Ok, I have seen bees foraging on catalpa flowers, many years ago. In fact here are some pictures I took, in 2005 (date stamp: 2005-06-18 10:31:24).
I remember this was in Okemos, behind the Quality Dairy Store near Grand River and Dobie.
At that time I did not notice that catalpa leaves also have extra-floral nectaries. So this year, for about two weeks now, I have been checking 6 trees and did it in the morning (9-10 am), afternoon (5-7 pm) and in the evening (8 pm yesterday). No luck seeing bees at the back of leaves, some leaves I can see nectar on the glands. In fact, no bees even on flowers.
I have a friend in London, Canada (3 hrs away!) who has shot many bees on catalpa leaves. Last night he saw even more, so I went again to a tree which was quite close to one of my apiaries. still no luck.
So, please! if you see bees foraging on the back of catalpa leaves, please let me know and I will come and take some pics, and treat you for lunch or dinner (depending on the time bees are there:). Otherwise I may have to drive to London to get these pics!