In the fall of 2014, we installed “bee houses” on the campus of our sponsor, the UU Church of Palo Alto, to provide potential nesting sites for Mason Bees. Mason Bees are solitary, and do not nest in hives like the more familiar Western Honeybee, so the bee houses consist of a series of separate holes drilled into wood.
Recently it looked like some of the holes are now or had recently been occupied by insects:
Since Mason Bees use mud and soils to plug up their nesting holes, and these holes do not look like they have mud in them — the color of the plugs is not the color of any of the nearby soils — I’m not convinced that Mason Bees are nesting here. Nevertheless, some organism has definitely moved in to these holes; perhaps further observation will reveal what that organism might be.