Enhancing Nest Sites for Native Bee Crop Pollinators

by Mace Vaughan & Scott Hoffman Black

The European honey bee receives most of the credit for crop pollination, but the number of managed honey bee hives is half of what it was in the 1950s; and this number continues to decline. Native bees, however, also contribute significantly to crop pollination and, on farms with significant natural habitat located nearby, may even provide all of the required pollination for some crops.

To support the native bee community, it is essential to provide nesting sites, in addition to floral resources. Unfortunately, heavily managed farm landscapes often lack the untilled ground, tree snags, plants, and small cavities that native bees require for nest construction. Improving the nesting opportunities for these important pollinators will increase their populations which can lead to higher crop yields, reduced dependence on imported European honey bees, and improved on-farm biodiversity.

Where Do Native Bees Nest

Native bees have very different nesting requirements from the more familiar European honey bee (introduced to North America from Europe in the early 1600s). Unlike, the large, comb-filled hives of a honey bee colony, they are generally solitary species, with each female constructing and provisioning the nest by herself. Only when the adults emerge from their hidden nests do we see them flying about pollinating crops and other plants. The rest of the year solitary bees are tucked away inside the cells of their underground or plant-tunnel nests. Most solitary bees are active as adults for only a few weeks each year and most have only a single annual generation. An exception are some social sweat bees, that can have several overlapping generations through the summer. These sweat bees are the most abundant native bees in some studies of crop pollination and build large populations over the summer growing season.

About 30 percent of our 4,000 native bee species are solitary wood-nesters that build their nests inside hollow tunnels. These tunnels may occur in the soft pithy centers of some twigs (e.g. box elder, elderberry, or various cane berries); they may be left behind by wood-boring beetle larvae or, in the case of carpenter bees, may be excavated by the bees themselves. Another small but important set of bee species, at least one of which has been documented as an important pollinator of watermelon, tunnel into soft, above-ground rotting logs and stumps.

Most (about 70 percent) of our native bee species excavate their nests underground. These ground nesting native bees all burrow narrow tunnels down to small chambers (the brood cells) six to 36-plus inches under the surface. Inside of these brood cells, the next year’s bees will develop. In order to build these nests, bees need direct access to the soil surface, often on sloped or well-drained sites.

The remaining bees – only about 45 species in the U.S. – are social bumble bees. Bumble bees are frequently our most effective crop pollinators. They construct nests in small cavities, often in old rodent burrows, either underground or beneath fallen plant matter, or occasionally above ground in abandoned bird nests. Queen bumble bees start nests anew each spring and by mid-summer their colonies can have dozens or hundreds of workers, all visiting nearby flowers. For this reason, doing what you can to encourage bumble bee nest sites in agroforestry practices can go a long way towards supporting crops that flower during summer months.

Managing for Potential Bee Nest Sites

The easiest approach to increasing the number of bees in a landscape is to look for potential nesting areas and then protect them as best as possible.

Specifically:

• Retain dead or dying trees and branches whenever it is safe and practical. Wood-boring beetle larvae have often filled these trees with narrow tunnels into which tunnel-nesting solitary bees will move. In addition, retain rotting logs where some bee species may burrow tunnels in which to nest.

• Protect sloped or well-drained ground sites where plants are sparse and direct access to soil is available. These are the areas where ground nesting bees may dig nests. We have found native bees nesting in orchards, front yards, along farm roads, and even in cultivated fields.

• Leave some areas of the farm untilled and minimize weed control tillage. Turning over the soil destroys all of the ground nests that are present at that depth, and hinders the emergence of bees nesting deeper in the ground.

• Protect grassy thickets, or other areas of dense, low cover from mowing or other disturbance. These are the sites where bumble bees might find the nest cavities they need, not to mention biennial or perennial forbs that can provide significant food resources.

Enhancing Nest Sites in the Field

Besides the nest sites that will naturally occur in conjunction with agroforestry practices, there are management practices that can be used to intentionally provide even more nesting opportunities for native bees. Here are some practical examples.

Tunnel-nesting Solitary Bees

• Using a hand drill and a variety of drill bit sizes (from 3/32″ (3 mm) to 5/16″ (9 mm), drill holes as deep as possible into downed dry wood sections and then erect the section upright like a fence post to simulate a beetle-tunneled snag. A variety of hole diameters will support a variety of different sized species of bees. Face the holes south as much as possible.

• Using the same drill and bits, drill holes in stumps or standing dead wood, so long as the wood is not rotting or saturated with water. Angle the holes slightly upward to reduce water entry.

• Plant shrubs or other plants that have pithy stems. Every year, cut back some of the new growth to expose the pithy interior of the stems. Elderberry, boxelder, blackberries or raspberries (Rubus spp.), sumac, or dogwood are all good choices. You may also experiment with end-drilling these twigs to encourage some bees. Drill ½- 1 inch into the pith from the cut end, and then drill a small side hole into the soft pith four inches from the end.

Nesting Sites for Ground-Nesting Bees

The precise conditions – soil type, soil texture, degrees of compaction and moisture retention – needed by most ground-nesting bees are not well known. However, the methods below will allow you to create conditions suited to a variety of species. Colonization of these nest sites will depend upon the bees already present in the area, their successful reproduction and population growth, and the suitability of other nearby sites.

• Wherever possible, avoid turning over soil to maximize areas where bees may nest. Bees need stable soil, and their progeny spend up to eleven months of the year underground. The more surface area left untilled, the more likely bees will find and colonize these patches of bare ground.

• Clear some of the vegetation from a gently sloping or flat area that is not under cultivation. The goal is to remove thatch, making it easier for bees to access the soil below but still leaving some clumps of grass or other low-growing plants to prevent erosion. The site should be well drained, in an open, sunny place, and, preferably, on a south-facing slope. Different ground conditions – from vertical banks to flat ground – will draw different bee species, so create a variety of bare patches and observe which ones best attract ground-nesting bees.

Bumble Bee Nests

Several studies conducted around farms and other landscapes demonstrate that bumble bees often occupy the grassy interface between open fields and hedgerows or woods. This has been attributed to the presence of abandoned rodent nests in which bumble bees nest. Areas of habitat suitable for bumble bees should include a mix of native grasses and forbs abutting shrubs or trees. The grass should be planted in a strip at least five feet wide and should only be mowed every two or three years. Always mow in the late fall or winter, after the colonies have died for the year and when queens are dormant. If rodents pose a risk to a farming operation, a well mown strip or road, which provides no cover, between this habitat and the crop will act as a barrier through which rodents are reluctant to move into a crop.

More details about how to construct these nest structures are available on the Xerces Society website, in the Pollinator Conservation Handbook, or from the references provided at the end of this publication.

Other Considerations

Besides the basic nest structures or features needed by native bees, a few other resources are important for successful nesting. First, different bee species – particularly tunnel-nesting solitary bees – need various materials to construct their brood cells and seal their nests. A few bees secrete a cellophane-like substance to protect their brood cells, but most use gathered materials, such as pieces of leaf or flower petals, mud, fine pebbles, or tree resins. Most likely these materials are already present, but providing a diversity of native plants and protecting areas with damp clay will help. Second, bumble bee queens need protected sites in which to overwinter. These often occur in the soft humus, leaf litter, or other sites protected from extreme winter weather.

Finally, a bee’s nest is a home base from which to scour the surrounding landscape for nectar and pollen. It is important to provide all of the nectar and pollen that bees need (see Agroforestry Note 33). The closer nest sites are located to pollen and nectar sources, the less energy female bees need to spend commuting back and forth, and the more resources they can put into their offspring. As a result, they will produce more offspring, and grow their populations over time. In addition, if nest sites are located close to abundant nectar and pollen (within 250 meters), the bees are less likely to forage where they may encounter insecticides or other hazards that are outside of a grower’s control.

Resources

Bosch, Jordi and William Kemp. 2001. How to Manage the Blue Orchard Bee As an Orchard Pollinator. The National Outreach Arm of USDA-SARE, Handbook Series, Book 5. Sustainable Agriculture Network, National Agricultural Library, Beltsville, MD. 88 pp

Shepherd, Matthew, Stephen Buchmann, Mace Vaughan and Scott Hoffman Black. 2003. Pollinator Conservation Handbook. The Xerces Society. Portland, OR. 145 pp

Stephen, W.P., G.E. Bohart, and P.F. Torchio. 1969. The Biology and External Morphology of Bees. Oregon State University Agricultural Experiment Station. Corvallis, OR. 140 pp.

Vaughan, Mace, Matthew Shepherd, Claire Kremen, and Scott Black. 2003. Farming for Bees: Guidelines for Providing Native Bee Habitat on Farms. Xerces Society. Portland, OR. 34 pp. http://www.xerces.org/Pollinator_Insect_Conservation/xerces_publications.htm

Xerces Society Pollinator Program, http://www.xerces.org/Pollinator_Insect_Conservation/

Logan Bee Lab website: http://www.ars.usda.gov/Main/site_main.htm?modecode=54-28-05-00 (click on the “research” button and look for links to nest blocks and stick nests).

Acknowledgements

The authors wish to acknowledge the very helpful comments provided by native bee scientists from across the country: James Cane, USDA ARS; Gretchen LeBuhn, San Francisco State University; T’ai Roulston, University of Virginia; Matthew Shepherd, Xerces Society; Connie Stubbs, University of Maine; Robbin Thorp, University of California, Davis; and Neal Williams, Bryn Mawr College.