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Saturday, February 28, 2009

Bird-Nest Wasps

Jeffrey Hahn, University of Minnesota Asst. Extension Entomologist


Photo 1: Birds-nest wasp. Jeff Hahn

Several people have complained of insects damaging their windows and have either described or sent me samples of grass that they found in it.  When asked when they first encountered this, I was told that this was not new from the winter but had occurred last summer or fall.  When the samples were carefully examined, tree crickets were found amongst the grass.  People never noticed any other insects.

However, the tree crickets are not responsible for the grass.  The actually culprit is a sphecid wasp known as Isodontia.  This insect is called bird-nest wasp or grass carrying wasp.  It belongs to a group of sphecid wasps known as thread-waisted wasps.  They have a very thin stalk or waist (actually part of the abdomen) connecting the thorax with the abdomen.  The most common species in Minnesota is Isodontia apicalis.  It measures ½ - 3/4 inch long, is dull black in color, and is active from July to September.

A bird-nest wasp uses a preexisting cavity to construct its nest.  Worldwide, they have been known to use hollow plant stems, rolled leaves, abandoned bee nests, and even crevices between stones.  In Minnesota, home residents occasionally find them inside window cases.  The bird-nest wasp flies to her nest site, carrying grass, sometimes several inches long.  This grass is used to form cells in her nest.  She provisions her nest with insects to feed her young.  Isodontia prefers katydids or crickets - a particular species will prefer a specific insect as its prey.


Photo 2: Grass and tree crickets. Jeff Hahn

Although the activity of a bird-nest wasp can be a little alarming or disconcerting, they are not actually damaging anything.  Just remove the grass and insects you find.  It is unlikely you will have an ongoing problems with this wasp.  If you saw it one year, you probably will not see it the next.  This is fortunate as it would be very difficult to exclude them or otherwise control them.


  1. I live in a suburb of Minneapolis in an area surrounded by wetlands and crop fields. I have found grass between the window and screen of a southeast crank-out window at the bottom of the window earlier this spring and again this summer. The first time I saw the grass, I also found grass on top of the actual window frame when the window was open. I cleaned the grass off and thought no more of it. Then today I found grass between the window and screen again. Upon further investigation, I also found "nests" with what looked like egg pods lining the inside the upper window frame/track of the window. I assumed that the grass nest I found was a grasshopper nest, because when I cleaned the grass out, small dead grasshoppers(?) came out with it. Can I assume that this nest belongs to a bird-nest wasp as you described in the article? I too saw no other insects other than the "grasshoppers".

  2. I just encountered these wasps in my backyard this summer. She was carrying a long piece of grass into the support bracket on my patio chair. Glad they are not aggressive and that they do some good insect control, however I think I will be plugging up those holes. They are pretty neat to watch but I am not sure how she would ever get a cricket or grasshopper in those tiny holes!

  3. I have these in about 10 of my windows around my house, and they have been there all summer. I went around and thoroughly removed each nest and cleaned. They are already back in multiple windows, one week later. So they will definitely come back to the same windows. I am finding it very difficult to get rid of them.


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