What are solitary bees and how can you recognize them?

When most people think of bees, they usually think of the social bees, such as honeybees or bumblebees. But did you know that in reality more than 85 percent of the 20,000 species of bees are not social but solitary?
In fact, the vast majority of bee species on Earth live solitary lives (1). As the name suggests, solitary bees usually do not live in colonies like bumblebees and honeybees, but rather live mostly alone. A solitary female builds her own nest, defends it from intruders, parasites and predators, and searches for pollen, nectar or flower oil as food for her offspring. Some solitary bees nest in groups of hundreds to thousands of nests, but each nest is managed by only one female, and each female is both worker and queen throughout her life.

Solitary bees are less conspicuous, less known, and less studied than social bees. Nevertheless, solitary bees exhibit interesting diversity in morphology, behavior, nest architecture, and host-plant associations. Moreover, solitary bees play a very important role in ecological systems, particularly in the pollination of crops and wild plants (2). For example, farmers in some parts of the world are already beginning to make specific arrangements for the collection of solitary bees for the pollination of crops (such as alfalfa) that are not effectively pollinated by honey bees.

What do solitary bees look like?
Most solitary bees generally do not look like honey bees, but are more like wasps, flies, or bumblebees. Because there are many different types of solitary species, they also vary greatly in appearance. Some species are shiny and hairless, while others are densely hairy. In addition, the hairs or spots and stripes can be all colors of the rainbow and vary in length from 1.5 to 46 millimeters.

Solitary bees are divided into nine families that occur throughout the world. Some examples are the Colletidae (Figure 1), or membrane bees, which are most numerous and diverse in the southern hemisphere; the Andrenidae (Figure 2), or digger bees, which are found mostly in the northern hemisphere; the Megachillidae (Figure 3), or leafcutter and mason bees, which are found worldwide; the Halictidae (Figure 4), also called sweat bees (because they are attracted to human sweat). And finally, the Anthophoridae (Figure 5), or carpenter and miner bees, found mostly in tropical places (2).

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The life cycle of the solitary bee
A female solitary bee makes her own nest of about 10 brood cells. Then she provides the brood cells with food (pollen/nectar) for the young and lays an egg in each cell and dies before the next generation. The eggs of the female solitary bee hatch into larvae, which eat the pollen and overwinter. They remain in the cocoon for about 11 months during the summer and winter (Figure 6). In the following spring, the larvae pupate into adult bees and emerge from their nest. Once outside the nest, the average lifespan is a short 4-6 weeks. You must be wondering, what do the male bees do in this whole story? Well, they have only one job and that is to fertilize a female and then die (1). The figure below depicts this life cycle.

Figuur 6: de levenscyclus van een bij

Nesting habits
So you’ve learned that solitary bees are very diverse. So are their nesting habits. For most species, the nest is the largest investment of time and energy a female makes in her life (1). The construction of a nest consists of several phases. First, she must find a suitable site. Then she must build the nest itself and form the brood cells. These must be supplied with pollen, nectar or flower oils collected in the surrounding nature. When she has succeeded in all these steps, she can finally lay her eggs and close the nest.

Most bees choose to line their brood cells with glandular secretions, but there are also bees that use various combinations of sand, pebbles, and plant products (leaves, petals, and resin). Most bees choose to build their nests in the ground. They make homes for their young in underground pits, tunnels and burrows (whether soil, sand or clay). However, many bees also dig out nests in wood, herbaceous stems, or in unexpected places, such as active termite nests. Then there are bees that build or use free-standing nests, such as abandoned bee or wasp nests and snail houses. With urbanization, there is a loss of habitat for bees and therefore a reduced abundance and diversity of solitary bees in urban areas.

Solitary bees versus honey bees: who is the best pollinator?
As mentioned, solitary bees differ externally from honey bees and usually do not live in colonies like bumblebees and honey bees, but usually live alone. In addition, solitary bees have a shorter activity period than honey bees. You may have already known these differences, but did you know that solitary bees pollinate plants more efficiently than honey bees? (3). In the adult stage, solitary bees are very efficient pollinators and require fewer individuals to pollinate an area than honey bees. Solitary bees pollinate different types of crops and non-crops that are not pollinated by honey bees. Honey bees can even be harmful to solitary bees, as they often compete directly for the same resources. They can also transmit diseases to other solitary bees or other native pollinators (4).

Furthermore, solitary bees are less picky than honey bees. In addition, they collect pollen less carefully on their “scopa” (which are stiff, branched hairs located on their legs, under their abdomen, or along the sides of their bodies) which makes it more likely that pollen will fall when they visit the next flower, and therefore more likely to pollinate that flower. Solitary bees are also very effective pollinators because some solitary bees carry less pollen with each charge, requiring them to travel back and forth from the flowers to their nest much more than honey bees and bumblebees. These extra trips result in many more flowers being pollinated (5).

How can you distinguish solitary bees from social bees?
One way to distinguish honey bees from other insect pollinators is by bee hotels/houses. These bee hotels are usually used by many different species of solitary bees and not honey bees (Penn, Hu & Penn, 2019). The marketing of bee hotels to promote pollination and conservation is widespread and expanding in North America and Europe (6). This is also necessary because these solitary bees, important crop pollinators in agriculturally dominated landscapes and essential pollinators of many wild plants, are in steep decline worldwide (3).

In summary, solitary bees consist of many different species, usually live alone, have a relatively short lifespan, have many different nesting habits, and are very effective at pollinating crops and wildflowers, even more effective than honeybees. Moreover, the number of species of solitary bees is decreasing worldwide and therefore it is very important to raise awareness about the importance of these species of solitary bees, the threats they face and their contribution to sustainable development to halt the loss of biodiversity.

References:

  1. Bryan N. Danforth, et al. The Solitary Bees : Biology, Evolution, Conservation. Princeton University Press, 2019.
  2. Batra, S. W. (1984). Solitaire bijen. Scientific American, 250(2), 120-127.
  3. Peterson, S. S., & Artz, D. R. (2014). Productie van solitaire bijen voor bestuiving in de Verenigde Staten. In Massaproductie van nuttige organismen (pp. 653-681). Academic Press.
  4. Penn, J., Hu, W., & Penn, H. J. (2019). Steun voor behoud van solitaire bijen onder het publiek versus imkers. American Journal of Agricultural Economics, 101(5), 1386-1400.
  5. https://beestrawbridge.blogspot.com/2014/09/why-solitary-bees-are-such-amazing.html
  6. MacIvor, J. S., & Packer, L. (2015). ‘Bijenhotels’ als instrumenten voor het behoud van inheemse bestuivers: een voorbarig oordeel? PloS one, 10(3), e0122126.

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