AntsTendAphidsCosmos-blog-compress
Ants tending aphids on Jesse’s cosmos

I suppose it is human nature to assume that things are more interesting “over the fence”…the “grass is always greener” phenomenon. If we lean too hard that way, we overlook that which is beneath our feet, in our own backyards. The Forest Unseen, a 2013 book by David Haskell, provides a wonderful example. For a year, Haskell kept close watch on a square meter of the forest floor in a woodland near his Tennessee home. His observations carry the reader through a remarkable voyage into the complex biology and lives of dozens and dozens of creatures and plants and their interactions with one another, all anchored to that square meter of forest.

Many years ago, while conducting my doctoral research at the Academy of Natural Sciences in Philadelphia, I had field plots near Newtown Square, a small town some 20 miles west of Philadelphia. The plots were in old, abandoned agricultural fields. One set of plots was in a field owned by a new Baptist church. Only a hundred yards from their parking lot, I could immerse myself for hours among the goldenrods and broom sedge, and dozens of other species that had become established after the last corn crop was harvested some seven years previously. For nearly three years, I conducted experiments and made observations there several times a week, attempting to better understand the dynamics of how a forest re-establishes through successional processes. Many of my observations involved counting tiny tree seedlings coming into the old-field vegetation. For hours, I would lie on my stomach, my face inches from the ground, systematically searching for seedlings that were no more than an inch or two tall. My closest companions during this time were the insects and spiders that lived in the old field. The invertebrates provided a distraction from intense focus on vegetation, but they also were an important dimension of the old-field ecosystem I was studying.

With my nose so close to the ground, I could hardly miss watching ants, a continuation of something I began doing as a four-year-old child. Ants are everywhere, it seems. If you doubt it, try laying out a picnic at random anywhere in the world, with the possible exception of on a glacier. Ants will soon join you. It is estimated that there are over 12,000 species of ants in the world, and the combined weight of ants would exceed that of world’s human population!

My brother and I spent several days one summer collecting insects around our childhood home, carefully pinning them, as our father had shown us, in an old cigar box we obtained from our grandfather, who favored a cigar from time to time. One evening, we were called in at dark, and forgot to bring in our insect collection. It was around noon the following day that we resumed our work on the collection. Opening the box, we discovered that during the ensuing hours, ants had found our insects, dismembered them, and hauled them off to parts unknown. Perhaps it was this that aborted any thoughts we entertained about becoming entomologists. When complaining to our mother about the ants, she suggested we make an ant farm using a small empty aquarium. She briefly described how we could dig up an ant colony. With a shovel, we dug out one of the ant hills in our yard and transferred it with the surrounding soil to the terrarium. We covered the terrarium with a sheet of glass. Within three days, we could see ant tunnels within the soil that we had disturbed. We fed the ants insects and crumbs. Unfortunately, the ants built their brood-rearing rooms too far from the sides for us to see them, but they established order in the terrarium quickly and seemed content with their new home. We later disassembled the colony to explore the compartments the ants had built, and we started a new ant-farm between two plates of glass only a couple inches apart, where we could see more of the ant activity.

I had spent little time watching ants since those early childhood days, but now I could hardly miss watching the ants going about their activities in my research plots. One species of small tan ant was especially common. They lived in the soil and made small pyramids of excavated soil around the openings that led into the ground. Although I couldn’t be certain, it appears that several nearby openings connected underground to a common ant city. These ants appeared to be scavengers, and I watched as they collected dead insects or insect parts, and perhaps other organic material which were hauled laboriously to one of their entrances. Tiny sticks or stones, or a leaf of grass were huge barriers to the laboring ants that often carried objects larger than themselves. Not uncommonly, two or more ants would work together to move something, sometimes pulling in different directions, but in time, with persistence, eventually getting it to their destination. I experimented by placing crumbs or dead insects of various sizes and watching how the ants worked independently or as teams. They sometimes dismembered large objects so they could be moved, but the ants’ ability to move things so much larger than themselves seemed to border on the impossible. Ant strength is deceptive because of their diminutive size. Proportionately, ants have much greater muscle mass than we or any other mammal. I don’t know how much more muscular ants are than humans, but it has been demonstrated that ants can move objects 50 times heavier than themselves. Can you imagine moving something weighing as much as a pick-up truck?

One day, as I was lying on my stomach, my attention was drawn by unusual activity among the tiny ants that had become my research companions. There were far more ants visible than usual, and they were running here and there more or less at random, acting frantic. Curious about the cause, I watched, and within a couple minutes, a column of larger, more reddish ants appeared from beyond my research plot. Marching two or three wide, the column stretched for several feet into the thick vegetation. It moved steadily toward the opening of the colony of smaller ants. Apparently, the tiny ants had sensed the approach of the enemy and spread the word through the colony. Before the first of the larger ants reached the entrance, they were attacked by the smaller ants, which were largely ignored. The large ants seemed undeterred by having two or three smaller ants attached by their mandibles, and they continued forward. Only in few instances, when half a dozen or more small ants clamped their mandibles around the legs and antennae of a larger ant, would it react and attempt to free itself. It was stopped, however, only if the smaller ants were successful in severing its head, a risky endeavor. Using its much larger and more formidable mandibles, the large ant could sever the head or abdomen from the thorax of a smaller ant with one bite. Interestingly, that didn’t seem to lessen the grip of the smaller ant, whose head would remain attached to the large ant. In rare instances, when the smaller ants were successful, others were quick to join in an attack. As the large ant writhed in apparent agony, more small ants joined until the large ant was completed subdued, but the army of big ants marched on.

Upon reaching the entrance, the large ants disappeared one at a time into the apartment of the smaller ants. Several dozen had gone into the ground before the first of the large ants reappeared, each carrying an egg case. Others soon followed. Within a short time, the number of larger ants returning with egg cases along the path they had come equaled the number still approaching the smaller ant colony. In five minutes, the war was over. The larger ants had captured the smaller ant colony, killed most that attempted to defend the nest, and made off with their eggs. Dozens of the smaller ants milled about, as if at a loss about what to do. Perhaps their queen had been killed.

Ants have evolved a remarkable variation of adaptations. All species are social, meaning they exist in colonies with one or more queens. The workers and soldiers, which may number into the millions in a large colony, are all sterile females. Only the queen and males have wings and engage in nuptial flights for brief periods when a colony divides or becomes over-crowded. When a young queen is inseminated during her flight she may return to the nest to augment the egg-laying duties of the old queen, or go off to establish a new colony of her own. Each queen participates in only one nuptial flight and stores enough sperm to fertilize her eggs for her lifetime. She tears off her wings once her flight is over and thereafter remains dependent on her workers. Within a typical colony, there are usually various chambers, including the queen’s chamber, a nursery, a toilet or area where wastes are placed, and food-storage compartments, all with connecting corridors. Worker ants often function in only one task such as attending the queen, or eggs and immature ants in the nursery. Others are involved in food gathering and storage. Guards or soldier ants defend the nest against intruders.

The ants I witnessed attacking the smaller ants were soldiers, with enlarged mandibles, whose job is defense of  their nest or capture of other ants to employ as slaves. Ant species that prey on other ants, a fairly common adaptation, may use the captured ants to augment work in the colony, and some species are completely dependent on captured ants to carry out all tasks except egg-laying. If the slave species is removed, the colony will die. More often, the captured species merely augments the work performed by its captors. This raises the question of how ants know who is from their colony, who is a slave of the colony, and who is an alien that might be a threat.  Ants produce many kinds of chemicals that are used for recognition of those belonging to their colony and to communicate important information about location of food sources, threats to the colony, and perhaps other information important to their survival. They may also communicate by physical touch in some instances. When two ants meet, they often touch antennae or secrete a tiny drop of liquid that contains various chemical substances. Volatile pheromones are also involved in ant communication.

Most ants seem especially fond of sugar. Other than invading homes or picnics, they may obtain sweets from flowers or nectar-producing structures on plants, from nests of other insects such as bees, or they may farm aphids to collect honeydew secretions. Each spring I can count on having one and usually two species of tiny ants coming into my kitchen. I’ve never been able to trace them through the walls or behind cupboards to their nests, which may be some distance away from the house. I dislike using poisons to control them, so try to keep the work areas around my kitchen as clean as possible to avoid attracting the ants.

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Photo by Roger Zimmermann

Hundreds of species of ants tend aphids (video), insects that feed on the sap of plants. Most aphids are specific as to what plants they attack and even which tissues of the plant. Likewise, ants are usually specific as to which aphids they adopt. Some ant-tended aphids feed on roots of certain plants, but those feeding on above-ground plant structures are the ones we can see in our backyards. If you find aphids on a plant, watch for a few minutes. I’ve never failed to find ants attending aphids. To concentrate sufficient nutrients, aphids process relatively large quantities of plant sap, not unlike our processing large quantities of maple sap to obtain a concentrated maple syrup. Excess liquid and sugars, called honeydew, is secreted by the aphids. When there are no ants to collect it, it will fall as sticky film to the ground, or on your car if parked under a tree where aphids are at work. More often, ants will tend the aphids much as we might attend a herd of cows, collecting the honeydew and taking it to their colony, where it is processed for storage or used to feed the queen or young ants in the nursery. Ants carefully move their aphids as needed to maintain production, and they protect them from harm, even moving them into their nests during unfavorable weather.

When I burn my prairie in the spring, I am always startled to see the great numbers of large ant mounds that dot the landscape, relatively inconspicuous until the tall grasses are removed. These mounds are primarily, if not exclusively the work of “field ants” of the genus Formica, with over a dozen species and subspecies. Taxonomy of ants is very complex and is being continually revised. Mound-building ants are opportunistic foragers, feeding on dead or live animals they can overwhelm, as well as tending aphids and leaf-hoppers for their honeydew. Some Formica species use other ant species as slaves. They kill vegetation around their growing mound by biting stems and injecting formic acid. Dead plants are then cut and removed.  Apparently, one reason for keeping the mounds open is to allow the sun to warm them. Inside the mounds, which may be two or three feet tall and as much as five or six feet in diameter, hundreds of thousands of worker ants may be engaged. Those working in the nursery move eggs and larvae as necessary to areas with optimal temperature. A large colony may have 1000 or more queens, and sometimes two or more mounds will be connected by underground tunnels, forming super colonies. The mound results from the soil that is excavated during construction of the underground compartments. From these mounds, worker or soldier ants may roam over an acre or more in search of food to maintain the colony.

One of the more complex and fascinating associations in nature involves lycaenid butterflies and mound-building ants. The Lycaenidae is the second-largest family of butterflies, with over 5,000 species, 30% of all known butterflies. They are sometimes called “gossamer-winged” butterflies. Among them is the Karner blue butterfly, a federally endangered species whose larvae feed only on lupine plants. Wisconsin has more Karner blue habitats  supporting these butterflies than any other state. Lacaenid butterflies have evolved a complicated strategy of fooling mound-building ants into protecting and nurturing their larvae. This is done by mimicking the chemicals ants produce to communicate and guide their behavior. When butterfly larvae secrete the chemical, ants that may be tending aphids on the same plant are fooled into thinking the larvae are a queen or a juvenile ant that requires protection and food. During favorable weather, the ants guard the larvae from parasitic wasps or flies, perhaps even birds. When needed, larvae may be moved from one plant host to another with more favorable foraging opportunities. In autumn when weather is becoming unfavorable for their development, larvae or pupae are taken into the ant nest, where they are maintained through the winter. In spring, when the plants are again suitable for the larvae, they are placed back on the plants, perhaps along with aphids from which the ants will collect honeydew. Because the ants invest energy and resources to protect and nurture the lycaenid larvae, yet receive no benefit, this is a parasitic relationship, unlike the mutualistic relationship between the aphid and the ants. Many species of butterflies and ants throughout the world have evolved similar parasitic relationships.

From our perspective, we see that mound-building ants help maintain soil friability and fertility, as well as overall diversity in the grasslands and woodlands where they live. Studies have shown that soil nitrogen, for example, is measurably higher near the ant mounds. We would expect this given the extensive concentration of animal tissue in one form or another being returned to the ant nest, where it is consumed and nitrogenous wastes are released. Meanwhile, wheelbarrow-loads of soil are excavated, and plant tissue is harvested in many instances and used in the nest, which results in keeping the soil porous and enriched.

Many species of ants directly utilize plants. The acacia ant is perhaps the most publicized example of mutualism. In the tropics and subtropics, many species of the acacia trees have evolved mutualistic relationships with ants in the Pseudomyrmex genus. Acacia trees and shrubs are armed with spines or thorns. Bullhorn acacias, for example, have very large hollow spines that resemble the horns of a bull. Ants cut openings into these spines and colonies live there in relatively safe environments. If the tree or shrub is threatened by some leaf-eating insect or mammal, the ants rush out to defend their “home” which, of course, is a great benefit to the acacia. Many acacias produce detachable structures on their leaves, called Beltian bodies, which are rich in lipids, protein, and carbohydrates. Attraction of ants appears to be the sole function of these structures. In essence, the acacias are providing housing and food in exchange for protection.

Another well-known example of ant use of plants is the leaf-cutter ants of the tropics, some 47 known species in two genera, Atta and Acromyrmex. Ants harvest fresh plant tissues by using their mandibles to cut out sections of a size one ant can carry. Thousands of ants may focus their activity in one tree or one small area, with narrow columns of ants coming and going along a trail to their mound, which can become enormous. Inside the mound, plant tissues are used to farm fungi, the primary food used by the colony. Second only to humans, scientists suggest that leaf-cutter ants have the largest and most complex animal society, having a central mound that may be 100 feet in diameter, with satellite mounds covering a tenth of an acre or more, with many millions of workers and thousands of queens.

A conspicuous group of ants that I regularly encounter are carpenter ants of the Camponotus genus. Especially when I am cutting firewood in the winter, I sometimes will expose a colony of these relatively large, nearly black ants in elaborate galleys carved out of decayed wood in an old tree. Carpenter ants do not eat wood like termites, but only use the rotted wood as an opportunity to excavate space for the colony. There are some 1,000 species of carpenter ants worldwide, with similar habitat preferences. Foraging mostly at night, the ants bring protein and carbohydrate foodstuffs back to the colony. It may be dead insects, carbohydrates from aphids, or even pieces of carcasses of dead animals. Like other ants, they lay down pheromone trails to guide their fellow ants to food sources. Inside the hollowed-out tree, stump, or log, the colony will be compartmentalized much as with other ants. When a colony is exposed during very cold weather, the ants will be lethargic or even moribund. It is during these conditions that the Pileated Woodpecker focuses on carpenter ants as a primary food. The aggressive attack by Pileated Woodpeckers on trees in the winter, evidenced by piles of coarse wood chips around the base, is a sure sign that carpenter ants have established a colony in the rotted core. In my woods, it is old white pines and aspen that attract the ants and the woodpeckers that sometimes weaken the truck to the point that the top breaks off. The Northern Flicker also is attracted to carpenter ants, but feeds mostly at or very near the ground. There, the flicker will encounter other species of ants, and it appears to have no favorite when comes to one ant or another. Flickers often will forage over the ground, where they will pick up field ants as well as other insects, but they have no hesitancy in attacking wood that may house carpenter ants.

The famed biologist E. O. Wilson became fascinated by ants during his childhood. He followed his fascination to become the foremost authority on ants and, from that springboard, one of the leading scientists of our time. He has stated: “Look closely at nature. Every species is a masterpiece, exquisitely adapted the particular environment in which it has survived. Who are we to destroy or even diminish diversity?” Even casual study of ants in our backyards can demonstrate the wisdom of his words.

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Have you ever watched what ants are up to in your backyard? Do you have a backyard story to share?  Share your stories, photos, and questions in the comments section below.

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