Photo by cottonmanifesto. Location: Olmsted Park, Boston.
Urban species #241: Great golden digger wasp Sphex ichneumoneus
Though its appearance may be fearsome to some people, the great golden digger wasp is not aggressive, and in fact, feeds only on flower nectar. Their young, however, deep in a vertical burrow in the soil, eat the paralyzed but still-living bodies of the animals provided by their otherwise peaceful herbivorous mother. This wasp is very particular about what food to provide for its larvae: it only chooses long-horned grasshoppers (Family tettigoniidae, which includes katydids). It seems hard to believe that there are enough katydids and their relatives to support this common and widespread wasp, but this group of grasshoppers is characterized by being extremely well-camouflaged. The adults feed on a variety of different flowers, though we always seem to find them on the blossoms of sweet pepperbush (Clethra alnifolia), a native shrub used in many city park plantings. Great golden digger wasps are found in most places in North America and in much of South America as well. As long as a given city has soil to dig in, flowers to feed on, and long-horned grasshoppers to provide for its larvae, it is likely also to have this species.
Many thanks to phlogiston for identifying this species. (If you would like to post your picture in the comments, that would be great!)