If during fall cleaning you find a handful of dried grass blades tucked between your window screen and window it is a sign that you were lucky enough to host a family of grass-carrier wasps for the summer. The female grass-carrier wasp collects blades of grass and carries these grass clippings to a nest cavity she has selected. The nest looks like a loose pile of brown grass clippings stuffed into a protected opening.
In nature the grass-carrier wasp nests are located in hollow stalks or stems of plants, galleries in wood, abandoned bee galleries and in vertical clay banks or bluffs. However, in the urban environment people who observe grass-carrier wasp nests find them in the sliding tracks of windows or in the space that is left between the screen or storm window and the house window frame. It's startling to be changing the screens or storm windows, or just opening the window and finding the grass clippings, often on the upper story of the house. See photo below.
After the nest is stuffed with grass the female adds tree crickets (Oecanthus sp.), a specialized food source for her offspring that will soon occupy the nest. The tree crickets are the 1-inch long, slender, light green insects found mixed into the grass-clipping nest. The wasp lays eggs on or near the tree crickets and the larvae feed on the paralyzed prey. After only a few days the wasp larvae are fully-grown and spin papery cocoons. There are one or two generations per year, the wasps spending the winter as a prepupal larva within the cocoon and emerging the following year
Grass-carrier wasps are solitary. That means each nest is the effort of an individual female. There are no workers to help with nest construction and larval rearing. As a rule solitary wasps do not aggressively defend their nests. They are capable of stinging but will do so only if harassed or handled.
No special controls for grass-carrier wasps are necessary. Discard the nests as they are discovered (usually when cleaning windows or changing screen windows). Chemical treatment is not necessary. Prevent future nesting in the area by installing tighter fitting windows and screens or by plugging gaps and openings leading to interior cavities.
Female grass-carrier wasp carrying a cricket to her nest. Photo by Clint Kelly, Iowa State University
Grass clippings, paralyzed food provisions and grass-carrier wasp larva. Photo by Shellie Specter.
Typical collection of grass clippings assembled by grass-carrier wasp.