What are the 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 reality, the vast majority of bee species on earth live solitary (1). As the name suggests, solitary bees do not usually live in colonies such as bumblebees and honeybees, but usually live alone. A single female builds her own nest, defends it from intruders, parasites and predators, and forages 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, known and studied than social bees. However, solitary bees exhibit an interesting diversity in morphology, behavior, nest architecture, and host-plant associations. In addition, solitary bees play a very important role in ecological systems, especially in the pollination of crops and wild plants (2). For example, farmers in some parts of the world are already starting to make specific arrangements for collecting solitary bees for pollination of crops (such as alfalfa) that are not effectively pollinated by honey bees.

How do solitary bees look like?

Most solitary bees do not generally look like honeybees, but look more like wasps, flies or bumblebees. Since there are many different types of solitary species they also vary a lot in appearance. Some species are shiny and hairless, while others are densely haired. In addition, the hairs or the spots and stripes can contain all the colors of the rainbow and they vary in length from 1.5 to 46 millimeters.

The solitary bees are distributed among nine families that can be found all over 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 mainly found in the northern hemisphere; the Megachillidae (Figure 3) or leaf cutter and mason bees that can be found worldwide; the Halictidae (Figure 4), also called sweat bees (because they are attracted by human sweat). And finally the Anthophoridae (Figure 5), or carpenter and miner bees, which are mainly found in tropical places (2).

The solitary bee life cycle

A female solitary bee makes her own nest of about 10 breeding cells. Then she stocks the breeding cells with food (Pollen/nectar) for the young and lays an egg in each cell and dies for the next generation. The eggs of the female solitary bee hatch in larvae, which eat the pollen and hibernate. They stay in the cocoon for about 11 months throughout summer and winter (Figure 6). In the next spring the larvae pupate into adult bees and emerge from their nests. 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 only have one task and that is to fertilize a female and then die (1). The figure below shows this life cycle.

Figure 6: the solitary bee life cycle

Nesting Habits 

So, you learned that solitary bees are highly diverse. Well, therefore so are their nesting habits. The nest represents for most species the largest investment in time and energy that a female makes in her lifetime (1). The construction of a nest consists of different stages. First, she needs to find an appropriate site. Next, she needs to construct the nest itself and form the brood cells. These need to be provided with pollen, nectar or floral oils collected in the surrounding nature. When she finally succeed 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 different combinations of sand, pebbles and plant products (leaves, petals and resin). Most bees choose to build their nests in the ground. They make houses for their young in underground pits, tunnels and caves (whether ground, sand or clay). However, many bees also dig out nests in wood, spicy stems or in unexpected places, such as active termite nests. Then there are also bees that build or use free-standing nests, such as abandoned bee or wasp nests and snail shells. Due to 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 above, solitary bees differ in appearance from honeybees and do not usually live in colonies such as bumblebees and honeybees, but usually live alone. In addition, solitary bees have shorter activity periods than honey bees. It may be that you already knew about these differences, but did you also know that solitary bees are known to pollinate plants more efficiently than honeybees? (3).  During the adult stage solitary bees are highly efficient pollinators and require fewer individuals to pollinate an area when compared to honey bees. Solitary bees pollinate different types of crops and non-crops that are not pollinated by honey bees. Honey bees can even harm 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). 

Solitary bees are less picky than honeybees. In addition, they collect the pollen less carefully on their ‘scopa’, these are stiff, branched hairs, which are on their legs, under their belly or along the sides of their body, making it more likely that pollen will fall when visiting the next flower, making that flower more likely to be pollinated. Also, solitary bees are very effective pollinators, as some solitary bees carry less pollen in each load, which means they have to travel much more back and forth from the flowers to their nest than honeybees and bumblebees. Because of these additional foraging trips, many more flowers are pollinated (5). 

How can you distinguish solitary bees from social bees?

One way to distinguish honey bees from other insect pollinators is through bee hotels/houses. These bee hotels are mostly used by many different species of solitary bees and not by honey bees (Penn, Hu & Penn, 2019). The marketing of bee hotels to promote pollination and conservation is widespread and expanding, at least in North America and Europe (6). This is also necessary, as these solitary bees, important crop pollinators in agriculture-dominated landscapes and essential pollinators of many wild plants are declining significantly worldwide (3).

In conclusion, solitary bees consist of many different species, usually live alone, have a fairly short lifespan, have many different nesting habits and are very effective in pollination of crops and wild flowers, even more effective than honey bees. Moreover, these solitary bee species are declining worldwide and therefore it is very important to raise awareness of the importance of these solitary bee species, 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). Solitary bees. Scientific American, 250(2), 120-127.

3. Peterson, S. S., & Artz, D. R. (2014). Production of solitary bees for pollination in the United States. In Mass production of beneficial organisms (pp. 653-681). Academic Press.

4. Penn, J., Hu, W., & Penn, H. J. (2019). Support for solitary bee conservation among the public versus beekeepers. 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). ‘Bee hotels’ as tools for native pollinator conservation: a premature verdict?. PloS one, 10(3), e0122126.

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