The winter ant, Prenolepis imparis, appears to be a fairly common and widespread species on Martha’s Vineyard. But it’s unusual in that it is active mostly during cold weather. It’s by far the ant you’re most likely to encounter outdoors during the winter, and temperatures around 40 degrees F, if the sun is out, are warm enough for this species to be active. Workers — the form you’ll encounter in winter — are about 3 mm long. Seen from above, they show a distinctive “hourglass” shape to the thorax (the middle section of the body), and their abdomens are broad but tapering toward the tip. Legs and antennae are usually lighter in color than the body, which can be a useful field mark.
This is also one of the first ant species to reproduce in the spring. As with most ants, mating involves “nuptial flights” of winged males and queens, both of which, especially the queens at about 8-9 mm length, are larger than typical workers. (The species name “imparis” means “unequal” and refers to the dramatic difference in size among the castes of this ant.) On Martha’s Vineyard, nuptial flights of this species can occur as early as the first weeks of April. Fertilized queens produce a new generation of workers, which forage through the spring; many of these workers store large amounts of fat in distended abdomens, and this stored energy, released as glandular secretions to other members of the colony, is the main source of sustenance for the colony during the warmer months when it is sealed underground. A new generation of reproductive males and queens matures over the summer, overwintering in the colony underground until they emerge in early spring to start the life cycle over again.
Despite its tolerance of cold, Prenolepis imparis is not particular northerly in its distribution; it ranges only as far north as southern Canada. Rather, a life history centered on the colder months appears to be a adaption to avoid competition from other ants: active when few other species are, Prenolepis has unfettered access to whatever resources it can find. It’s a generalist in terms of diet, eating plant juices, flower secretions, decaying plant material, or worms and other invertebrates (mostly scavenged). Prenolepis colonies are often deep underground: researchers have found colonies extending as much as six feet down, insulating colony members from high temperatures during the summer.
If you’re curious about this odd ant, you can try attracting it by putting suitable food out on or near the ground on mild days in winter. Bits of meat or pet food might work, or you can use slices of fruit, like I did. If there is a colony nearby, odds are good that some of the workers will discover the treat you left out for them. Docile and slow-moving, these ants will be easy to observe closely or photograph if they do show up. If you find Prenolepis on the Vineyard, please let us know by commenting on this post or entering your sighting into iNaturalist.