Here is a little trick I use. You can adjust this to your own yard. Just keep a little log of what opens when and when you spot your first mason bees; they will be males in nature. Example: When I see the buds on my plum tree just crack and show a little white on close inspection it is time for my release box to go up. This is the first tree I found males on 3 years in a row. By putting my release box up then there is plenty of time for the bees to emerge naturally.