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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.

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