Digger Bees

Small hairy or metallic bees that dig into the soil to nest, hence one common name, digger bees. This is a diverse group that comes from different families and the term digger bee can include the andrenid bees, halictid bees, and colletid bees such as the plasterer and yellow-faced bees.

Digger bees are 1/4 to 1/2 inch long and variable in color (mostly shiny metallic or dark, but some with markings of white, yellow or reddish brown). There is one generation of digger bees per summer and once the adults finish perpetuating the species by laying eggs of the next generation there will be no activity till the following spring.

Digger bee nests are commonly located in areas of the landscape where the grass is sparse, either from too much shade, previous drought conditions or other stress. It is tempting to blame the bees for causing the turfgrass to be thin but it is the opposite; the bees are in the yard because the grass was already thin. The entrances to the tunnels (mounds of soil) are disruptive and annoying to the homeowner but are not usually damaging to otherwise healthy turf.

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 in 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.

Life Cycle: 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.

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.

When new queens are nearly ready to emerge, the old queen will leave the hive with thousands of worker bees. This large mass of bees is called a swarm and their purpose is to look for a new place to form a hive. These swarms often rest in exposed sites such as trees and shrubs (Figure 2) before finding a suitable cavity for building a new nest. Although swarms are less defensive than an established colony, it is best not to disturb them. In Oklahoma, swarms are most common during late April-June.

Pollination: Digger bees are important pollinators of several native plants and spring crops. Coexistence rather than eradication is encouraged where possible. If that’s not going to happen then rejuvenate the turfgrass by leveling the area and re-establishing a thick turf or ground cover tolerant to the site. Small numbers of burrow openings can be treated individually with insecticide dust (Sevin or permethrin). Larger infested areas can be sprayed or treated with insecticide granules to discourage the bees when the problem warrants a response.

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