The osmia is a pollinating bee belonging to the Megachilidae family. The osmia is also commonly known as the “mason bee” due to its ability to create nests using plant material or mud combined with its saliva. It is a peaceful solitary pollinator as it does not live in colonies and does not produce honey.
Morphologically, they have a rust-black colouring accompanied by a thick down that allows them to retain the nectar and pollen they collect. In Europe there are mainly two species of wild bees: the horned osmias, known as Osmia cornuta, and the rufas, whose scientific name is Osmia bicornis.
The horned osmia has thick reddish-black hairs and ranges in size from 12-16 mm. A peculiarity is that males are smaller than females and can be recognised by a tuft of light-coloured hairs on the front of the head. The characteristic name of this species is due to the presence of small horns on the female’s head. The males emerge from the cocoon before the females, usually in March, and after mating they entrust the care of the eggs to the females. The early period of unfurling and the resulting tolerance of cold mean that the horned bee species is suitable for pollinating early flowering fruit trees.
The osmia rufa is smaller than the horned osmia, being only 10-12 mm in size. This species is reddish-brown in colour and has an abdomen covered in raven hairs. It is easy to distinguish a male from a female because of their morphological characteristics: females have a head and thorax covered in dark hairs, while males (smaller) have whitish hairs. This species usually flickers, i.e. emerges from its cocoons, in the spring period of April, making them suitable for pollinating late crops. The rufa species can also be found at altitudes above 1000 metres and tends to create nests in unconventional places such as in locks and garden pumps.
In general, following the mating of the species and the laying of the eggs in the nest, the latter hatch 6-10 days after laying, inside a cell rich in nutrients such as pollen and nectar collected by the females. When the food supply begins to run out, the larva creates a cocoon in which it passes to the prepupa stage followed by transformation into an adult (the length of this period varies according to species). When sfarfallamento (the moment when the insect, now an adult, emerges from the cocoon) takes place, the males emerge before the females, with the flickering of the females, the mating phase begins.
At the end of reproduction, the males are destined for death, while the females are responsible for creating the characteristic nests made up of cells rich in pollen and nectar in which they will lay their eggs. The female eggs are placed in the first cells that are built, while the males are placed in the last cells (closer to the exit of the nest) to allow the males to flicker more quickly. When construction is complete, the nest is closed again to protect the eggs from parasites and temperature variations. Osmias are only able to create one generation per year.
As already mentioned, Osmia cornuta and Osmia rufa are used in agriculture as pollinating insects, particularly for early flowering fruit trees such as peach, almond, pear, apple and apricot or for later maturing fruit trees.
Given the ability of osmias to nest in cavities, it is possible to facilitate their breeding and use for pollination by using artificial nesting materials. These consist of groups of tubes made of various materials (reeds, paper, perforated wooden blocks), which can be brought between the crop to be pollinated just before these pollinators flicker.