Biology and ecology
Solitary bees are bee species in which every single female is fertile and makes an individual nest she constructs herself. Some species are gregarious, preferring to make nests near others of the same species. In some species, multiple females share a common nest, but each one makes and provisions her own cells independently. Moreover, sociality is present in several bee groups (i.e. Halictidae), at different levels of complexity.
Solitary bees usually nests in pre-existing cavities, like hollow reeds or twigs, holes in wood or stones, or tunnels in the ground; however sometimes they dig new ones by themselves.
Solitary bees are often oligolectic, in the sense they gather pollen only from one or a few species/genera of plants, unlike honey bees and bumblebees which are generalists.
The life cycle
The life cycle starts in spring/summer (depending on the species) when males and females exit from the cocoon and mate. In most of the species males emerge before females (proterandry). After mating, the female searches for a proper cavity as nest, often using the same from which she has emerged. She prepares several sequential chambers along the cavity, using various materials (pieces of leaves, resin, sap, rotten wood or others, depending on the species) then she fills each chamber with a mix of pollen and nectar, laid one egg in each chamber and finally seals it off with the same materials.
The female does not take care of the brood once the egg is laid, and usually she dies after making one or more nests. The first eggs laid along the tunnel are fertilised and will develop into females, the last are unfertilised and will develop into males. Therefore, males are located in the external cells and will emerge before females, in order to be present when the females emerge from other nests, and optimize the chances for meeting and mating.
Taxonomy and distribution
Solitary bees belong to the superfamily Apoidea and to 7 families: Colletidae, Andrenidae, Halictidae, Melittidae, Megachilidae, Antophoridae and Apidae. They include species with variable dimensions: from 4 mm of some Colletidae and Halictidae, to 30 mm of Xilocopa (Apidae). The members of Megachilidae family are distinguishable by a peculiar pollen-carrying structure (called ventral scopa), which is located in the ventral surface of the abdomen (rather than on the hind legs, like the corbicula in the other bee families).
According to a recent estimate, the number of solitary bee species in the world is 20.000 – 25.000. In Italy there are over 960 bee species, which represents more than one third of the European species. In fact, solitary bees are most common and abundant in hot-temperate and semi-arid regions of the world, like the Mediterranean basin.
Many solitary bees are very efficient pollinators; Megachilidae bees of the genus Osmia, known as orchard bees, are better than honey bee in pollinating fruit trees. To date, few species of solitary bees are produced on a commercial basis for pollination service: Nomia melanderi, reared in the USA and New Zealand for alfalfa pollination, Megachile rotundata, used in several countries also for alfalfa; Osmia cornifrons, used in Japan and the eastern USA for apples and almonds; Osmia lignaria, tested in North America for a number of fruit crops, notably almonds and apples. Some other Megachilidae species are managed in other countries for pollination purposes (e.g.Osmia cornuta in Italy), but their artificial rearing is performed mainly for research purposes, while the commercial use is just at the beginning. On the other end, providing nesting boxes for solitary bees in private gardens is becoming increasingly popular.