For most people, the word bee conjures up a picture of a colony of honey bees (Apis mellifera) consisting of a queen and thousands of workers. But as it happens, the great majority of bee species lead solitary lives. In these species, each nest is occupied by a single female who plays the role of both queen and worker. This female builds her own nest, and defends it against parasites and predators. She also collects pollen for her offspring.
Solitary…but often community-oriented!
Solitary bees build their nests in a wide variety of substrates, such as soft wood, hollow stalks or preexisting cavities. However, about 70 percent of solitary bees make their nest underground. In Québec, mining bees (Andrena), plasterer bees (Colletes), sweat bees (Lasioglossum) and halictids (Halictus, for example) are the best known of the ones that nest in the ground. Moreover, it’s not unusual to see them burrow into our flowerbeds or alongside our walking paths.
When they dig, these bees accomplish true technical achievements. Among plasterer bees, for example, the female first drills a vertical tunnel between 10 and 30 centimeters deep. Then she makes little galleries, which she waterproofs by coating them with a secretion that resembles cellophane. Among halictids and mining bees, the gallery walls instead are hardened with a liquid, waterproofing secretion. Now well insulated, each of these little cells is ready to receive an egg accompanied by a serving of pollen that the future larva will use for food. That larva will develop in its cell, will turn into a pupa and then become a bee ready to breed. Finally, although certain species are solitary nesters (sweat bees, for example), some – like the halictids – form communities that may contain several hundred nests and are true “nesting grounds.”
Organizing a nesting site
Solitary female bees rarely colonize openings already present in the soil. They prefer to dig their own nests, normally choosing a sunny spot with southern exposure, because their activity is highly dependent on temperature. The place chosen tends to be insulated, and above all contains a minimum of 35 percent sand. Sand makes their excavation work easier and generally offers better drainage. It also protects against fungal infections and lowers the risk of larvae drowning during heavy rains.
As a rule, solitary bees don’t forage very far from their nesting place: often the distance traveled varies by an average of no more than a few meters. For that reason, it’s important that their food resources – plant nectar and pollen – can be found in the immediate vicinity of the nesting site.
Populations in decline
According to scientists, bumblebees aren’t the only ones to be suffering reductions in their populations. Unfortunately, there’s little in the way of data for solitary bees. But a European study revealed that a majority of them were under threat. That’s the case, notably, for 12.9 percent of Colletidae and for 18.9 percent of Melittidae, another family of excavating bees. In addition, habitat fragmentation and destruction, exposure to pesticides, and the impact of introduced plants and bees are among the principal threats to the conservation of solitary bees.