has been constructed by bees and consists of a hard shell of sand grains cemented together by a waxy substance. I haven’t been very successful finding anything in the literature about this, and I will keep a watchful eye, especially next spring.
Cemented sand grains forming a casing, 2 x 3 cm in dimension, built and maintained by bees. The bee seen here is approximately 5 mm long.
Here’s a brighter macro shot with scale taken the following day of this amazing casing. It is located on the southern side of a small boulder in the rock garden, so it will be exposed to full sun through the winter.
UPDATE: Thanks to Dr. Harlan Gough for suggesting that this sandy cell has been constructed by a bee belonging to genus Dianthidium, or pebble bees.