I usually amend my container soils with commercial compost, but this year it wasn't available, so I used my own compost from the garden - a place where insects rule supreme. I've never used any pesticides and come summer the place is buzzing with bumblebees and many many insects I don't even know what they are.
The upshot... my "pollinator excluded" greenhouse has its own bees this year.
Bees I didn't know about or even recognize, but which Carolyn had mentioned as important pollinators of tomatoes: "sweat bees" aka halictid bees. They are tiny, metallic instead of furry, and nest in soil.. yep, I must have brought them in with the compost. Besides the green one shown in the picture, I also have seen a bright blue one, and a shiny brown one, like this one resting on leaves. I haven't seen them buzzing the flowers at all (some species may be nocturnal or 'vespertine' afaik?) If you look at the side of the green bee in the picture, you can see by the yellow bulges, this one has been busy collecting pollen, so the deed has been done if not witnessed.
I guess my question is, what rate of crossed seed is expected where sweat bees are common?