Over much of the western range it is sympatric with the equally tolerant and variable Stenamma occidentale.
Stenamma species can be difficult to tell apart. The following is modified from Snelling 1973. It discusses differences between Stenamma diecki and Stenamma occidentale and also highlights some of the problems in identifying Stenamma collections to species.
Stenamma diecki: the median lobe of the clypeus possesses a fine transverse carinula below the summit of the declivity between the two lateral carinulae; the median lobe, in profile has definite dorsal and anterior surfaces, the latter clearly "undercut" below; the sides of the pronotum are sharply longitudinally rugulose, with distinctly shiny interspaces; the mesopleura, although with numerous fine punctulae, is moderately shiny and with several distinct oblique rugulae; the upper half of the side of the propodeum bears several longitudinal rugulae; and the anterior face of the metanotal groove, in profile, is vertical or nearly so, the groove flat along the length.
Stenamma occidentale: the median clypeal lobe lacks a clearly defined transverse carinula and in profile the lobe is evenly, strongly convex, without defined dorsal and anterior surfaces, the latter not sharply "undercut"; the sides of the pronotum are densely punctate, longitudinal rugulae largely effaced; the mesopleura barely shiny, densely punctate and without evident rugulae; the upper half of the propodeal sides closely punctate, dull and without defined longitudinal rugulae; the metanotal groove, in profile, either broadly V-shaped, or if long, the anterior slope oblique rather than vertical.
These differences between the two types, while perfectly obvious in many specimens, are subject to much variation and many specimens are thus difficult to correctly place. The variation of greatest significance is perhaps that of thoracic punctation. The sides of the pronotum are longitudinally rugulose in both species, the rugulae very fine and irregular in Stenamma occidentale, coarse and sharp and widely spaced in Stenamma diecki. The interspaces are, in Stenamma diecki, usually concave, smooth, shiny and without evident punctulae. Specimens do show up, however, which do have punctulae in these interspaces; the punctulae are fine, often visible only in oblique lighting, and most often separated by more than a puncture diameter. Those specimens which do have punctulate propleurae have the punctulae confined to the lower half or less and the punctulae do not obscure the decidedly shiny overall aspect of the segment. In those insects which I interpret to be Stenamma occidentale the entire propleura is closely punctulate and the punctures evenly distributed and the overall appearance is of a dull, or barely shiny, segment.
The mesopleura of Stenamma diecki is variable, but is usually more or less closely punctulate, varying from moderately to strongly shiny, dependent upon the density and depth of the punctulae. Although occasionally reduced or absent, rugulae are usually present and distinct. These rugulae may be strictly longitudinal or may extend obliquely from the upper anterior angle toward the lower posterior corner. In Stenamma occidentale the mesopleura is barely shiny, closely and deeply punctulate. Usually absent, occasionally one or two fine, indefinite longitudinal rugulae may be present.
The propodeal sides of Stenamma diecki always bear coarse longitudinal rugulae over the entire surface; these are sometimes reduced over the posterior portion of the upper half. The interspaces are punctulate, the punctulae most often fine and sparse, the surface moderately shiny. In this species, too, the basal face of the propodeum is definitely shiny, with fine, obscure, sparse punctulae. Sometimes fine transverse rugulae may be present and in a few specimens the basal face is reticulate-rugulose. Dense punctation on both lateral and basal faces is characteristic of Stenamma occidentale, the lateral face rarely with a few fine, obscure longitudinal rugulae on the sides at or below the level of the spiracular opening.
The profile of the metanotal groove is variable in both species, but in Stenamma diecki it is usually long. The posterior slope of the mesonotum, which sets off the groove anteriorly is usually vertical, or nearly so, as is the anterior slope of the basal face of the propodeum. In this species, the "floor" of the metanotal groove is, in profile as long as the groove is deep or longer. In Stenamma occidentale the groove is most commonly broadly V-shaped in profile, without a definite "floor." Some specimens have been seen in which this is not true; in these, however, the posterior slope of the mesonotum is strongly oblique, rather than vertical, and the anterior slope of the basal face of the propodeum is similarly inclined. In a few specimens the basal face of the propodeum almost entirely lacks the anterior slope and the mesonotal groove is thus very poorly delimited and is quite shallow.
Other differences exist between the workers of these species, but are too variable to be used reliably. Eyes tend to be larger in Stenamma diecki, the OMD correspondingly less; cephalic rugulae are usually coarser in Stenamma diecki; the pronotal humeri are usually more pronounced in Stenamma diecki and in this species the nodes of the petiole and postpetiole are usually shiny and occasionally lightly rugulose. These conditions are usually contrasted in Stenamma occidentale which is also, on the average, a smaller species.
