The grass-carrying wasp: A solitary wasp that builds nests in unusual places

Grass-carrying wasps build their distinctive nests in the tracks of storm windows as well as abandoned insect galleries and hollow plant stems.

Photo: Grass-carrying wasp (<i>Isodontia exonata</i>) with a prey item to provision a nest. Photo by Johnny N. Dell, Bugwood.org.

Photo: Grass-carrying wasp (Isodontia exonata) with a prey item to provision a nest. Photo by Johnny N. Dell, Bugwood.org.

As a Michigan State University Extension educator who accepts homeowner samples of insects, plants and pest damage for identification, I have the opportunity to learn about many interesting and unusual creatures. One such opportunity arrived with a lady who was preparing to install her storm windows for winter. She came across a clump of dried grass with a number of cocoons in it lodged in the track of one of the windows. Curious about what it was, and a little concerned that it might be something harmful, she brought it to my office.

After examining the clump of dried grass and stray insect parts, I opened one of the cocoons. What looked like a wasp was developing inside. Intrigued, I did some digging to find more information. It turns out the insects in the cocoons were grass-carrying wasps.

Grass-carrying wasps prefer to lay their eggs in nests made above ground, unlike some other solitary wasps. In nature, the females of these solitary wasps use hollow plant stems or abandoned galleries of other species to build their own nests. Apparently, in urban environments the tracks of storm window frames are a convenient place to use, too.

Females carry blades of grass to the chosen brood cavity where they lay their eggs in cells. The brood cells are prepared with a lining of grass and provisions of tree crickets for the larvae to feed on. Once the larvae have completed their development, they create papery cocoons and transform into pupae where they wait out the winter before emerging as adults in spring.

Grass wasp cocoons

Dried grass and cocoons from a grass-carrying wasp nest found in a window track. Photo by Diane Brown, MSU Extension.

Grass-carrying wasps are native to North America. They belong to the family of thread-waisted wasps (Sphecidae) and are in the genus Isodontia. The adults are about 0.75 inch long and shiny black. They don’t sting anything other than their prey (tree crickets) and are not aggressive unless you try to handle them. Most of their adult life is spent pollinating flowers and sipping nectar, except when the females are busy building and provisioning nests. Native plants they are attracted to include goldenrods, rattlesnake master, mountain mint and boneset, among others.

As the interest in preserving pollinators continues to grow, it is helpful to find out more about the variety of native wasps and bees that can be found in our own backyards. You may encounter a grass-carrying wasp in a bee hotel if you have created one with a variety of different-sized tubes. In tubes occupied by grass wasps, you will likely find blades of grass sticking out of the tube. A site dedicated to rearing native bees and wasps mentioned that grass-carrying wasps were found nesting in holes that were 8 millimeters (0.3125 inch) in diameter.

A detailed article about grass-carrying wasps written by Heather Holm, author of the books “Bees: An Identification and Native Plant Forage Guide” and “Pollinators of Native Plants,” is posted here: “Meet the Grass-Carrying Wasp, a Gentle Pollinator of Summer Flowers.” The article contains excellent photos and a list of the nectar plants preferred by grass-carrying wasps.

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