Mound-building ants of the Black-Ash swamps in the Macoun Club’s Nature Study Area

Ant mound DES-10 in Black Ash swamp, in 2010

When we look under rocks or logs, we almost always find ants. We have developed a special interest in the small, yellowish ones, particularly those that build large, solid mounds in swampy places. These ants are usually species in the genus “Lasius”.

In this genus, each antenna has 12 segments (counting the straight section). Viewed from the side, the back has a stepped appearance, the rear part (the propodeum) being sharply lower than the forward part. Compared with other similar ants, the surface of the propodeum is also quite short on top and much longer sloping downward. Lasius ants have one scale-like node (the petiole) between what are commonly thought of as the thorax and the abdomen (the alitrunk and the gaster). They do not sting, but have an acid pore fringed with hairs instead.

Features distinguishing Lasius from other ants

Members of this genus are pretty common in our Study Area. We have identified a dozen species there, finding some species’ colonies under stones (like Lasius flavus), others in leaf litter, and our favourite, Lasius minutus, which builds huge mounds that number in the hundreds in the biggest of our Study Area swamps, wherever ash trees occur.

• Lasius alienus
• Lasius claviger
• Lasius flavus
• Lasius latipes
• Lasius minutus
• Lasius nearcticus
• Lasius neoniger
• Lasius niger
• Lasius sitkaensis
• Lasius speculiventris
• Lasius umbratus

The mound-builder is the one that we are studying. Its name is Lasius minutus. The features that distinguish its workers from those of other species are subtle differences in the size of the eyes and the lengths of the straight part of the antennae and palps, and the number of palp segments and their relative lengths. Identification is easier if the queens can be found with the workers. They are distinctly smaller than in other species, both in absolute size (4.5 mm long) and in relation to the workers (less than 1.5 times as long as a worker).

Well established mounds are domes of very solid dark earth that rise as much as 32 inches (80 cm) and be as wide as 97 inches (245 cm)! Some of them appear to be very old, because they are covered with lichens and mosses. We have read that a queen of another Lasius species has been kept alive for at least 29 years in an entomology lab, so our mounds could well be several decades old. By digging into mounds and making estimates, each queen appears to have at least 10,000 workers.

Immature root aphids in ant mound

Like most other species in the genus, Lasius minutus is subterranean. It is so committed to living underground that it doesn’t even swarm to the surface in alarm when its mounds are disturbed. Like many ants, it looks after “herds” of aphids and feeds on their sweet honeydew secretions. Each colony protects its aphids from parasites and predators, and the workers move them about to best advantage. Lasius minutus does this all underground. Its aphids, which are in the genus Prociphilus, draw sap from plant roots. In all but one of the past 12 years we have found immatures, and immatures only, of these root aphids within the mounds.

Lasius minutus, queen ant

The big event in each colony’s year is the launch of the reproductive generation they have spent the summer raising. We have seen that in late August, these reproductive members of the colony are brought to the top of the mound and kept for a time in new galleries just under the surface. When the time and the weather are right, the workers open large holes in the mound’s top and bring out the queens and drones. These ants, whose wings shine in the picture above left, take flight and disperse. Once they have left, the workers retreat inside again, but they leave the surface openings that show what has happened.

Winged adult root aphids in ant mound

In 2015, the ants had produced some winged queens and drones by late summer, but they seem to have kept them indoors, for we were finding them just under the surface to the end of October. It seems an unlikely time of year to release them. In just two of a dozen mounds checked, we have also — for the first time in seven years — found small aggregations of winged root aphids.

In our Study Area, many hundreds of these mounds occur across a quarter-mile of mixed swampy ash and cedar forest. Focusing on about one acre (50 x 80 metres) of fairly open, purely Black Ash swamp, we tagged the mounds and then mapped them with tape measure and compass. The results are shown just below. The labels are the initials of the Macoun Club member who found each mound. One can see that sometimes they are in groups of from three to five mounds. This suggests that these clusters are related in some way — probably each being a single colony with several interconnected mounds.

Will the ants die out with the destruction of the ash swamps ?

Naming and mapping ant mounds, as shown at left, has enabled us to more easily find our way back to individual mounds so that we can monitor them from year to year. This has become very important as we track the viability of this swamp’s Lasius minutus population under the impact of invasion by an ash-tree killing beetle accidentally imported from northern China about 20 years ago.

From Michigan, where it was first recognized about 2001, the Emerald Ash Borer (Agrilus planipennis) spread to the city of Ottawa, where it was finally noticed in 2009. In 2012 we saw the first signs of it in our Study Area, and by 2014 ash trees standing among the ant mounds showed signs of infestation — woodpeckers were flaking away the outer bark to get at the beetle’s larvae. By 2018, most of the big ash trees (mainly Black Ash, but also Red Ash) had died; only saplings still survived.

As of 2019, Black Ash still had a precarious presence here — the saplings were under stress from beetle attack — but the plant community that could be characterized as a “Black Ash Swamp” has ceased to exist as a living entity.

Black Ash Swamp canopy in summer of 2015, before EAB had killed any trees

Black Ash canopy trees in early June 2018, after 4 years of attack by the Emerald Ash Borer; sapling putting out new leaves

As the big ash trees died, we saw an apparent disruption in the reproductive cycle of the ant mounds: queens were not being released at the end of the summer, or apparently, even being produced in some mounds. More and more mounds have been lapsing into inactivity, so that by 2019, about half of them looked “dead” (the ants hadn’t refreshed the earth covering of the mounds).

The ants that build the mounds, being dependent for their food on root aphids that seem to feed specifically on ash trees, may be failing through starvation. Laisus minutus ants in other places and environments have probably found other kinds of root aphids, but not here. All our aphid samples from the mounds  have been tied together by DNA fingerprinting (without matching any of the known species that have been barcoded).

Those mounds that are still active may be surviving thanks to the living roots of ash saplings, which have been slower to succumb to the ash-borer invasion. But even they are showing stress by producing epicormic shoots up and down their trunks.

Site developed by Robert E. Lee in December 2009 and July 2010, based on fieldwork he did in 2008 and 2009; and updated in February 2020. All photos by Macoun Club members and leaders, past and present. Coding revised July 20, 2013 and May 12, 2016.