Female Mason bees use their butts, as well as their head (mandibles) and two front legs to form mud balls. They carefully collect several mud balls and put them together at their nesting hole, creating walls to separate the chambers for each egg.

I took video of the bees on April 19, May 9th and 10th. This season has had a lot of rainfall and it was rare that I found any day to record bee events. In reviewing my film clips I discovered that the mason bee uses her butt to dig and position the mud. She curls her abdomen between her legs, toward her head and then uses her two front legs and mandibles to shape the mud into balls.
Field notes: Female Mason Bee Making Mud Balls
with their butt
Date: May 28, 2010
Place: Washougal, WA