On a walk by Starkweather Creek last week I saw a number of these little clusters of box-elder bug nymphs (Leptocoris trivittatus), varying considerably in size:
When I drew too close, they would scatter, revealing in one case a dead millipede, and in another a dead sow-bug, at the heart of the cluster. Which.. is actually a little puzzling, because every account has them feeding on plant sap, favouring that of the box-elder and other Acer species, hence the name. Coincidence? Defending a food supply from interlopers? No idea.
Historically the box-elder bug appears to have been native to the western United States and to have spread eastward; the Ohio Journal of Science for 1915 has:
The species is known to have spread eastward thru Kansas, Iowa and Illinois… the advance within the state appears to be independent of all railway lines.
In his American Insects, Vernon L. Kellogg inclines to the think that they travelled along with ornamental use of box-elders:
… with this increased supply of trees the box-elder bugs have come to be very abundant. In late autumn they gather under sidewalks or, often, in stables and houses to pass the winter, and have led many housewives to think a new and enlarged kind of bedbug had come to town.
And I suppose I do see a bit of a resemblance. The bright colour suggests they are toting some form of chemical weapon, confirmed in the EOL entry, complete with citation:
… boxelder bugs are redolent and will release a pungent and bad-tasting compound upon being disturbed to discourage predation; this allows them to form conspicuous aggregations without being preyed on.
Fortunately I don’t seem to have been sufficiently disturbing to provoke redolence. Apart from being nosy with the camera, I made an effort to step around the conspicuous aggregations.