Garden Allies: Solitary Bees

By: Frederique Lavoipierre
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Frederique Lavoipierre is Education Program manager at Santa Barbara Botanic Garden. She also teaches classes and workshops on many aspects…

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An Halictid bee. Illus: Craig Latker

According to one of my favorite books, Evolution of the Insects, if we eliminated bees, ants, and termites from the planet, all terrestrial life would collapse.  Whenever someone expresses a horror of insects, I pull this astonishing assertion from my bag of tricks. Without bees (and a few other six-legged pollinators), I continue, our diet would become horribly bland; we would be living on potatoes, corn, wheat and a few other crops. No apples, berries, oranges, zucchini, or tomatoes. No mint, thyme, oregano, or parsley. No chocolate (midge-pollinated), tea, or coffee. Many of the beautiful flowering plants that we take for granted would also disappear. Insects and flowering plants co-evolved, leading to an explosion of species, and the creation of a multitude of niches for other animals. Bees played a central role in increasing this biodiversity.

The familiar honey bee is not native to the United States, but originated around the Mediterranean. Yet, flowering plants were being pollinated long before the first European settlers brought hives to North America. As honey bees struggle with a variety of woes, our native bees are receiving increasing attention. Unlike the highly social honey bee, native bees usually live a solitary existence. Here, we discuss the families that include principally solitary species (not all are native). Social bees exhibit a division of labor, with only certain females reproducing. Solitary bees may occasionally live in aggregations, but they do not help each other in nest-making and the rearing of young. Each solitary female bee builds her own nest (there are no worker bees), and provisions it with the food necessary for the larva to develop.  Solitary bees produce neither honey nor wax.

A leaf-cutter bee (Megachilidae) Illus: Craig Latker

A Short Life

In temperate zones, especially where the ground freezes in the winter, solitary bees may have short life spans. Some species come out at the first hint of warmth, and rely on early flowering plants for sustenance. Bees are either oligolectic (feeding on just a few or, rarely, a single species of flowering plants) or polylectic (with a broader, more general diet). Oligolectic bees may be far less discriminating when seeking nectar than pollen. Many of the solitary bees are oligolectic, and are found flying only during the flowering season of their favorite plants. All bees require protein (pollen) and sugar (nectar); only females gather pollen and nectar for the larvae in their nests.

A few bees carry pollen internally, but most bees have external structures called scopae for carrying pollen. These are hair-like brushes, often on the hind pair of legs; in the leaf-cutting bees, scopae are found on the abdomen, a distinguishing character for the Megachilidae. Some (honey bees, bumble bees) have corbiculae, or pollen baskets, instead of scopae.

Nesting Strategies

Bees can also be grouped according to nesting ecology. Many solitary bees are ground nesting, while others seek out hollow stems or cavities in wood. Plasterer bees (Colletidae) are solitary species (although some nest in aggregations) that build their nests in the ground and characteristically line their nests with a cellophane-like material. Mason, leaf-cutting, and carder bees are also primarily solitary species, with a few species that nest in aggregations; these megachilids nest mainly in hollow stems and cavities in wood, although a few are ground nesters. Mason bees use mud as a nesting material, while leaf-cutting bees build nests lined with leaves (and sometimes rose petals). Carder bees line their nests with plant fibers.

Mining bees (Adrenidae) are solitary ground dwellers; some species may use a common entrance to the nest, but each female still provisions her own offspring. Scopae are usually present on the entire length of the hind legs, and many species in this group are oligolectic. This large family of bees contributes much of the bee diversity of temperate zones of the Pacific Coast. By contrast, the Melittidae (mellitid bees and the oligolectic clarkia bee) is a small family of mostly ground-nesting solitary bees with a restricted distribution; they often have shaggy scopae. [The Halictidae (sweat bees) and Apidae (honey bees, bumble bees and carpenter bees) exhibit a variety of social behaviors and will be discussed in future columns.]

Providing habitat for bees is quite easy. Bee plant lists are readily available; choose plants that flower over a long season to serve the greatest diversity of bees. Select plants that will attract native bees in addition to honey bees. Annual native wildflowers are easily established and are an important source of early spring nectar for native bees. Ground-dwelling bees prefer bare ground and are often found nesting in sun-baked banks, so leave some areas mulch-free for their benefit. Populations of many species of wood-nesting bees can be established using readily available nest boxes, or you can make your own boxes. Different-sized holes in the wooden nest boxes will accommodate a variety of species.

Solitary bees are highly unlikely to sting. The sting is a modified ovipositor; thus, only female bees can sting. Most bee species are not aggressive and only sting in self-defense.

Surely, few things can give more pleasure in the garden than the gentle hum of flower-visiting bees.

In a Nutshell

Popular Names:

Plasterer bees (colletids), mason, leaf-cutting, and carder bees (megachilids); mining or digger bees (andrenids); melittid bees (no common name, but includes clarkia bees).

Scientific Name:

Order: Hymenoptera (ants, wasps, bees). Superfamily: Apoidea (bees and sphecoid wasps).

Common Families:

Primary solitary bee families on the Pacific Coast include Colletidae, Megachilidae, Andrenidae, and Melittidae. (Halictidae and Apidae also include some solitary species.)

Common Species:

Megachilid bees such as the native blue orchard bee (Osmia lignaria) and the non-native alfalfa leaf-cutting bee (Megachile rotundata).

Distribution:

Worldwide, there are about 20,000 species of solitary bees. About 3,500 species are native to North America; California alone is home to probably over 1,500 species.

Life Cycle:

Holometabolous (a complete metamorphosis from egg to larva, pupa, and adult)

Appearance:

Eggs: small, translucent, sausage-shaped; in solitary species, the egg is generally laid on a pollen ball that resembles a lump of ear wax. Larvae: white, legless, grub-like; relatively inactive. Pupae: Exarate (legs and appendages are free); many species spin a cocoon around the pupa. Adults: Various, from 1/8 to over 1 inch; brown, black, or sometimes metallic green or blue; may be striped yellow, white, orange, or red. Bees can be distinguished from wasps (which they sometimes resemble) by the branched hairs, especially on the thorax, that can be seen under a microscope.

Life Span:

Solitary bees have an annual life cycle; generally only a short time is spent as an egg, larva, and adult, compared with the much longer pre-pupal and pupal stages

Diet:

Pollen and nectar in both larval and adult stages.

Favorite plants:

Nectar- and/or pollen-rich flowers for sustenance; hollow stems, leaves for nesting materials. Bees may favor blue or white flowers, but will visit flowers of any color.

Benefits:

Pollination, resulting in the development of seeds and fruits

Problems:

Leaf-cutting bees leave cuts in leaves and petals. Mining bees occasionally cause problems in lawns.

Interesting facts:

Other insects, such as flies, beetles and diurnal moths, may mimic the physical appearance or activity of bees. Solitary bees produce fewer offspring than most other insects, sometimes fewer than twenty-five during a life cycle.

Sources:

Orchard mason bees are readily available for purchase. Abundant floral resources will attract many bee species; some can be attracted if nesting blocks are provided.

More information:

Evolution of the Insects, 2005, by Grimaldi and Engel; a beautifully illustrated text.

Bees of the World, 1991, by O’Toole and Raw, is a great resource, delving into the natural history of all the bee families.

http://nature.berkeley.edu/urbanbeegardens/. An essential resource maintained by UC Berkeley’s Dr Gordon Frankie and his team.

www.attra.ncat.org/attra-pub/PDF/nativebee.pdf

www.xerces.org/native-bees. Excellent information on bees and other pollinators.

 

See also these past articles in Pacific Horticulture:

Bees in the ‘Burbs, April 2003

The Melissa Garden: A Sanctuary and Season for Honeybees, July 2009