What is the day-to-day life of a solitary bee?

When we talk about bees, the honey bee (Apis mellifera) often comes to mind. A social species, with a very marked hierarchy, that forms large hives and produces honey; however, be careful, because this is not the reality of all bee species. To give you an idea, in Catalonia alone, we can find more than 700 species of bees , and in fact, the vast majority are solitary and do not make honey. So, how do these tireless solitary pollinators live?

Following the day-to-day life of a solitary bee is quite complicated, as it spends most of its life inside the nest undergoing metamorphosis, and during the few weeks it takes to fly, it hardly ever stands still. To add even more intrigue, each species has adapted its flight period, morphology, behavior and life cycle to the conditions of its habitat; so many species, so many stories.

Bees travel from flower to flower to feed themselves and, in return, pollinate them, helping them produce fruits and seeds. The problem is that today they are in general decline in Europe, among other things due to climate change, intensive agriculture or habitat fragmentation. So knowing their 'secret life' can help design measures to promote their presence in the natural environment, agricultural landscapes, urban parks...

The honey bee has a reputation for building honeycombs of wax with a multitude of hexagonal cells where it raises its offspring and stores honey, and all this is the result of the cooperative work of the workers. In the case of solitary bees, on the other hand, it is the female who is in charge of building her nest, so they have had to be tricked into inventing the best hiding places to be able to produce offspring.

The vast majority of solitary bees nest underground. They excavate small galleries, often in areas where vegetation does not grow, in bare soil, either on vertical slopes or on the ground. There are other species that nest in holes that already exist : either in holes in dead wood, between stones, inside reeds and other hollow stems or in empty snail shells.

We could say that solitary bees are not claustrophobic, since the holes where they nest have their own size: we are talking about between 3 and 9 millimeters in diameter! Within these narrow galleries, the females lay their eggs, but not in groups, but in small isolated chambers. This means that the females cannot visit and care for the eggs they lay, since the last one covers the previous one. Therefore, they have to leave the food for each larva before laying the next egg. It may seem complicated, so a picture is worth a thousand words:

The females supply each egg with a mixture of pollen and nectar , two resources that they collect from the flowers they find around the nest. Each species has its preferences when choosing the menu for its offspring: some are very selective and only collect pollen from a few floral species, others, on the other hand, are good at everything, they are generalists. To gather such a quantity of provision, the parents have to make many trips: in the specific case of Osmia cornuta, an average of thirty trips has been recorded for each egg.

The female, once she lays her egg on the provision, seals the gallery by building a partition and continues collecting provisions for the next one. Each species seals the nests with a different material , and this is where the bees get creative: some separate the chambers with mud (which they collect near the nest), others do it with pieces of leaves or petals (which they themselves, with their jaws, cut), with plant fibers, and even with resin.

Once the nest is full of eggs, each with its own supply, the female seals the entrance to the nest permanently and leaves. Like a fine wine!

The number of eggs per nest varies depending on the species, environmental conditions and food availability, but the arrangement of these within the nest always follows the same pattern. If you look at the following image, the provisions of the eggs on the left are larger than those on the right. It could be coincidence, or that at that time there were few flowers with pollen and nectar around the nest, but this is not the case. The first eggs that the female lays in the nest - those that remain at the bottom of the hole, on the left of the image - are the ones that have the most provision, since they are fertilized eggs, and will become females. On the contrary, the last eggs to be laid, have less provision and are not fertilized and will be males.

This arrangement of the chambers determines the order of exit: males emerge first and females last, thus ensuring fertilization. Males live for a few days and their only function is to fertilize females. When they emerge from the nest, they wait impatiently for the females to do the same. Once the females emerge, a battle without rules begins to transfer their genetic material to the other sex.

The vast majority of solitary bees are active during the spring and summer months . So, during the colder months of the year, where are the bees?

Most solitary bee species in Catalonia have a single generation per year, known as univoltine . Adult males live for a few days, and females for a few weeks: in less than a month they have time to mate, build a nest, lay eggs and provide for them. Once all this is done, they die.

The next generation of bees will spend the rest of the year developing inside the nest. Inside each chamber, one of the most beautiful miracles in the animal world will take place: metamorphosis, where these eggs will finally become adult bees. First, the eggs will develop into small larvae that, little by little, will feed on the provisions that the parent has served them, and they will grow. When they reach the last stage of development, they will form a cocoon, inside which they will transform into pupae. Just as butterflies do, the following year, the adult bees will emerge from the cocoon and instinctively pierce the partition of their chamber towards the outside. Once they take flight, a new countdown will begin. Time flies!

The loss and homogenization of habitats, intensive agriculture, excessive use of pesticides, urbanization and climate change, among other factors, are causing populations of wild pollinators to fall at a European level, and also in Catalonia. This decline is worrying, because it endangers the proper functioning of ecosystems and also food security. So knowing in detail how solitary bees live and what they need, and the rest of wild pollinators, can help us design measures to favor them.

From CREAF, the research group in insect ecology and pollination (GEIP), has several projects underway to study pollinator populations in different areas of the Catalan and global landscape. For example, through the European initiative RestPoll they work to implement and evaluate simple practices that benefit pollinator communities in various types of habitats, such as agricultural areas, vineyards and pastures. There is also BeeConnected SUDOE underway, which is restoring the margins of greenways , to better connect habitats.

On the other hand, monitoring the status of wild pollinators is also essential to know their status and decide where and how to apply measures. That is why the team also coordinates the deployment of the Intersectoral Plan for Wild Pollinators of Catalonia (PIPOL) . On a more international scale, the GEIP has also participated, through the SPRING project, in the design of the future Pollinator Monitoring Plan in Europe.