There is increasing evidence that wild pollinators are encountering a severe decline, both in the abundance as well as in the number of species, with high risk of local extinction.
Several causes have been identified: climatic changes, soil use changes, habitat fragmentation, use of pesticides in agriculture and environmental pollution in general; but the most common and widespread reasons for the decline of a pollinator species are the scarcity of floral resources and the scarcity of nesting sites.
Scarcity of floral resources
A bumblebee colony is active from March to September and because of this reason it requires a continuous succession of flowers. Furthermore, bumblebees do not store large amount of honey and pollen in their nest, like honeybees do, and therefore they are extremely vulnerable to discontinuity in the food supply. In particular the nest establishment phase is the most delicate, when the queen alone has to gather sufficient forage to feed her first batch of eggs, and the availability of flowers in this period is essential.
Solitary bees are mostly univoltine and have a short life cycle, often lasting one or two months. Consequently, the availability of flowering species during the flying period of a species is essential not only for its maintenance, but even for its occurrence in an area. Furthermore, most solitary bees are oligolectic, which means that they forage on few botanical species. The decline or disappearance of one or more of these species could lead to the disappearance of the pollinator specie in that area, and vice versa.
The fate of plants and bee pollinators are strictly connected: if bees decline, the plants that they pollinate set less seeds and there is consequently less food for the bees in the following year; this in turn leads to a higher decline in bee populations. This situation, named as “extinction vortex”, in which mutually dependent species drive each other to extinction, has been recently described for an increasing number of plant-pollinator relationships.
Scarcity of nesting sites
The scarcity of nesting sites is a very critical and widespread threat in intensively cultivated areas, where the substrates for cavity-nesting species are scarce and the absence of hedgerows and corridor between crops prevents many species from finding suitable nesting sites.
Furthermore, some solitary bees need particular materials or substances to build the nest: some Megachile species cut foliar pieces from Rosaceae leaves to cover the nest internally; someChalicodoma species need mineral salt to reinforce the nest; other Megachilid species, like Heriades, use vegetal resins or gums to seal the nest. The scarcity or the complete absence of these substance in an area can cause the decline or the disappearance of the specie, which becomes unable to construct the nest.