Interesting study Myriam!
I have found some plants from the southern hemisphere to be very attractive to bumblebees. I grew Leptospermum nitidum from seed, when the plants were a good size and I had developed more ground to have a place to put them, I planted them in the ground. The first of those was in the autumn of 2004, some other plants had to wait until I made another flower bed in 2006. The bumblebees which they attracted were cuckoo bees, Bombus campestris. These bees did feed on other plants such as the Geraniums, I have crosses of Geranium versicolor and what I think is Geranium endressii Wargrave Pink as you know. The latter gave a certain robustness and survivability to the crosses and have proved popular with many insects including the Large Skipper, Ochlodes sylvanus. While the Leptospermum was around, Bombus campestris came. Unfortunately the harsh winter 2010/11 killed all the plants along with Callistemon rigidus which had been in the ground for a good ten years or more, it was a magnet for many bumblebees, mainly males but also attracted many different species of social wasps. The last time I saw Bombus campestris was in 2013, feeding on a very popular self set Geranium pratense which may have come from a named hybrid. I often think I should grow the Leptospermum and Callistemon again, maybe we won't have another such harsh winter for 20 or 30 years at which time I doubt it would bother me.
I wonder where these bumblebees are feeding now, there's very little growing in the 'wild' here.
The species mix of Dahlias which I had, and originally grew from seed, have all about disappeared after that harsh winter with the exception of the cut leaf one which I still am not sure of the name, it is gradually setting more plants around from seed and is very hardy. At the same time I grew the species dahlias I also grew some from seed called Bishop's Children, they have also almost disappeared but they crossed with the species and set themselves in places around the garden, which is amazing in this climate. I have some which seem to have appeared long after the originals disappeared and which have the same characteristics, one is white with a hint of pink but maybe shorter than the species, the species flowered very late and so does the self set! These late flowering dahlias have helped feed later bumblebees such as Bombus pascuorum and the more recent arrival, Bombus hypnorum. It is essential of course that you grow dahlias which have single or semi-double flowers which have a large boss of stamens.
My early flowering Camellias provide food for early emerging bumblebees and honey bees. Mahonia is very popular with bumblebees, honey bees and some early emerging solitary bees.
There are certain insects which seem to prefer native species, but they will feed on other non-native flowers so the answer is certainly to have as varied a mix of flowers as possible which flower over as long a period as possible.
