Mason Bees Nesting in Roof, and Embryos Falling Out

Asked July 8, 2020, 12:24 AM EDT

Hi, I've had bees that I'm pretty sure are mason bees building nests in the overhang of my roof for years. I thought this was okay because I know they are beneficial but I have a couple of concerns: 1. I just learned that they can't make these holes themselves, so I'm wondering what is making these holes. And is the creature that's making these holes damaging the structure of my house? My house is all wood with a metal roof. Built in 1966 in an area at the interface of pines and oaks. 2. Yesterday and again today, I found what I believe to be baby bees that aren't fully hatched on my deck, under one of the holes where the bees go in. Is something kicking them out? Or what's happening to interfere with their normal development? We don't use any pesticides. I'm attaching two photos of the bees that I found on my deck. Thanks for your help. Pauline Black, Ashland

Jackson County Oregon

1 Response

Hi Pauline, I love how you've been supporting these native bees over the years. Yes, those are pupae of some species of bee. I am unable to identify them from those photos. In order for you to determine what is going on, you will need to inspect the area. There are many possibilities for what is causing the pupa to fall out, but you need more information.

The identity of the bee is necessary to understanding what is making the holes. It is possible that the bees are not mason bees but rather carpenter bees, but we cannot tell without a positive identification. If you could take close up clear photos of the top, sides, and bottom of the bees perhaps the OSU Insect ID clinic would be able to identify it for you. Maybe woodpeckers made the holes. Maybe it is simply imperfections and cracks in to wood. The possibility list is almost endless. Again, you need to make a closer inspection. Native bees typically are active in the spring and early summer when they make and provision their nests, lay their eggs, then die. Only the pupae live in their cocoons over winter.

Here is more information on native bees; I’m including links to two publications, from OSU Extension on Nurturing Mason Bees in your Backyard in Western Oregon. And from the Oregon Department of Agriculture is the publication, “Common Bee Pollinators of Oregon Crops”.