What’s that dried grass doing in my window track?

By Mary Free, Extension Master Gardener

Dried grass and cocoons removed from a window track in Arlington, Virginia. Note food remnants in open cocoon. Photo © 2020 Mary Free

Now that temperatures have warmed, we eagerly work outdoors in our gardens, while indoors, with perhaps less enthusiasm, we tackle our home’s spring-cleaning projects – like window washing. As you slide the screens or storm windows up or down the tracks, you may be surprised to see hay-like pieces fall on the sill. Inside a track, you likely will discover dried grass packed within the cavity along with cocoons.

Grass-carrying wasp, Isodontia mexicana, feeding on Pycnanthemum muticum. Photo © 2020 Mary Free

Uh-oh, you might wonder. What’s inhabiting those cocoons, and should I be worried? The answer is Isodontia, the solitary, grass-carrying thread-waisted wasps. Don’t worry about being stung, though. Unless you mishandle them, the female wasps reserve their stingers for the insect prey (Orthoptera) that will serve as food for their offspring.

In its natural habitat, each female Isodontia builds her own nest in cavities like hollow stalks, dead wood crevices, and abandoned insect nests. In the urban and suburban environment, however, these wasps have an affinity for window tracks.

On Dec. 21, 2018 and May 21, 2019 a raised window screen revealed grass-carrying wasp cocoons in a window track. In May, an exit hole marks the empty cocoon; in the occupied cocoon, the dark shape and antennae of a soon-to-emerge wasp are visible. Photo © 2020 Mary Free

A female wasp builds her nest in the summer, carrying and cramming pieces of grass, hay, and other plant fibers into a cavity, like a window track. She then hunts for small tree crickets or katydids, which she will sting – paralyzing but not killing them – and carry back to her nest. She lays her eggs near these insects and her larvae will feed upon them. When mature, each larva creates a cocoon in which to overwinter. It will emerge from the cocoon as an adult wasp in spring.

On May 21, all the cocoons were removed from the window track above. The one occupied cocoon was placed in a fold-top plastic bag to see if anything would yet emerge. On May 28, Isodontia mexicana was discovered and freed. Photo © 2020 Mary Free

Since these beneficial insects spend their adult lives feeding on nectar and pollinating plants – when females are not busy building or provisioning their nests – you might wait until all the wasps have emerged before cleaning out your window track. (If the cocoon has a hole in it, the adult wasp has flown away. If the cocoon is intact, the wasp is still inside.) The nest and cocoons are easily removed and can be thrown away; pesticides are not needed!

Isodontia favor white flowers like mountain-mint (pictured), hyssop-leaf thoroughwort (Eupatorium hyssopifolium), and boneset (E. perfoliatum) as well as yellow-flowered goldenrod (Solidago). Video © 2020 Mary Free

Meanwhile, you get satisfaction knowing that you aided some pollinators by hosting them at your home for the winter, and that’s a nice way to start off the growing season. And, if they found your home hospitable, you may be privileged to host a grass-carrying wasp family again this year.

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