrow solely for her own offspring, without the assistance of a worker caste. Each female digs several tunnels, provisioning her own larvae. These solitary species are generally less aggressive toward humans because they do not have a large, established colony to protect.
Anatomy of the Underground Nest
Social wasps often select a pre-existing void, such as an abandoned rodent burrow, a hollow beneath a tree root, or a space under a rock. Inside this cavity, the wasps construct a nest made of paper, which is a pulp created by chewing wood fibers and mixing them with saliva.
This paper structure, which contains multiple tiers of hexagonal brood cells, is completely enclosed by a protective outer envelope. The only visible sign of a social nest is typically a single, small entrance hole in the soil, through which a steady stream of worker wasps flies in and out. Solitary wasps, however, construct a much simpler burrow that is essentially a tubular tunnel dug directly into the soil.
The female solitary wasp excavates the tunnel, which may extend several inches to over a foot deep, and carves out individual chambers, or cells, off the main passage. The entrance to this burrow is often a pencil-sized hole in loose, sandy soil, and it may be camouflaged by the female after she enters to prevent detection. Each cell within the burrow is provisioned and sealed individually, creating a simple, functional structure for a single larva.
Behavior and Life Cycle in the Burrow
The soil acts as an insulator, offering a consistent temperature and a buffer against extreme heat and cold fluctuations above ground. For solitary wasps, the burrow is the site of a unique provisioning behavior, where the female captures and paralyzes prey with a sting.
The paralyzed insect or spider is then dragged into a cell, where the female lays a single egg on it before sealing the chamber. This living, immobilized food source ensures the larva has fresh sustenance immediately upon hatching. For social wasps, the burrow is the center of their seasonal life cycle, which begins in spring when an overwintered queen selects a nesting site.
The queen initiates the first cells and raises the first generation of sterile workers, who then take over the labor of enlarging the nest. By late summer, the colony reaches its maximum size, and the queen produces male wasps and new queens. These new, fertile queens leave the subterranean nest to mate and seek out sheltered locations, such as under bark or in structural crevices, where they will hibernate until the following spring.