Creature Feature: Thatch Ants

by Tom Lisle

With wonder or horror, many of you may have noticed mounds of conifer needles and bits of dried grass teaming with ants at various places on the North Coast. These are nests of ‘thatching’ or ‘red wood’ ants, which range over mid to high latitudes of the northern hemisphere worldwide. They are native to our area and can be found from our coastal dunes to the high Trinity Alps. Thatching ants are fairly large with red heads and thoraxes and dark gasters. (Identifying the many similar species of thatch ants is best left up to ant taxonomists; barring that, it may be safer to assign casual specimens to the Formica rufa group.) Their distinguishing characteristic is the monumental size of the nests they build. Their largest nests can exceed five feet in diameter, two feet in height and extend even deeper underground. 

Thatch Ant nest at the Lanphere Dunes. Photo by David Mast

Why go to all this work to build such an edifice? Insects function best when their muscles and organs are about as warm as ours. A primary purpose of the thatch nest is to provide warm brood chambers in winter to raise workers and fertile queens and drones (males). Interior nest temperatures in all seasons commonly range from 20o to 30o C. In comparison, humans also like our nests to be around 20o C.  

How do thatch ants manage to keep their nests at optimum temperatures? First, nests are established in sunny spots with a southern exposure. The well-insulated nests passively absorb radiant heat during the day and store it under shade and after nightfall. Secondly, ants bask on the nest surface and cycle in and out of the nest to transfer their raised body heat to the interior. Thirdly, the thatch generates heat as it decomposes. Ants regulate the rate of composting by exchanging new and old material from the top of the nest to underground passages. During summer, the ants weave an open thatch to allow air to circulate. During winter, they tightly weave the thatch surface and plaster it with soil material, leaving only a few entrances.

You may have noticed swarms of flying ants of other species on warm days in early fall. The new queens in these swarms mate, drop to the ground, shed their wings, and find a place in the ground to hunker down for the winter with prospects of laying eggs and starting a new colony the following spring. Thatch ants, however, have a different strategy. New queens and drones spend the first part of winter in their toasty thatch suites, tended by their ladies-in-waiting (workers), and wait until spring to emerge full of vigor and ready to fly off to find a mate. If you visit a nest late on a sunny morning in March or April, you may witness the grand sendoff, with new queens (big red heads) and drones (small black heads) climbing to the tips of grass stems and launching themselves into the air as the workers swarm over the nest below. Once fertilized, a new queen is at the height of her powers to start a colony at the beginning of the season of plenty. Some new queens return home and serve in reserve in case the reigning queen dies, or march off with a cadre of workers to start another colony and a new nest. Both colonies will share territory and remain compatible, even after the new queen produces her own offspring. Queens typically live for about a decade, and nests can persist throughout  multiple generations of queens. With so much capital invested in building a big nest, this makes ecological sense.

Thatch Ants. Photo by David Mast

Thatch ants, like other ants, are major players in their ecosystem. As generalists targeting high-energy food sources, they prey on and scavenge other arthropods. They also tend aphids who suck nutritious fluids from plants and pass it through to ants as “honey dew.” In exchange, the ants ward off predators and may keep dairy herds of aphids in their nest during winter. 

On the other side of the coin, notable predators of thatch ants are flickers and bears. Flickers can tolerate the caustic formic acid that ants use in their defense and commonly feed on them by excavating a hole on the side of a nest and wait for the ants to swarm out for the attack. More wholesale and sometimes lethal destruction is caused by hungry bears emerging from hibernation in early spring to target nests right when they are loaded with eggs, larvae, and reproductives.

Thatch ant nests are not that hard to find. Ideal habitats for this fascinating species are coastal grasslands bordered by conifers and shrubs. An effective way to locate a nest is to find a column of thatch ants crossing a trail and trace it back to their nest, which may be as far as 100 feet away. Take the time to look deeper into the world of small creatures, and you will be rewarded with wonder and beauty.