The Sting of the Wild

by Justin O. Schmidt

  A legendary story relating to harvester ant longevity began in 1942 with a publication by Charles Michener, the famed bee scientist who revolutionized the modern biology of bees and continued that revolution in 2015 at age 96. Michener began his publishing career at age 16 with broad interests in insects, including ants, and only turned to bees at the ripe old age of 26. In his 1942 article, Michener described in detail the history and behavior of a California harvester ant colony in his backyard that he had been watching since age 6. He observed the colony for 16 years, at which time it succumbed to attack from Argentine ants, bad winter weather, or both. Mich, thus, documented a longevity of 16 years.10 In a famous, final small-print footnote, he mentioned “one [colony] of which, according to the report of the owner of the property on which it was situated was at least forty years old.” Several subsequent authors focused not on Michener’s factual 16-year number, but on the hearsay of a neighbor as evidence that harvester ants can live 40 years. What then is a realistic number for the longevity of a harvester ant colony, that is, of the queen harvester ant? The answer is still unclear, but our best estimate comes from detailed studies by Kathleen Keeler of Nebraska who studied 56 mounds of the western harvester ant over a 15-year period. The western harvester ant is the species suspected to live the longest of all harvester ants. For her 56 colonies, she calculated that the last colony would live to 44.9 years of age.9 That is the best answer we have: any takers for a longer-term study? Even Keeler’s study still begs the question of what causes harvester ant colonies to die of old age. Is it because the queen runs out of sperm from her original and only mating flight, which causes the demise of fire ant colonies? Does her body simply wear out? Or is it some other cause? The tools for answering those questions are available for future studies, but for now, we are left with the conclusion that harvester ant colonies live a long time.

  TO LIVE UP TO 45 YEARS, a queen harvester ant must remain amazingly safe and secure. How does she accomplish this? A surrounding mass of nasty stinging and biting workers is a start. Further, she never leaves the nest alone, and if she does leave, say, because the nest becomes uninhabitable due to flooding or shade, she leaves amid a cohort of workers as they move from the old colony location to the new preconstructed nest. These defenses pale in comparison to her best defense: simply hide deeply within her fortressed castle of a nest.

  In my graduate student days, I needed live colonies of harvester ants to compare ants from the extreme edges of their range with others within the central part of the range. The westernmost population of the Florida harvester ant lives isolated in the small town of Amite in eastern Louisiana, home of the National Football League linebacker Rusty Chambers. Two fellow graduate students joined the challenge of excavating an entire mature colony, collecting all ants, including the queen. Fortunately, the soil in Amite is nearly pure sand, perfect for digging. One person would dig a shovel load of dirt with ants and pour it on the nearby ground. The other two would collect the ants with aspirators, those indispensable tools for myrmecologists that consist of a bottle with an incoming copper tube to suck up ants and an outgoing screened tube connected to a rubber tube to the mouth. Aspirator use is an acquired skill. One sucks just hard enough to lift the ant and pull it into the bottle, but not so hard as to bring a batch of dirt along with it. For precaution, the user learns to direct the stream of air from the rubber tube onto one’s tongue. That way the dirt sticks to the tongue (rather than going to the throat or lungs) and can be conveniently spat out. (Aspirating ants is not an ideal activity in the presence of fine company.) After a few shovelfuls of dirt are removed, a cone of sand forms, allowing the ants to be exposed and readily collected as the ants and sand tumble down the sand volcano. Several hours and thousands of ants later the hole was 6 feet deep and 3 feet in diameter. Still no queen but lots of workers present. By now, a normal shovel is useless and that other essential myrmecological tool, the army shovel, becomes the tool of choice. Real army shovels, those of World War II vintage, are coveted not only for their sturdiness but also because they can be locked into a position with the shovel blade at a 90-degree angle to the handle. One person squats in the hole, digs a full load of sand, and, holding it perched like a food tray, hands the army shovel up to the aboveground people to grab by the handle. Eight feet down we found the queen with the last workers. That seems to be a safe place for the queen. I seriously doubt, even if they lived in Louisiana, that an aardvark would bother digging that deep.

  We wore shorts and a light shirt, hardly armor against stings. Nevertheless, we received only three stings, a low price to pay for a full colony of harvester ants. At least that day, the Florida harvester ants were showing their Southern hospitality.

  Armed with one success we headed to Lucky, Louisiana, in the northwestern part of the state and the easternmost location of the Comanche harvester ant. Lucky, a village with fewer than 300 inhabitants, is best known for notable resident Joslyn Pennywell, a model and contestant on America’s Next Top Model. We didn’t run into Joslyn, but we did find the ants. The digging was similar, and the queen was located at a depth just under 8 feet. We were having such a good time, nobody remembers whether we got stung, and, as a parting gesture of goodwill, we tossed a couple of abandoned tires into the bottom of the hole and filled it with sand; thereby preventing small children from falling in and also providing two fewer tire-breeding sites for mosquitoes.

  As we demonstrated the hard way, harvester ants are famous for the depth of their nests, which rank among the deepest of ant nests. Determining harvester ant nest depths is not an easy task. Most nests are not in pleasant sand but instead are in dry, hard rocky soil. Bob Lavigne at the University of Wyoming found an innovative solution to the hard soil and depth of harvester ant colonies. He brought in a backhoe to excavate 33 nests.11 The human marathon excavators Bill and Emma MacKay, as part of Bill’s Ph.D. dissertation for the University of California at Riverside, took the effort a step further. They totally excavated by hand 126 harvester ant colonies, the record having a total depth of 4 meters, with the queen located at 3.7 meters (12.1 feet).12 This may well be the record depth of a colony, although H. C. McCook in 1907 wrote of one nest “fortunately exposed by a deep cutting, the galleries and chambers were traced to a depth of fifteen feet.”13

  Extremes within biological systems provide ideal investigative opportunities because extraordinary features are the result of extraordinary adaptations and behaviors. Over the years, several explanations for the extreme depths of harvester ant colonies have been proposed: protection from freezing or broiling heat, protection from wildfires, protection from dryness, and protection from predators. Protection from freezing is unlikely for two reasons. First, many areas, including the southwestern deserts, Mexico, and Louisiana or Florida, have mild winters with freezing temperatures that penetrate into the soil no more than a few centimeters, yet colony depths reach at least 2 meters. Second, even in the coldest parts of their range, including in the 1,600-meter-high grasslands around Casper, Wyoming, the soil never freezes below 60 centimeters; again the harvester ant colonies reach a depth of more than 2 meters, much deeper than necessary to avoid freezing. Protection from the fierce summertime sun and surface heat is also implausible for the extreme colony depth. I have measured soil temperature at various depths in open sandy loam in Willcox, Arizona, for many years and have never recorded temperatures above 32°C at a depth of 30 centimeters, a temperature well below the lethal temperature of at least 40°C. Likewise, extreme colony depth as protection against fire seems unnecessary. Soil is an excellent thermal insulator and will prevent heating to a lethal temperature below a few centimeters, unless a flaming dead tree falls and burns directly on top of a nest. Even then, lethal temperatures would fall short of 2 meters. One study revealed that fire can actually benefit rough harvester ants by providing a supply of toasted dead insects to supplement the usual diet.14

  Protection from desiccation and defense against predators are the two remaini
ng explanations for the extreme colony depths of harvester ants. These roles are not mutually exclusive. Likely, both factors come into play. Clues come from other extreme desert-dwelling ants. Both the Mexican honey pot ant and Veromessor pergandei, another seed-collecting ant related to the famous biblical ants, construct deep nests; the former to at least 4 meters depth and the latter to greater than 3.4 meters. All three species share features of living in hot, dry desert areas and having very deep nests. Two of the three species, the harvester ants and the Mexican honey pot ants, also have extreme queen longevities. The life span of the third, Veromessor, is unknown, but it would be unsurprising if its life were long. Soil moisture generally increases with depth. These features support the observation that extreme depth serves the dual purposes of queen protection from predators and protection of long-lived species from the vagaries of seasonal and yearly variation in dryness.

  Harvester ants, as their name suggests, collect seeds. They are good at finding seeds. Even in parched, wind-swept deserts where the human eye sees mainly bare ground and a few shrubby plants, the grass long since eaten by hungry cows, harvester ants sally forth huge distances in a quest for seeds. Some species engineer long, broad trails that can extend 30 meters or so from the nest entrance. These ant autobahns function much the same as human highways to speed transport of goods and to reduce bumps and potholes in the path. On the outward trip from the nest, a forager can race along the smooth surface like a sports car. On the return trip, an ant burdened with a seed, sometimes several times her own weight, is like a tractor-trailer truck, lumbering along much better without obstacles in the way.

  A name sometimes clouds reality. The name “harvester ant” flashes mental images of a peaceful, dedicated vegetarian ant farmer methodically harvesting its grain. Subconsciously, we tend to accept as the norm this view of the “agricultural ant,” so named in the popular 1879 book on the red harvester ant by H. C. McCook.15 Other behaviors are viewed as odd exceptions not worthy of more than tucking into a back corner of the brain. In reality, harvester ants can be active predators as well as scavengers, capitalizing on opportunities to collect dead insects or parts. During much of the year in arid regions, few insects are available; consequently, harvester ants focus on finding seeds, and that is the activity we see. When the summer rains arrive, providing a bounty of insects, harvester ant foraging switches to aggressive predation. Foraging ants tend to abandon the beaten path of their trunk trails to forage in the intervening spaces. Insects encountered are attacked, and small insects are carried back to the nest. If the insect is huge, for example, a caterpillar hundreds of times larger than an ant, teams of ants gang up to attempt to subdue the prey and drag it back to the nest. In the summer, western harvester ants exhibit two personalities. During the day, they act mostly as traditional foragers for seeds. During the night, they turn into fierce predators, focusing more on finding insect prey. These disparate behaviors make sense: During the day, few insects are on the ground where temperatures reach 40°C to 60°C or more, and most insects present on the ground are other ants. During the night, temperatures are cooler, and many insects are now present on the ground surface.

  In the Sonoran Desert of southwestern North America, the first major summer monsoon rain brings great excitement to the parched inhabitants—both wild animal and human. Insect life explodes. Beetles fly, many ants send forth massive swarms of reproductive alate females and males, arachnid and insect predators emerge from their hidden refugia, and termites swarm. When termites swarm, all other bets in the lives of predators are off. Winged male and female termites become the focus of all species with even an inkling of predation. Some birds swoop the air for flying termites, others land to peck ground-running termites, lizards patrol the ground eating any termite that moves, spiders pounce, and ants of all varieties pour from their nests in feeding frenzies. Harvester ants appear to be insomniac as they forage continuously day and night. Why are termites so special? To harvester ants, termites are simply mobile seeds loaded with juicy fat and protein, and these “seeds” are soft and easy to eat, unlike the usual hard, dry seeds they must normally gnaw. Termites are about the same size as a large seed, are abundant, and are perfect for harvester ants adapted for seed harvesting.

  The first monsoon rain is exciting and perilous. I, like other entomologists, love this time and drop whatever else I am doing, sometimes to the displeasure of my wife, and head to the field. The presence of rattlesnakes activated in response to the rains and the resultant increased activity of rodents present one peril. The harvester ants are no less a peril. Normally, I walk in sandals unimpeded in the desert, stopping to inspect ants and colonies, rarely worrying about stings. The greatest risk is a wandering ant climbing onto my foot and inadvertently getting trapped between my sandal and the sole of my foot. My tender feet lack protective callouses—ouch. Otherwise, harvester ants frequently crawl on my bare feet and almost never attempt to sting. During the rains, this changes. Harvester ants are now omnipresent on the soil surface. They move at a faster clip than usual and readily climb on most anything, such as my foot. If they encounter something that appears to be an animal, they bite and sting. No matter how vigilant I become, inevitably I am stung. That is a price for having a love for harvester ants.

  Their habit of collecting seeds has strangely led to an alleged competition between harvester ants and humankind. From the nineteenth century into the late twentieth century, people who grazed livestock on western lands perceived a low amount of grass forage to be a result of harvester ants collecting so many seeds that too few were left for producing a good crop of grasses. Add to this the emotional response of numerous anthills “marring” the aesthetics of the landscape, and we have a prescription for war against harvester ants. Lost in the fray were data on the basic biology of harvester ants. Just what percentage of the seed crop is collected by harvester ants? Far from the predictions of woe, a surprisingly low percentage is actually harvested. Estimates range from a wildly exuberant 10 percent of the seed crop to a more accurate estimate of 2 percent, a figure based on a carefully controlled experiment involving marked seeds.16 Even using the highest estimate of seed removal, the impact of seed harvesting is minuscule compared with the damage to the grasslands caused by overgrazing and subsequent soil erosion. Opponents of harvester ants, not content with basing justification for harvester ant wars simply on seed loss, alleged serious damage from the clearing of vegetation around ant colonies, removal of young crop plants, attacking horses and livestock, stinging agricultural workers, and, worst of all, damaging farm mowers and other equipment when they hit the elevated ant mounds. Toss in damage to airplane runways and roads from undermining their surfaces and an ironclad case against harvester ants was formulated. Pesky little benefits of harvester ants, like soil aeration, nitrogen, and phosphorus enrichment of the soil around the nests, and the increased plant lushness near ant mounds can be ignored. Also forgotten were the joys of little and not-so-little children playing with horny toads, which are highly dependent on harvester ants as their main food source.

  The long-established herpetologist George Knowlton of Utah State University wrote, “[Harvester] Ants are responsible for keeping thousands of acres of western range land bare of vegetation, land which otherwise would support forage plants and livestock” and anointed the harvester ant “the most economically important ant pest in Utah.”17 Initial efforts to eliminate harvester ants commenced years before modern insecticides. These early efforts were limited to powerfully toxic, nonspecific poisons. A particularly detailed study described in 1908 showed that the best killing agent was carbon disulfide, a volatile, flammable liquid that forms a vapor two and a half times heavier than air. Carbon disulfide worked best because its density caused the fumes to penetrate deeply into colonies to kill queens and inhabitants. The authors went on to detail how hydrogen cyanide gas was useless because it, being lighter than air, would not sink into mounds to kill queens or most workers and that gasoline and keros
ene were even less effective, again, because they did not penetrate far into the mounds. After sealing the evidence for carbon disulfide, their final piece of advice was not to light it because “the explosion which followed did not … blow the vapor far down into the chambers.”18

  Not wanting to be left out, other control enthusiasts hauled out the materials of last (or was it first?) resort, various arsenic compounds, including London purple, a by-product of the garment-dyeing industry. These heavies, a step backward from carbon disulfide, failed pathetically. Soon thereafter, industrial chemists created miracle insecticides—DDT, chlordane, aldrin, dieldrin, heptachlor, toxaphene (you name it)—certain they would work. Again, the depth of the colony often prevented these insecticides from reaching the queen. Never fear, a page from the fire ant battle manual should do the trick. Enter Mirex, Kepone, and Amdro. Amdro is still used to kill harvester ants. At last, materials to win the war were available, but how many ranchers and farmers cared to use them? One wonders if all this effort was really necessary.

  HARVESTER ANTS STING. Why do they sting? To defend themselves and, particularly, to defend the queen and nestmates. Harvester ants seem to have more different kinds of predators than other ant species and a glimpse into their lifestyle reveals why. The more populous and successful harvester ants live in open bare areas where they are conspicuously visible and where other prey are scarce. Their huge colonies with numerous individuals provide a bounty to any predator able to exploit them. An added disadvantage is that harvester ants live in the same colonies in the same places year in and year out and cannot easily move the colony to a safer location. Finally, harvester ants are relatively large, providing nutritional potential worthy of the attention of both larger and smaller predators. The sting is by no means the only harvester ant defense, as it is augmented by powerful biting mandibles, a variety of behavioral defenses, and alarm/recruitment pheromones.