Tree bees. 1. Holly.

These days I am looking up trying to find bees in trees. Insect pollinated trees can attract large numbers of bees, as they offer a highly concentrated resource: hundreds or thousands of flowers are present in the same spot. The drawback for the bee watcher is that, unless you carry binoculars, identification is not easy. There are two holly trees in my garden and I had noticed patrolling Red mining bees and feeding Bombus hypnorum in them before, but I could not take good shots. Today I found a holly hedge in bloom and I could get close to the bees that pollinate this tree. Holly flowers are small and inconspicuous and I have rarely noticed bees visiting them, but this little bush had many bees in it. Hollies are dioecious, which means that there are male and female trees, self-pollination is not possible and fertilisation requires insects visiting first a male tree and then a female tree. I am not sure if male holly flowers produce nectar as well as pollen. On the photo above a male Red Mining bee sits on the flowers of a male holly, its antennae covered on pollen.
In their Holly monograph, G. F. Peterken and P. S. Lloyd stated:

Entomophilous. Apis mellifera L. is the commonest insect visitor, but the following bees have been observed in southern England (O. W. Richards): Andrena wilkella Kby. (Andrenidae), Osmia rufa L. (Megachilidae) and Bombus lucorum L. (Apidae). B. lucorum and syrphid flies have been seen at the flowers in northern England. Nectar is secreted from tissue at the base of the ovary.

A honeybee visiting the flowers.

An Red Mining bee at the left of the flowers with scopa full of yellow holly pollen

Reference:

Peterken, G., & Lloyd, P. (1967). Ilex Aquifolium L. The Journal of Ecology, 55 (3) DOI: 10.2307/2258429