Spring is the season for digging: gardeners do it, moles do it, even bees do it. Two weeks ago, Carrie Dunn of Seattle was working her garden when it occurred to her that little larvae she was tossing aside might be bees. She wrote:
These were a few inches below the surface of the soil. It is a well-drained, protected spot without plants nearby. I think they are about 1/2-inch long and simply white. I noticed some yellow deposits . . . so I wondered if that was pollen. The [larva] in the middle came out of that smooth indentation to the right. There is also a yellow piece on the left side of the picture that went with the white [larva] on the left. . . . I had thrown a couple out before I thought about maybe it being a burrowing bee and stopped. I love the bees that come by my yard . . .
These are definitely some kind of mining bee larvae. I was awed by Carrie’s photo because it illustrates many details of the mining bee lifestyle. Although the life cycles of the various mining bees differ from species to species, some aspects remain constant.
After she mates, the female digs a tunnel with one or more chambers branching off the main tunnel. The chambers are smeared with a secretion from her Dufour’s gland in her abdomen. This secretion protects her nest from water and mold, which is why the chamber looks smooth and shiny on the inside. She uses her pygidial plate to smear the stuff around. (I ran a photo of a pygidial plate in the recent post Andrena mining bees. It’s the part that looks like a small black triangle.)
The female then goes out and collects pollen which she forms into a ball at the base of each chamber. The pollen is held together with a little nectar and is just the right size for one larva. She then places a single egg on top of the pollen ball and then seals up the chamber. Once a chamber is complete, she starts another and keeps doing this until she dies, a total of about 4 to 6 weeks.
Once the larva hatches, it feeds on the pollen as it grows. When it reaches full size, the larva defecates and then spins a cocoon. The bee overwinters in the cocoon stage or as an adult, depending on the species.
Carrie’s photo clearly shows the smooth inner surface of the chamber, the oval shape of a typical chamber, the larvae and the partially consumed pollen balls, and the type of sandy/gravelly soils the bees like to build in. At the far right you can see a larva still in its chamber.
I’m so happy Carrie stopped digging when she realized what she found . . . that by itself is very cool indeed. And I hope you enjoy the photo; it’s one of the best I’ve seen.