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With the reduction in numbers of native populations of honeybees over the last decade, the presence of alternative pollinators has increased in importance. Many species of solitary bees pollinate native plants, and ornamental and crop plants, such as the various bumble bees, carpenter bees, mason bees, sweat bees and the digger bees.
Digger bees in the genus Andrena are ground dwelling, solitary bees that are exceptionally good pollinators for a variety of ornamental plants and vegetable crops. There are many species of Andrena; most about ½” long and are brown to black with whitish bands of abdominal hairs. Many species fly in the spring, and the adults are generally short-lived (4 to 6 weeks). They are more active than honeybees during lower temperatures, which makes them better pollinators during the cooler spring weather. For example, they are excellent pollinators of many early blooming wild flowers and native azaleas in South Carolina. Digger bees are not aggressive, generally are not defensive of their nests, and rarely sting.
Digger bee (Andrena sp.)
Photo by Joseph Berger, Bugwood.org Forestry Images
When the adults emerge during April or May, they mate, and then the females will dig ¼” holes in the soil into which they place pollen and lay eggs. The young larvae remain in the nest and develop into adult bees which emerge the next spring. There is typically only one generation per year. One may find many nests in very close proximity to each other, but they are solitary insects. They may continue to use the same sites again the next year.
There is generally a small mound of soil excavated from the underground nests, and these nests are usually made in areas with sparse vegetation, such as a thin section of the lawn. It is not that the nest digging damaged the lawn, but that the lawn is usually thin where the nests are made. Nests also may be constructed near shrubs if there is not an adequate layer of mulch over the soil.
Digger bee (Andrena sp.) nest
Photo by Whitney Cranshaw, Colorado State University, Bugwood.org Forestry Images
To some gardeners, digger bee nest construction may be somewhat unsightly (similar to a small ant nest), but control in home landscapes is generally not necessary. If needed, nest building activity may be discouraged by irrigating the area. Digger bees prefer dry soil, so irrigate at 1” of irrigation water per week. Correct the problem causing the weak or thin lawn. Test the soil and apply the correct amount of lime and fertilizer to promote a more healthy lawn. Mow the grass species at the correct height to thicken the lawn. If the lawn still will not grow well in those areas, perhaps due to shade and tree root competition, consider mulching the area to discourage digger bee activity.
Remember that these are relatively harmless insects that play an important role in plant pollination, so chemical control should not be necessary. Try the cultural controls first to have them nest elsewhere.
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This information is supplied with the understanding that no discrimination is intended and no endorsement by the Clemson University Cooperative Extension Service is implied. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.