On the Crematogaster of the DMV area
The five Crematogaster present in the DMV area are all in the Crematogaster (Crematogaster) subgenus. In the field, all five species appear quite similar.
The following list is of our species in no particular order:
Crematogaster pilosa Emery
Crematogaster lineolata Say
Crematogaster ashmeadi Mayr
Crematogaster vermiculata Emery
Crematogaster cerasi Emery
Of these species, two pairs fulfill the same niche; Crematogaster lineolata and Crematogaster cerasi both nest rather generally (under rocks, in logs, etc) and Crematogaster ashmeadi and Crematogaster vermiculata are both primarily arboreal.
From most common to least common, in my experience:
Crematogaster lineolata, from my observation, seems to be the most conspicuous species throughout our area. The colonies, often containing several egg-laying queens, can be found in open areas to the forest edge under rocks and in wood, both decaying and live. It appears to be sparse to absent from the mixed mesic hardwood forest common in our region. The workers have many scattered short, erect setae on the dorsal of the alitrunk. Flights occur in late August to early September and the queens are nearly indistinguishable from those of C. cerasi. I have had males come to my blacklight.
Crematogaster ashmeadi seems to be the second most common. Often collected from the trunks of many different species of trees, colonies are primarily arboreal, resulting in less collections and observations than C. lineolata. It is present throughout most habitats in the DMV. The species may be separated from its congeners by the short, downward-curved propodeal spines and the smooth pronotum. Flights occur on warm days in late April and is the only species of Crematogaster here which flies this early (except unk. vermiculata). I have collected the much smaller, in comparison to the other Crematogaster, queens of C. ashmeadi along with flights of Camponotus pennsylvanicus in the afternoon after short rain.
Crematogaster pilosa is most commonly found in wetlands; indeed, this widespread habitat is the only one in which I have collected the species (Occoquan Bay, along Rock Creek, etc). However, in the areas in which the species is present, it is extremely abundant. Colonies nest in a variety of ways; I have found large colonies under rocks, in fallen logs, and in wooden posts. It is likely one of the most distinctive species of Crematogaster in the DMV; the workers are heavily pilose on the dorsal surface of the head and mesosoma. In photos taken by casual observers on iNaturalist where the hairs can be difficult to see, it can be differentiated from its congeners by the several anterior-curving setae on the occipital margins of the head. Flights occur in mid-summer and is the only Crematogaster species here which flies at this time (except unk. vermiculata).
Crematogaster cerasi is a species that I have been unable to collect in our area (although this does not mean it does not exist here!). It is also certainly the largest species of Crematogaster in our area. For years, those in the ant-keeping community have assumed all Crematogaster collected in the Eastern and Midwestern US to likely be this species; an incorrect assumption. Thus, the identification of C. cerasi was placed on many Crematogaster in our area, even with subpar photos. I have disagreed with the majority of these but there are still many false observations throughout the US, which should be brought back to subgenus Crematogaster if the photo does not clearly display any characteristic features . Like C. lineolata, C. cerasi can be found under rocks and in decomposing wood, and foragers can be found on all habitats. When I collected this species in Richmond, it was distinctly larger than C. lineolata and the two long setae on each pronotal shoulder were diagnostic (however, smaller specimens will have short, erect setae, with similar length to those of C. lineolata). The species has flights in August and early September.
Crematogaster vermiculata is another species that I have been unable to collect in our area, although I have found this species several times in Mississippi and southern Virginia. It is found in the palustrine and possibly estuarial wetlands, or swampy areas the cypress is abundant in, often found on those trees. It has been theorized to be arboreal and nesting in cypress, although I have collected workers on other species of plants. In the DMV, I assume this species may be found along the Chesapeake Bay. Workers are easily differentiated from its congeners by the namesake vermiculate mesosoma. I am not familiar with the flight time of this species although flights likely occur in the first half of the season.
On the distinction of these species on iNaturalist; These species are all very distinct when collecting in the field. In fact, many of the similar species that typically require microscopy to distinguish from each other can be identified just by their general habitus when seeing live workers in person. However, identifying to species from photos on iNaturalist is more difficult. For Crematogaster, C. ashmeadi and C. pilosa can be identified to species from decent photos of the spines or the head pilosity, respectively. C. lineolata and C. cerasi are more difficult (as mentioned above). Some things to note in the field are the size (The max size of C. cerasi is very large, although worker size is variable; I have also collected cerasi workers that were smaller than lineolata), mesosomal opacity (cerasi appearing smoother, lineolata rougher), and bicoloration (some cerasi may be bicolored, potentially being confused with C. pilosa, although both species may be concolorous). The best way to differentiate these two species on iNaturalist is to include photos of the setation of the dorsum of the mesosoma from a side profile.
A short note with photos from AntWeb inserted of these species: https://docs.google.com/document/d/12QYCEDvLBwThHf8u6ufhcrtT6Sthxg6C8krnERiWG_Q/edit?usp=sharing
This post merely serves as a short synopsis for the Crematogaster of our region. For more information on each specific species, I recommend reading the species page on AntWiki, and, for those more dedicated to learning these species, to read papers regarding each species on AntCat.
