Bees need flowers and plants primarily for food. In particular, they gather nectar and pollen from the flowers with which to provide the nutrients they need to feed themselves and their young.
However, it may surprise people to learn that depending on the species of bee, flowers and plants (including trees) may have other uses for bees too! These uses typically include:
Aside from the nectar and pollen, leaves, petals, plant hairs, plant stems, oils and resins may also be gathered. Nests may be created in hollow stems, holes and crevices in tree trunks.
Nectar contains sugar (carbohydrate) which provides bees with lots of energy. You can read more about this topic on my page: 'Why do bees need nectar and pollen?'.
Pollen contains essential protein and fats and provides essential food for developing larvae.
Some species use segments of leaf or petal for creating nest cells - leafcutter bees are well known for this activity. In the very short video below, you can watch a leafcutter bee returning to its nest cell with a segment of leaf.
Likewise, other bees such as the silvery leaf cutter, and the poppy mason bee use pieces of petal to line their nest cells, among other species.
It's not only the flowers that are of interest to bees. Some species are also interested in the plant hairs found on some leaves and stems. For example, wool carder bees collect hairs for their nests from leaves and stems of plants - typically Lamb's ear - Stachys byzantina.
For some species of bee, floral oils are very important, in particular to Macropis - or oil-collecting bees. These bees have extra long, specially adapted hairs on their hind legs which they use to gather the floral oils. The floral oils are used to line and waterproof their nest, and are mixed with flower pollen to feed their young.
Not just any floral oils will do, however. These bees forage on loosestrife flowers which somewhat results in this species being restricted to locations where the flower can be found. In the USA there are four known Macropis species, in the UK just one.
Some bees occupy old insect holes, such as small beetle burrows in wood, and gather resins with which to line their nest cells. An example is Heriades truncorum - the large headed resin bee - as you can see, a very tiny bee - pictured above.
It is also well known that some bees shelter or create their nests in hollow plant stems. For this reason, I advise against cutting and burning dry plant stems. Instead, put them in a pile at the back of the garden out of view. Hollow stems can be used to create insect houses, which may attract bees, such as red mason bees.
The red-tailed mason finds thin stems of dried grass close to a nest she creates in an empty snail shell, and creates a thatch over the shell, presumably to conceal her nest. Watch the red-tailed mason bee - Osmia bicolor.
Some bee species happily create nests in the crevices and holes of trees. Below is are images indicating the location of a wild honey bee nest in a tree trunk.
It's not uncommon to see bees resting, or even apparently snoozing inside or on a flower - particularly where bumble bees are concerned. This was the case with the Bombus barbutellus male below.