Approximately 70% of native bee species in North America are solitary and construct their nests in the soil. These ground-nesting bees provide substantial pollination services integral to local ecosystems. A female bee digs a main tunnel with side branches, each ending in a brood cell provisioned with pollen and nectar. The egg laid on this provision develops through larval and pupal stages underground, often spending 9 to 11 months before emerging as an adult the following season. This extended subterranean life cycle makes the developing bee highly vulnerable to mortality factors originating both above and below the surface. This article examines the primary threats that kill these developing bees while they are still protected within the ground.
Chemical Agents and Soil Toxicity
One of the most pervasive threats to developing bees underground comes from chemicals applied to the landscape, particularly systemic insecticides. These compounds are absorbed by a plant’s vascular system, making the entire plant toxic to feeding pests. When a mother bee collects pollen and nectar from treated plants to provision her nest, she inadvertently contaminates the food source for her offspring. The resulting larva consumes this toxic provision, leading to direct mortality or sublethal effects that impair development.
Neonicotinoids, a class of systemic insecticides, are highly persistent and can remain toxic in the soil for years, creating a chronic hazard for future generations of ground-nesting bees. Studies have shown a high likelihood of chronic exposure to lethal doses of chemicals such as clothianidin in agricultural fields. The adult female bee is also exposed to these residues while digging the nest, which can lead to impaired navigation and reproductive capacity.
Physical Destruction of Nesting Habitats
Human land management activities, especially mechanical manipulation of the soil, represent a direct physical danger to subterranean nests. The most significant threat is agricultural tillage, including plowing and rototilling. Since brood cells are typically located between 6 and 16 inches below the surface, deep tillage easily crushes or displaces the fragile nest structures. This mechanical breakage results in the immediate death of the immobile egg, larva, or pupa within the cell.
Research has demonstrated that tilling fields significantly reduces the emergence of adult bees in the subsequent season. For instance, deep tilling in squash fields decreased the emergence of squash bee offspring by approximately 50%. Even conservation tillage results in a lower incidence of bee nesting compared to undisturbed field edges. Other activities, such as heavy construction, soil compaction from machinery, or excessive foot traffic, can similarly destroy the integrity of the nest tunnels and cause offspring mortality.
Specialized Predators and Parasites
The ground environment hosts a diverse array of specialized biological enemies that have evolved to breach the protection of the subterranean nest. Cuckoo bees, which belong to various genera like Nomada, are cleptoparasites that enter the host’s nest to lay their own egg. The cuckoo larva hatches quickly and either kills the host egg or larva directly, or outcompetes it for the limited pollen provisions gathered by the host mother.
Another specialized group is the blister beetle, specifically its first larval stage known as a triungulin. Some triungulins use a chemical lure that mimics the host bee’s sex pheromone to attract male bees. The triungulins attach to the male, transfer to the female during mating, and are carried back to the nest. Once inside, they consume the host egg and the pollen ball.
Parasitoid bee flies (Bombyliidae), such as those in the genus Anthrax or Bombylius, also pose a threat. They hover over the nest entrance and flick their eggs down the burrow. The fly larva then makes its way to the brood cell, where it develops by feeding on the host larva and its provisions.
Climatic and Hydrological Factors
Natural environmental extremes, particularly those related to water and temperature, are major causes of death for developing bees in the soil. Flooding and excessive soil saturation are highly lethal because they deprive the developing larvae and pupae of oxygen. When water fills the air spaces in the soil, the bees, which rely on gas exchange, effectively suffocate.
Prolonged flooding events have been linked to high mortality in ground-nesting species. Conversely, extreme drought can also be detrimental by hardening the soil, which prevents the fully developed adult bee from successfully excavating its way out to emerge. Additionally, temperature extremes in the soil, whether excessively hot or cold, can exceed the physiological tolerance range of the dormant pupae, compromising their survival.