Grass-Carrying Wasp

Classification

Common name: Grass-carrying wasps

Scientific name: Isodontia species

Family: Sphecidae (thread-waisted wasps)

Order: Hymenoptera (bees, wasps, ants, and kin)

Description

Six species of grass-carrying wasps are found in North America, five of which occur occur in Pennsylvania. Adults wasps are ~0.7 inches (18 mm) long. The wings are tinted a smoky black or red-brown color, and may have blue reflections. Body color is black (Figure 1) or reddish brown (Figure 2). The abdomen is positioned on a long, thin pedicel. Long hairs occur on the thorax that can be pale, dark, or golden. Characters for distinguishing the different species can be found on BugGuide. The nests are constructed from grass packed into tight spaces.

Figure 1. Isodontia mexicana. Note the smokey black wings with bluish reflections. Photograph by dillonmr via iNaturalist, used under a CC BY-NC 4.0 license.

 
Figure 2. Isodontia auripes. Note the reddish legs and brown wings with bluish reflections. Photograph by Allison Zhang via iNaturalist, used under a CC BY-NC 4.0 license.

Natural history

The adult wasps emerge from their cocoons in early summer, mate, and the females locate a suitable nest site. Nests are built in pre-existing holes such as abandoned carpenter bee nests (Figure 3), hollowed branches, bee hotels (Figure 4), holes in soil banks and between rocks, or pitchers of American pitcher plants or in tight spaces such as window runners (Figure 6) and folded patio umbrellas. These spaces are packed and plugged with grass, which may extend beyond the nest by two inches. When nests are being constructed, female wasps can sometimes be observed carrying long blades of grass. Female wasps hunt and paralyze tree crickets, katydids, and other orthopterans to provision nests with (Figure 5). The larvae, which feed on the paralyzed crickets, are grub-like with no legs and are a yellow-cream color, while the cocoons are elongate/ovoid and yellow-brown with a papery texture (Figure 6). Isodontia mexicana typically produce two generations per year in Pennsylvania.

 
Figure 3. Grass-carrying wasp building a nest in a pre-existing hole, possible an abandoned carpenter bee nest. Photograph by Teresa Mayfield via iNaturalist, used under a CC BY 4.0 license.

Figure 4. Grass-carrying wasp building a nest in a bee hotel. Note the finished nests at the top of the photo indicated by grass sticking out of the bamboo tubes. Photograph by Sandy Thomas via iNaturalist, used under a CC BY-NC 4.0 license.

Figure 5. Grass-carrying wasp carrying a paralyzed tree cricket back to a nest. Note the medium size and non-defensive behavior as it rests on a human hand. Photograph by David Jeffrey Ringer via iNaturalist, used under a CC BY-NC 4.0 license.

Figure 6. Grass-carrying wasp nest in a window track. Note the grub-like larvae and partially devoured katydids. Photograph by  Ahn Feldman via iNaturalist, used under a CC BY-NC 4.0 license.

Management

Grass-carrying wasps are at worst a minor nuisance. Female wasps can sting if handled but are not aggressive and do not actively defend their nests. Nests that can be left should be. Nests that need to be removed, e.g., in window tracks, can be removed by hand. Pesticides are not needed or recommended. Because they only live for a season, specific wasps do not return to nest in the same area year after year. However, if an area is suitable habitat one year and nothing is changed, it will continue to be suitable habitat and may be nested in by wasps in the future.

Authored by: Steve Jacobs, April 2009, reviewed January 2014. Revised by Michael Skvarla, September 2023.