Observations of Male Mason Bee (Osmia) Aggregations

 Mason Bee males in an aggregation in a wooden post in Sweet Home, OR 6 April 2017. Photo Credit Richard Little.

Mason Bee males in an aggregation in a wooden post in Sweet Home, OR 6 April 2017. Photo Credit Richard Little.

by Rich Little

During the first week of April 2017 I checked my Mason Bee nesting condos every evening to determine the number of females that had returned to the nesting blocks to spend the night. I was trying to determine how many females were surviving during this prolonged cool & wet spring. I also checked the condos during daylight to determine the daytime activity level of the bees. During this time in early April on rainy/overcast days I observed very little flight activity.

On those days with little or no flight activity I saw about two dozen females in ‘their’ tubes. Most were facing outward while in their hole. I observed very few males during this time. On those days with some sunshine and the females were active, I observed some males on or near the nesting blocks. I was wondering how the males were surviving and where they were spending the evenings. I did some searching around my yard for these males. After several days and evenings of searching I observed a wooden post and notice a large empty bolt hole in which there were about 10 male Mason Bees that had formed an aggregation within the hole.

Several times over different days I observed this male aggregation. It may be that the males had found a sheltered location close enough to the bee condos where they may have been able to determine when the females were becoming active. While this wooden post is no longer in the yard, I will place in future seasons some artificial holes of various sizes and location around my yard and see what happens.