Wild Side: Winter ants

Prenolepis imparis is virtually the only ant you would find out and about this time of year.


In a recent column about feeding wild birds (bit.ly/30c79CX,) I pointed out that food you put out for birds often ends up feeding other types of wildlife. At the time, I was thinking mainly of skunks, rats, and deer, all of which, for ecological or public health reasons, are species Vineyarders should not be encouraging.

But unexpected consequences are exactly that, unexpected, and some such consequences are more welcome than others. The point was driven home to me during a few mild days at the end of February, when I tried a modest experiment in my very limited bird feeding regime.

Finding a mushy specimen in a bowl of tangerines on the dining room table, I wondered if any birds would like a little citrus treat. Red-bellied woodpeckers, which occur in my neighborhood, sometimes come into fruit. And Baltimore orioles, primarily a summer species here but increasingly regular lingering into winter, are notorious for liking oranges. Even my blue jays, I figured, might go for a little tangerine.

As it happened, no birds at all expressed interest in the fruit quarters I left on our deck railing. A few blowflies showed up, which didn’t surprise me since these often overwinter as adults and are routinely active in mild winter weather. But in keeping an eye on the fruit wedges, I was delighted to find them heavily visited by tiny ants, each about an eighth of an inch long. Ants in February! This made me happy.

A set of photos I took allowed a firm identification, which in retrospect was no surprise: Prenolepis imparis, sometimes called the false honey ant or, more appropriately, the winter ant. Knowledgeable friends confirmed my ID and assured me that Prenolepis was the only ant I should expect out and about in near-freezing winter weather.

The winter ant is unique among ants in our region for preferring cool, even cold weather to warm. Research has shown that Prenolepis workers happily go about their business in temperatures down into the mid-30s. And at temperatures above about 60, they cease to be active and begin to show physiological changes indicating heat stress. Like the winter moth and the winter crane fly, Prenolepis has evolved a complete commitment to being most active in winter.

One trait that follows from this ant’s dislike of warmth is a tendency to dig very deep nests. Prenolepis nests in Florida have been documented extending almost 20 feet beneath the surface, a depth where even amid subtropical heat, temperatures remain cool and even. In our region, such extravagant excavation is not needed. But our local Prenolepis presumably dig their colonies deep enough to get below the frost line and below the depth to which summer heat can penetrate.

Prenolepis reproduction also takes place in cool weather. Winged males and queens emerge en masse in early spring (I have found mating swarms in mid-April). After mating, the queens return underground, producing a new generation first of workers, and then of males and queens in late summer (these remain in the nest until the following spring). Once temperatures rise above the comfort zone, the nest is sealed, and the entire colony remains underground until things cool down in fall.

Prenolepis workers are versatile foragers, eating or bringing back to their colony anything from fruit (clearly!) to flower secretions to seeds to dead worms and other invertebrates. Some workers store huge amounts of energy in the form of fat in hugely distended abdomens; released to other colony members in the form of glandular secretions, these fats are the main source of sustenance during the summertime period when the colony is sealed.

Surprisingly, cold tolerance does not translate into a northerly distribution in this species. The range of Prenolepis extends north only into southernmost Canada. Rather, evolving to be active in winter appears to be a strategy for avoiding competition. Few other ants are active in winter; there may not be much to eat then, but what you can find is all yours if you’re able to be active in the cold.

This species can also secrete chemicals that are toxic to other ants, making it an ant that should prove highly resistant to competition even from aggressive ant species. With its resistance to competition and its ability to dig below the effects of hot weather, Prenolepis appears well equipped for enduring climate change and the spread of invasive species.

Like many ants, Prenolepis plays a useful role in recycling nutrients from plants or invertebrates that have finished their lives. Winter ants, by virtue of their fondness for foraging on seeds, play an important role in helping the dispersal of plants, dropping a certain percentage of the seeds they find and begin to carry back to their colony. So this is not just an amazing species in terms of its odd biology — it’s a useful one as well. Getting to know it was well worth the effort of putting a few tangerine wedges on the deck.