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Trapnesting Wasps and Bees

Q:  I am trying to build wild populations of Osmia where I live. Last summer I put out many trap nests, holes drilled in hardwood blocks and bundles of bamboo dowels of the appropriate diameter. I did not have time to monitor them daily, but at the end of the season found that something--probably bees--had nested in them. I'm wondering what species of Osmia live in my area, and have not been able to find any publications on this. Can you locate this information for me?
- Leif 
  Burlington, VT

A:  It's great that you've been putting out trap nests.  How exciting that someone has been nesting in them! Even those of us who put up trap nests for research purposes rarely bother to monitor them daily. That would be a great deal of work, and bees tend to spend much of their time away from the nest, so you might have missed them anyway.

As for your question about Osmia, a 1962 Opus of bee taxonomy, "Bees of the Eastern United States" by T.B. Mitchell lists 6 species as definitely occurring in VT, and another 7 that are probably found in VT because they are found in adjacent states. Thus, there are potentially about 13 species in your area.  Many of them are smaller than O. lignaria and are more likely to use trap nests with smaller diameter tunnels than the recommended diameter for O. lignariaOsmia species can be difficult to identify. One needs adults, ideally of both sexes, and an expert in bee taxonomy might need to look at them to be sure of the identification.  If your trap nests are  west of the Mississippi there is no comparable opus on bee taxonomy.  However, some information on distributions is available in Krombein, K. V., P.D. Hurd, D. R. Smith, and B. D. Burks, 1979.  Catalog of Hymenoptera in America North of Mexico.  Smithsonian Institution Press. 

The active period for Osmia bees is generally spring to early summer. If you put trap nests out in early spring but don't check them until the fall, you may have other genera of solitary bees and solitary wasps that nest during the summer, after fruit tree bloom. These include the leafcutting bee genera Megachile, Hoplitis, Anthidium, and Heriades, and the yellow-faced bees, Hylaeus.  There are also solitary wasps including the genera Stenodynerus and Euodynerus which feed small caterpillars to their larvae, and other wasp species that feed their offspring aphids, spiders, crickets or other insects. The tunnel diameter, along with the plug of the nest tells you something about what is likely to be nesting in it.  Leafcutting bees in the genus Megachile plug their nests with round pieces of cut leaves. Many Osmia and Hoplitis, plug their nests with dark green chewed leaf material, or leaf material mixed with mud.  Many of the wasps, and a few Osmia, including O. lignaria, plug their nest with mud. If you are looking for O. lignaria for orchard pollination (found throughout most of the US), open the nests and see if there was pollen, rather than small caterpillars in the cells. 

Photo by Karen Strickler
Megachile rotundata nest in a split trapnest, showing cells lined with petals and leaves. 

Other species plug their nests with resins (e.g., the bee Dianthidium), twigs (Spider wasps), cottony plant material (the bee Anthidium), or glandular secretions (the yellow-faced bee Hylaeus, the wasp Diodontus which collect aphids). Plug characteristics as well as nest contents provide clues about the family and genus of bee or wasp that is nesting in a trap nest. To determine the species requires rearing the adults, and using a good microscope, a key, and comparison with specimens in a good insect museum. Often an expert should be consulted to be sure.

To learn more about the characteristics of the nests of different species that use trap nests, get a copy the original "bible" of trap nesting: K. V. Krombein, 1967, Trap-nesting wasps and bees; Life histories and nest associates. Smithsonian Press, Washington D.C.  This book is out of print, but can be obtained through interlibrary loan or from used book dealers.

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Updated Jan. 23, 2001.
Updated July 3, 2007.

Copyright 2001, Karen Strickler. All rights reserved.