When you think of bees, what comes to mind? It might be the sweet taste of honey or an ever present buzzing next to your ear. It may even be the 2007 cinematic masterpiece, “Bee Movie.”
What you might not expect is that the grand majority of bee species do not produce honey or even live in hives. Roughly 77% of the bees on earth are classified as solitary. This classification exists in contrast to social bees, which are those that organize themselves into hives, with each bee serving a role to protect and sustain the community. On the other hand, solitary bees do it all themselves, with females building and provisioning nests with no assistance.
Solitary bees are wildly fascinating and incredibly diverse in their morphology and behavior. However, they have long been the underappreciated relatives of social bees. No more! Let’s dive into what makes these little creatures so special, and why we should protect them.
Bees emerged from a group of hunting wasps over 120 million years ago. These wasps feed on arthropods and insects, whereas bees get their nutrition from protein-rich pollen. This means that, in the simplest terms, bees are vegetarian wasps.
So what pushed this shift from predation to “vegetarianism?” One idea is that eudicots, which represent 75% of flowering plants, emerged in the fossil record at almost the exact same time as bees, roughly 125 million years ago. While there were forms of flowering plants and pollinators before this dual emergence, eudicots and bees represent an explosion of biodiversity and the origin of pollination as we know it today.
Even more interesting, bees predated the Cretaceous–Paleogene extinction event, in which the dinosaurs went extinct. After the asteroid impact, bees saw a massive rise in their rate of diversification, leading to the roughly 20,000 described species on earth today.
Of those 20,000 species, roughly 15,400 are solitary. These bees build individual nests for their larvae in an awe-inspiring variety of materials. Solitary bees are known to build nests in everything from wood to soil to reed structured plants. In urban and suburban settings, they will even create homes in fences and in the walls of sheds and homes.
To build their nests, the female bee must excavate the substrate material and provision the nest with floral oils, pollen and nectar. She will then lay her eggs within the brood cell, which will be home to her offspring while they mature. If her nest is poorly constructed, it could expose her larvae to flooding, parasites, predators or freezing, all of which could result in the death of a generation of her offspring.
It is important to note when collecting resources for food and nest provisioning that not all pollen is created equal. Or at least, not all bees like all pollen. Solitary bees fall onto a spectrum between oligolectic to polylectic. The former means that a bee species will only feed on one plant family, or perhaps a few genera of plants. The latter is the opposite end, where a bee will feed on many plant families.
Why does this matter? As it turns out, most solitary bees are oligolectic. This may be for a few reasons. One is that bees will generally choose to feed on plants with the highest pollen nutritional value. Another reason is that by specializing in one plant family, bees can adapt to collect pollen from those plants in the most efficient manner. A final theory is that by harvesting from only one type of plant, bees require less neural capacity to survive.
Whatever the reason, the majority of solitary bees being oligolectic means that bee species are very vulnerable to the loss of their host plants. In fact, habitat change, be that in the form of fragmentation, loss or conversion, is the leading human-imposed threat to bee populations. This issue is faced by both social and solitary bees; however, it seems to affect solitary bees most heavily. In fact, the majority of threatened bee species in Europe are solitary.
Why is this the case? Besides being oligolectic, solitary bees lack the social infrastructure of a hive, making it more difficult for them to forage far away from their nests. A study by Gathmann et al. found that the foraging range for solitary bees averages between 150 and 600 meters. This shows how important local habitat structure and host plant availability is to solitary bee conservation.
It is not enough to have a few host plants for each solitary bee species in an area. A study by Müller et al. showed that, for 85% of solitary bee species examined, the entire pollen content of over 30 flowers is required to provision and rear one larvae. This does not account for the fact that only 40% of a flower’s pollen is accessible to a bee at any given time, which makes the number much higher in natural conditions. And so, if we want to build habitats that can sustain solitary bees, we must ensure a local abundance of proper host plants.
Knowing all of this information is critical to protecting, building and conserving solitary bee populations. With 90% of plants on earth requiring pollinators to reproduce, this mission could not be more important.
So next time you see a bee, get excited instead of scared. After all, it has way more important things to do than sting you.
