Hi and thanks for contacting Ask an Expert. Since you asked how to improve Mason Bee colonization, I am going to assume that you have some Mason Bees.
The best way I know to improve the pollinator population is to make sure you have flowers blooming from right now through the first frost or from 'frost to frost' in your yard. That's for all pollinators. Mason bees don't fly very far maybe a few hundred feet. However they are much better pollinators than honeybees.
First the flowers, lots of them, open faced like daisies are best. The pollinators can get to the pollen easily. They are especially good at pollinating fruit trees but need other flowering plants as well. Native plants are best but pollinators will go to anything in bloom usually and there are a lot of flies and other ‘bees’ in your yard that pollinate.
If you do not have a nest box or tube holder, I would suggest you get one and put it in a lightly shaded area but one that will get some sun and protected from rain. Mark the front of the insert with a design. I use a sharpie to put in X’s or a spiral or an S. Something that makes it stand out. Research has found that this helps the bee to find her nesting tube. Make sure you have a water source and mud nearby. It could be a shallow bowl with rocks in it for the bees to perch on (not deep water as the bees will drown) and either some wet dirt or an area that the bees can pick up some dirt and make mud to enclose the cocoons near the nesting house.
In order to increase your population, remove the insert around June 1st and put it in a paper bag and seal it. PAPER BAG. Put it in the garage or a place out of the sun where it will not be disturbed for the summer. In the fall, you can remove and clean the cocoons and store in the refrigerator for release right about now – make sure the nights are not freezing. Removing the inserts or tubes on June 1st prevents predators from invading the cocoons and eating the larvae.
I just put out a few cocoons in 'emergent tubes'. I refer to them as 'birthing tubes' near the Mason Bee insert. I will continue to put out cocoons as the weather warms up and my fruit trees start to blossom. I have attached a link that was written a year or so ago with a lot of details on Mason Bees. Linn County Master Gardeners have free workshops in the fall showing how to remove cocoons and clean them for storage in the refrigerator. It is simple and prevents parasites from eating or killing the larva. Usually by September they have their workshops listed. Check at www.extension.oregonstate.edu/linn
Please write back if you have further questions.