Real Estate Developers for Ants
Here comes the neighborhood
This essay is dedicated to our lifelong friend, V.C. (Cliff) Moran whose friendship we have treasured for over five decades. Cliff’s contagious enthusiasm and love of life has always been fun, and his brilliant contributions to the biological control of invasive plants in southern Africa will forever be an example of how entomology should be done. Separation by oceans has never been a barrier to our long friendship.
In most American towns, whole new suburbs appear when developers buy a tract of land and build houses on it, hoping to be followed by families moving into these houses. Purchase and occupation is contingent on whether the house meets the needs and expectations of the prospective buyer. In a general sort of way, this is also the situation for cavity-nesting insects, including bees, wasps, and ants. These insects shop around, inspecting available cavities until they find one that meets their needs, whereupon they buy and move in. Over several years, I unraveled a complex story of a cavity-nesting ant, an endangered woodpecker that ate these ants, and several wood-boring insect “developers” that created the new suburban homes, including some that made the world safe for young ant families. This tiny excerpt of forest entomology hints at the intricacies, opportunities and mutual dependencies that play out in forests.
The Found Object for this essay is a piece of bark from a longleaf pine engraved with complex, branching galleries. I saw many of these on the outer bark of pines in the Apalachicola National Forest in northern Florida, and wondered what creature had carved them, and for what purpose. The bark of pines flakes continuously, and when these galleries become visible at the surface, the logical conclusion is that they were formed deep within the pine’s bark and gradually moved outward as new bark formed. The fate of these galleries is to fall off as the outer bark is sloughed off.
What if I could shave off the bark to reveal the newly formed galleries deeper in the bark? Because the bark also had a few fairly large holes that looked like exits, it suggested a connection between these and deeper galleries. But how do you give a huge pine tree a complete shave?
Initially, I used chisels to follow the exit hole deeper into the bark and discovered that they were all connected to galleries like the one below--- elongated or branching tunnels with lateral deeper pits alongside them. As my interest in this project heated up, I developed a bark shaver, like a large vegetable peeler, fondly nicknamed “the pine-o-matic” (New! From Ronco!) With it, large millimeter-thick swaths of bark could be quickly shaved, like peeling a carrot. After shaving many longleaf pines, we found many such galleries that were most abundant low on the trees and declined in frequency up the trunk. The mystery of what made these galleries was still unsolved, but the makers clearly preferred the thicker bark at the base. Sometimes shaving an area without exit holes revealed galleries just outside the living cambium, and sometimes these galleries contained a pale caterpillar. Aha! This must be the creature that excavates these galleries, but why? Bark would seem like an entirely unsatisfactory diet, indigestible, like eating cork.
The meaning of the lateral pits became clear upon watching a caterpillar in an excised piece of bark with attached cambium. The drawing below shows how it works. The caterpillar carves out its “dwelling gallery” just outside the cambium, leaving a single layer of bark separating the two. Being a caterpillar, it must eat plant matter, but the only edible matter, the nutritious cambium, is lying just beneath the thin layer of bark. When the caterpillar gets hungry, it chews a hole through the thin protective layer and feeds on the juicy, nutritious cambium, creating a tunnel in that layer by doing so. But pines have defenses in the form of an oleoresin system, and this sticky, toxic stuff begins to ooze into the gallery, eventually curtailing the caterpillar’s dining pleasure. No matter. The caterpillar withdraws into its dry gallery, but its problems are not yet over, because the oozing oleoresin follows it, threatening to overwhelm it in its own home. But now the advantage of being a caterpillar comes into play, for caterpillars can secrete silk from their salivary glands, and with this silk, the caterpillar spins a lid over the feeding hole, stopping the oleoresin from entering its home. The lateral pits are the healed-over feeding holes.
But who is this clever creature, secretly nibbling on pines? Clearly, in the adult form, the caterpillar must be a moth, but moths lack chewing mouth parts, so how would the adult exit its larval prison? They must create the exit before they pupate, and either drop to the ground to pupate, or wait in the now-open gallery to transform into a moth. We inspected hundreds of pines and found several fresh exit holes that held a moth pupa behind a silken door. Rearing a few specimens to adulthood allowed us to identify the moth as Givira francesca, belonging to the family Cossidae, the carpenter worms, many of whose species mine in wood.
With the departure of the caterpillar/moth from its burrow in the bark, the story shifts to the ant, Crematogaster pinicola, the pine-loving acrobat ant (formerly C. ashmeadi). We in the north temperate zone tend to think of ants as mostly nesting in the ground, but in the tropics, about half the ant species nest in trees, either using preformed cavities, or building nests out of “carton” made of chewed up plant fiber. The farther from the equator, the smaller the proportion of ant species that nest in trees, and if you live in Finland, it’s not hard to understand why. In the pines here in Florida, there are only 4 or 5 species (of about 100) that nest in pines, and the star of this crowd is definitely Crematogaster pinicola whose workers are only found on living pines. Like many ants that nest in trees, C. pinicola is very desiccation resistant, a fact my student, Greg discovered when he found live and happy ants in a vial he had forgotten in his backpack for over a week. Trees are (effectively) deserts when it comes to nesting.
This ant now becomes the “homebuyer” that scopes out the “houses” built by the caterpillars of Givira francesca. After all, the caterpillar left the door open, creating an opportunity. The quality of this real estate probably depends on how long ago it was created, because as I described above, over time, the houses (galleries) move outward as new bark forms until they are sloughed off. My measurements of bark thickness, sloughing rate, and new bark formation indicated that a “house” is good for about 25 years. But there must be other criteria for choosing, for there are always many unoccupied galleries.
The ants may do some minor interior renovation and sanitation, but they are simply buyers, not builders. Because there is more real estate on the lower parts of the trunk, there are also more ants in chambers there. Occupied chambers revealed varying amounts of brood, with type and brood stage depending on season. Winged sexuals (males and females) were produced in the spring and flew on mating flights in June. Low on the trunk, chambers often contained a lot of eggs, suggesting that the queen might be in the vicinity. And sure enough, I found queens mostly in ground level (or even below ground) chambers, surrounded by piles and piles of eggs. This brings a remarkable fact into focus--- brood can be found in chambers all the way up the pine tree as high as 50 to 70 feet, but they were all laid by the queen at the base of the tree. I reasoned that workers must transport eggs and brood on the tree’ surface, but even though I shaved rings around trees so that ant trails would have to cross this barren zone in full view, I never saw any.
Doing research on arboreal ants has its challenges, and “Swedish ladders” that could be safely attached to the tree gave us access to the whole pine (see image below). The ant foragers readily recruit in large numbers to filter paper bits dipped in a slurry of Kozy Kitten cat food, so we stuck baits every couple of meters all the way into the crowns and established that ants occupied the entire tree. But was it a single colony, or were there several? Knowing that these ants instantly attack any ant from another colony, we simply moved bait covered with ants up to the next higher occupied bait and allowed them to mix. If they fought, they were from different colonies, if they did not, they were from the same colony. Surveying several trees was pretty dramatic (see below), but we showed conclusively that each tree was occupied by a single colony.
How big were colonies? To answer this question, I captured queens and counted how many eggs they laid in a day (average about 250). Assuming the colony was not growing, these eggs represented worker replacement (average about 250 per day). By assuming that workers live for about half a year, I calculated that mature colonies had between 30,000 and 80,000 workers spread up the entire tree.
In queenless colonies in the lab, workers’ ovaries developed, and they began to lay eggs (being unfertilized, these could only develop into males), yet on their home tree, no workers had developed ovaries, no matter how high on the tree, and therefore no matter how far from the queen they resided. Another ant mystery comes into focus--- how does the queen exert her inhibitory influence from her royal residence at the base of the tree up to the very top? And how does she do this when I never detected any substantial traffic moving up and down the tree? I even checked in the middle of several nights!
How new colonies get started is less of a mystery. After mating in June, the newly mated winged queens seek out young longleaf pines whose lowest dead branches have been mined by bark beetles. Their vacated galleries are available as shelter. A few galleries that hosted such queens are in the image below. If more than one queen succeeds in colonizing the little pine, these colonies must fight until only one survives, because each pine hosts only one colony. Most young longleaf pines contain a colony of Crematogaster pinicola before they are 10 to 15 feet tall.
Pines can live for centuries, but how long do colonies live? Again, we bait-surveyed over 1000 trees three times annually for 6 years. A gap in ants coming to baits indicated that the resident colony had died, and the tree had not yet been recolonized. The frequency of such gaps suggested a life span of about 15 years. Clearly this is far less than the life of the host pine. I don’t know how trees get recolonized, but it must surely be by way of newly mated queens taking over the vacated suburb.
The final player in this caterpillar-ant-pine drama is the endangered red-cockaded woodpecker, a bird that makes nest cavities in living longleaf pines. By flushing the stomachs of these birds, Chuck Hess showed that most of their diet consisted of the ant, Crematogaster pinicola. So, even this small excerpt of the forest ecosystem is composed of several overlapping dependencies among unrelated animals. But are they dependencies or opportunities? It is hard to tell, really. Surely none of the players “intend” to benefit the others. The woodpecker depends on ants for food, but is this a true dependency on this specific ant, or is it mostly an opportunity because C. pinicola is so abundant in pines? The abundance of this ant is itself dependent on several species of wood-boring insects--- a caterpillar that creates the domestic real estate market, and bark-mining beetles that make the “starter homes.” Is this too a dependence or an opportunity? Would the ant be this abundant without the other insects? Maybe not. The woodpecker’s livelihood is therefore the indirect result of the abundance of several insects that create cavities that in turn allow these ants to carry out their lives. The caterpillar and the bark beetles probably get no benefit from the ants, although it is possible that the ants keep away predators and parasites. The well-being of the ants and woodpeckers in this drama depend indirectly on the actions and lives of players they have little direct link to or effect on, but together they nevertheless create a functioning ecosystem. Having revealed this much complexity, how could I doubt that there is more to be discovered?
Loved seeing how this story has developed since I found Crematogaster in my forgotten vials, undesiccated.