Tree Bumblebee nest

Unusual yellow structure in bird box These alien looking stuctures were found in several of the bird boxes this spring. It's not the best of pictures but if you look closely you should be able to see yellow, organic structures in the center.

This is the nest of the Tree Bumblebee. You can just make out a few bees in the picture too. The Tree Bumblebee makes its home in structures such as nest boxes. The bees will leave the box by the end of the summer.