I run a 5 greenhouse farm, with outdoor beds as well. We have always planted many flowers inside to attract pollinators, and have been successful. This year, I would like to plant lots of flowers outside as well. If I plant flowers outside, will the pollinators feel the need to still come inside to pollinate and predate pests?
Some bee species can not pollinate every type of plant, plants can be pollinator specific, such as some fruittrees, berries, vining plants have flowers designed so only the correct pollinator insect (or bat or fly) can do the job.
On our farm we have many different pollinator species including bumble bees, honey bees, mason bees, some wasp species that pollinate (this was a new one for me) and we have hummingbird moths that pollinate.
My feeling on all these different pollinators is that it is better to encourage all the different types to hang around. We have honeybees that live in three trees plus a top bar hive we set up for when they first swarmed.
We have wood piles that harbor the bumble bees (underground tunnels) and since we keep lots of "wild" spaces, I'm fairly sure that we give as much home space types as possible.
I've noticed that the mason bees are using trees that the wood peckers have drilled out to get the grubs, so I leave these trees standing until they fall on their own and then they are positioned to help with water flow control but otherwise not touched.
I don't think trying to depend on one species is the best idea given natures preference to variety.
In my experience, food availability draws those things that feed upon it.
I don't know the specifics, but as to your hives, I know that there will probably be an optimal way to orient the openings of the hives such that the greenhouse will be an easy, and obvious, early target.
I am not entirely sure about the various other pollinators, but I am with Redhawk on this one. Encourage a diversity of pollinators and beneficial predatory insects. I wouldn't worry about those outside finding their way into your greenhouse, as long as it's not sealed to them.
You have basically concentrated a buffet of food resources, and placed it in a carefully climate-controlled environment. I wouldn't be surprised to find them sheltering in your greenhouse during the least hospitable times of day. Motivation is certainly not an issue.
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein
Mason bees will definitely make 1/3" wide holes in wood. Trees or 2x4's they're not picky. Wait till they're gone & spray with WD40. I recently heard that works perfectly but I just let them be(e) unless it will cause structural damage. I plug with mud or rocks rather than WD40. Not into chemicals.
Blue is bees favorite color. Borage is good choice of hardy flower. Edible & bees like it too.
If you plant it, the pollinators will come if they can. Diversity of species, plants and bees is key.
The only thing you have to bee careful of is if the bees enter a greenhouse, they might not be able to find their way out.
I had bees at a freind's house, on his open air porch, on the roof, he had recessed glass panels to let more light shine down. Bees would get trapped in that one foot deep space, and heading for the light so they could orient themselves, they would bounce into the glass overhead for hours until they dropped from exhaustion. And if they find their way in from a fan or vent, or other small opening, they might not make their way out.
I've found that when it's over 100 F inside the greenhouse the polinators don't stay in there for long. But the tomatoes like it that hot, so I keep it up there. If there are too many delicious flowers inside they may not get around to your vegetables, unless you have hives nearby.
Just to be on the safe side, I keep herbs inside the greenhouse, and some green herbs outside, like rosemary that may or may not be in bloom, but still look good.
I've also found that bees are attracted to colored glass, oranges, yellows, blues, reds, and ornaments bring them in, then the only flowers are the vegetable flowers. I've been meaning to try the red flash tape to keep the birds from eating the pepper flowers, because they sneak through the chicken wire. Maybe the red flash tape will attract the bees, maybe other colors, too. I've heard the most popular color is blue/purple.
Don't fall for the My-Place-Is-Special, It-Won't-Happen-Here Syndrome.