Hollow sticks, cracked stones and other places bees call home: nesting habitat for bees

Nests are bee nurseries

Female bees build nests to protect their offspring from bad weather, predators, parasites and disease. Bees nest in cavities that they either find or create. Each of the 4,000 bee species found in North America has a unique life history and nesting preferences. Most North American bee species (>70%) nest underground. Ground nesting bees dig tunnels or look for abandoned holes made by other animals. Other bee species prefer to nest aboveground. Females look for hollow twigs, chew tunnels in dead logs or take advantage of preexisting burrows left by wood-boring beetles. Some species search for naturally occurring crevices in stones and trees and will even occupy pinecones or empty snail shells! A few species construct exposed nests made of soil and chewed plant materials plastered together with saliva or plant resins. Finally, rare parasitic bee species forgo nest making altogether and lay their eggs in other bees' nests.1

Females modify nest cavities to fashion a perfect nursery environment. Bees line their nests and build interior partitions with a variety of materials including mud, plant hairs, tiny pebbles and plant resins. Some bees even “wallpaper" their nests with pieces of leaves or petals. The females of many ground nesting species coat their nests with waxy secretions produced from specialized glands. These coatings act as a barrier against excess moisture and microbes. This allows some bees to nest in floodplains or seasonal lakebeds.1,2

Nests are partitioned into cells, with each cell serving as an individual bee's developmental chamber. Females lay an egg in each cell and leave behind a pollen pellet held together with nectar. (In social systems, family members actively feed immature bees). Eggs hatch into grub-like larvae and eat the pollen stores. Larvae undergo metamorphosis as pupae and emerge as winged adults. Ground nesting bees partition their tunnel or dig lateral offshoots to house cells. Offshoots can house one or many cells, with cells arranged linearly or in Bee Habitatclusters. Bees that nest in narrow cavities or hollow twigs align cells end-to-end. Partitions between cells are sealed with mud, wood-pulp, leafy envelopes or other materials. When mature, adult bees chew their way out of their cells and exit the nest. In these linear nests, females create cells in sequence, starting at the back of the nest and moving towards the front, until the final cell is sealed at the nest entrance.1,2

Life cycle and sociality are reflected in bee nest construction

The overwhelming majority of bee species are solitary. In these species, newly emerged adult males and females mate. Females then choose a nest site, construct cells, lay eggs, provide food stores for each egg and seal the cells. Females may build one or many nests in different locations over the course of their lifetimes and mothers usually do not meet their offspring. Depending on the species, solitary bees can have one or many generations per year. The final generation usually completes its development and overwinters in the adult stage inside the nest and waits until conditions are favorable for emergence.1

Solitary females may nest alone or in large aggregations. If nesting conditions are favorable, hundreds of bees may nest as close neighbors! Sometimes females share the same nest entrance but build cells in separate internal offshoots.1

Unlike solitary bees, social bees share the same nest and cooperatively divide tasks. A select female (the queen) and males reproduce while other female workers dedicate their lives to building a home and providing food for their close relatives. Cells in social bee nests are usually made of wax, and social bees nest in cavities that are large enough to accommodate their sizable families. Interestingly, some bee species display nesting habits that span the solitary to social gap, and some are able to flexibly switch between solitary and social behaviors depending on environmental circumstances.1

Nesting site habitat loss and 'bee rustling' are threats

With recent honey bee declines, the agricultural contributions of wild native bees and other managed bee populations are gaining recognition and becoming increasingly important to our nation's food security. Loss of pollinator habitat, including blooming floral resources, threatens managed and wild pollinator populations alike. Because the pollinator supply is limited, the trapping, sale and relocation of native bees from public lands is an emerging threat. For example, the native blue orchard bee (BOB, Osmia lignaria propinqua) is used for fruit pollination along with another introduced mason bee (Osmia cornifrons).3 Disturbingly, some businesses place 'trap' bee nests on public lands to attract nesting BOBs.4 Once the nests are occupied, they are removed and sold. This is problematic for three reasons: 1) BOBs play an important role in their natural habitat's pollination web. Their removal could undermine the stability of the habitat that they were taken from; 2) Many nontarget bees and wasps are also caught in trap nests. The capture (and subsequent disposal) of these nontarget species likely damages wild populations and the habitat that they came from; and 3. Trapped bees are shipped to regions where they are not naturally found. Broad dispersal could not only spread diseases and parasites, but also lead to the introduction and establishment of bee species in nonnative ranges, causing problems for local native pollinators.

How can you provide nesting habitat for bees?

  1. Play in the dirt. Different bee species nest in different soil types, but generally favor patches with southern exposure and good drainage. Bees might be attracted to your lawn, dirt driveway, or bare garden path. If you notice ground nesting bees in your yard, just leave them undisturbed (e.g., do not run a lawn mower over their homes). You can encourage bees to nest in your yard by providing different types of soil nesting habitat: build mounds of soil, clay, or sand to attract different types of bees. If you live in an area with poor soil drainage, offer bees planters filled with different types of dirt. Notably, many bees that nest aboveground still use soil to line their nests and build cell partitions. These bees will benefit from the soil you provide, even if they do not nest in the dirt piles.
  2. Brush up sticks. Old stumps and logs make excellent homes for wood nesting bees. Bees will either chew their own nests or occupy vacant tunnels left by other wood-boring insects. Plant native, hollow-stemmed perennials or provide brush piles to attract bees that nest in twigs.
  3. Rock on. Provide some loose rock piles or build a rock wall to attract bees that nest in rocky crevices.
  4. Manage other aspects of bee habitat. In addition to nesting sites, bees also need food (nectar and pollen from flowers) and water. Do not spray pesticides or other chemicals near nest sites or on food sources. Read about combating pollinator habitat loss here.
  5. Be a good bee host. With the decline of honey bees, human efforts to support, manage and even exploit wild bee populations have become increasingly popular. Human activities range from local nest box or 'bee hotel' installations to commercial enterprises that rear, harvest and sell native bee species. While most human activities are done with the best intentions, poor management of artificial bee nests can lead to parasite and disease build-up. If you choose to build (or buy) a bee nest box or bee hotel, it is your responsibility to follow best management practices, including cleaning or replacing nest materials at appropriate times to remove parasites and microbial pathogens. Bee nests can also temporarily be relocated to uninsulated buildings during certain times of the year to protect bees from the elements, parasites or predators. Familiarize yourself with the bee parasites that attack bee species in your area; when appropriate, screen nest contents and remove parasites. Read more about parasatoids and cleptoparasites of native bees.
  6. Do not trap or relocate native bees or their nests from public lands. If you are purchasing BOBs or other pollinators, carefully research where the bees are obtained from and determine if they are native to your range. You can consult with your local university extension program if you have questions.
  7. When bees move into your home. Bees will happily nest in manmade structures, sometimes to a homeowners' dismay. If carpenter bees are boring holes in your home, deck or fence, read this article to learn how to manage them (often painting wooden surfaces will deter bees). Of note, paper wasps and yellow jackets are relatives of bees and can also nest in or around human dwellings. If you must remove paper wasps or yellow jackets, more information is available: Getting rid of paper wasps and yellow jacks; and European paper wasp.
  8. Read more articles about bee life cycles, their ecology and their role in pollination: 1. Spring bees: who are they and where do they live?, 2. Bees in Pennsylvania: diversity, ecology and importance, 3. Conserving wild bees in Pennsylvania, 4. Who are our pollinators?, 5. Orchard pollination: wild bees, and 6. Wild bees for Pennsylvania cucurbits
  9. Participate in citizen science. Help scientists locate ground nesting bees and study their diseases by contributing data to Bee Germs. If you decide to install an artificial bee nest or hotel, determine if there are ongoing citizen science projects that are requesting data from artificial bee nests. The Minnesota Bee Atlas is one example. Many other citizen science programs request pollinator observations in the field. (Citizen Science)

References

  1. Wilson, J. S. & Messinger Carril, O. J. The Bees in Your Backyard. The Bees in Your Backyard (2016). doi:10.1515/9781400874156

  2. Michener, C. D. Evolution of the nests of bees. Am. Zool. 4, 227–239 (1964).

  3. Biddinger, D. Orchard Pollination: Solitary (Mason) Bees. Penn State Extension (2018). (Accessed: 1st March 2019)

  4. Tepedino, V. J. & Nielson, D. Bee-Rustling on the Range: Trap-Nesting for Pollinators on Public Lands. Nat. Areas J. 37, 265–269 (2017).