Osmia tergestensis: The Gaudí of the bees

Do bees use flowers only as food sources? Of course not. Male orchid bees in tropical America collect scented oils from flowers to attract mates; male carpenter bees often use flowers for a good night’s sleep; and the females of several species of leafcutter bees use flower petals to build brood cells, i.e., nests to lay their eggs into, resulting in tiny colorful works of artistic craftsmanship.

Encounter in Trieste

In the summer of 1896, in Trieste, 21-year-old Adolpho Ducke, an Italian naturalist of German descendants who would later become a leading advocate for the conservation of the Amazon rainforest in Brazil, discovered a new species of leafcutter Osmia bee, which he named Osmia tergestensis (Osmia of Trieste). No bigger than a ladybug, this bee is shiny black and sparsely covered with long greyish-yellow hairs that offer a somehow glowing impression.

The construction stars among the solitary bees

This brilliant little architect has gained its reputation owing to its particular nesting behavior. Solitary bees provide parental care to their offspring by securing that, once they hatch, their little ones (the larvae) will find enough food to sustain them until pupation. To achieve this, they construct brood cells in various types of cavities (e.g., crevices, old wood, into the ground, etc.), in which they lay one precious egg, put some food next to it for the hatchling, and seal the opening, usually with mud. Leafcutter bees are exceptional among solitary bees because they have evolved to use fresh leaves for building the brood cell walls; for this, they have developed special razors on their jaws, which make leaf-cutting an easy task. Yet, one group of species within leafcutters has specialized in using flower petals instead of leaves, resulting in some of the most elegant and charming constructions made by insects.

Artists at work 

How do you build a brood cell from flower petals? First, you need an already existing cavity; O. tergestensis often prefers rock crevices and fissures. Second, you need 50-60 petals from plants like geranium, rockrose, hawkweed, flax, or restharrow. Third, you need mud and tiny pebbles. To begin with, you coat the natural wall of the cavity with a first layer of petals, so that each petal partly overlaps with the adjacent one. Then, you overlay mud and pebbles, and before this mixture dries up, you cover it with another layer of petals. Once you have laid your egg inside the chamber and added the necessary food provisions (nectar and pollen), you seal the opening with a plug made of moist soil. Finally, you camouflage the nest with more petals and other materials from the vicinity.

Bedded on roses

This petal-mud-petal sandwiched brood cell is not only a pleasure to the human eye, but also a sophisticatedly engineered artifact, allowing baby bees to survive until adulthood. Although, to date, the ecology and evolution of this behavior have not been sufficiently studied, we can hypothesize about the advantages of using petals for brood cell construction. For instance, because petals have excellent water-repellent properties, they can protect the eggs and the larvae from flooding, but also preserve the desired levels of humidity inside the chamber. Moreover, the multiple layers allow the cell to float, in case it is carried away by water. In addition, the slender petals are easier to handle than leaves, making the life of the mom bee a little less painful: petals are easier to cut, carry, fold, arrange, and glue together. Bee scientists believe that another possible benefit is due to the scent of the petals, which often includes several compounds with antiseptic properties, offering an extra protection to the larvae.