By Whitney Cranshaw, Colorado State University Cooperative Extension, Entomology
One interesting group of native bees nest in soil, the digger bees (Family: Anthropodidae). These are solitary bees that individually rear their young within the soil tunnels they construct. Such nesting is restricted to only certain sites found to be optimal, based on features such as slope, aspect, soil type and drainage. As a result they often appear to occur in "colonies," often numbering hundreds. In these colonies, each female is hard at work digging out the nest cells and collecting pollen for her young, often in very close proximity to many others, giving the appearance of a nest. However, digger bees do not have a social structure, as do honeybees or social wasps such as yellowjackets.
The digger bees are also very non-aggressive and will not sting unless handled or trapped in clothing. Even then, their sting is reported to be much milder than that of a honeybee or yellowjacket. Male bees cannot sting at all.
One generation per year occurs with the typical digger bees found in Colorado. Winter is spent as maturing larvae within an underground cell. They pupate in late spring and emerge in early summer. Upon emergence, the females begin construction of the nest, which usually consists of a central chamber extending a few inches below ground and terminating in a series of cells. In dry soils the bees may collect water and return it to the soil to ease the excavation. After each cell is complete, the female bees collect pollen which they pack into the chamber and then lay an egg in the completed, furnished cell.
The males meanwhile cruise over the site during the day looking for a chance to mate. At night they perch some distance away, often resting in groups on shrubbery. They are not involved in either nest construction or provisioning.
Individual bees live about 3-4 weeks and the nests are often sealed when complete. Several parasites, such as bee flies, velvet ants, certain blister beetles and various parasitic bees try to steal into the nests.
Although the appearance of large numbers of active bees swarming over an area can be distressing, digger bees pose very little risk of stinging. If they occur in an area where they are not wanted, the best long term solution is to somehow change the surface of the nest area, such as incorporated mulch or increased watering to make the site unfavorable for nesting. Insecticidal control can be difficult since the "colony" actually consists of a great many individual nest cells, each requiring eradication.
Photo: Judy Sedbrook
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010