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by John H. Klotz, Laurel D. Hansen,
Byron L. Reid and Stephen A. Klotz
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Cover Photo: A mature colony of carpenter ants has thousands of workers, all infertile sisters.
John H. Klotzis an assistant professor of entomology in the Department of Entomology, University of California at Riverside, Riverside, CA 92521. Laurel D. Hansen teaches at Spokane Falls Community College, Spokane, WA 99224 and co-authored the Kansas School Naturalist on "Collection and Maintenance of Ants" Vol. 41, No. 1. Byron L. Reid is an entomologist and lives at 5311 Oakview, Shawnee, KS 66216.
Stephen A. Klotz provided all artwork and is Chief, Section of Infectious Diseases, Kansas City Veterans Affairs Medical Center, Kansas City, MO 64218.
by John H. Klotz, Laurel D. Hansen, Byron L. Reid and Stephen A. Klotz
Carpenter ants are some of the most fascinating of all ants. Entomologists have studied many aspects of their life and discovered important relationships which carpenter ants have with other living organisms including humans. The primary role carpenter ants play in our North American forest ecosystems is only beginning to be appreciated.
WHAT IS A CARPENTER ANT?
Carpenter ants belong to the large genus of ants, Camponotus, which is distributed worldwide in tropical and temperate regions. The common name, carpenter ant, is derived from the preference of some of the more notable Camponotus species to excavate nests in wood. However, most species of Camponotus are not "carpenters" at all, preferring to nest under rocks, in the soil, or in living or dead non-woody plants.
Figure 1. Economically important species of carpenter ants and their North American geographic distribution.
The first North American ant ever to be described scientifically was the black carpenter ant. In 1773, Baron Carl DeGeer, a student of the great Swedish botanist Linnaeus, described Formica pennsylvanica, now known as Camponotus pennsylvanicus (DeGeer). This ant is common in the eastern half of the United States (Figure 1), and though large and conspicuous, it goes unnoticed because of its nocturnal lifestyle. Nocturnal living likely arose in response to competition from other ants and predation by birds. Birds, except for owls, are active in daytime.
Figure 2. Camponotus modoc starts the recycling process in a forest in Washington. This dead tree is being cored out by a colony of carpenter ants.
Although carpenter ants are common insects in woodland habitats and often go unnoticed, their activities have far-reaching effects on forest communities and humans. Their social behaviors, such as group foraging and food sharing, support large colonies which by virtue of the queen ant’s longevity, make them permanent residents of forests. Among the many species of ants, the carpenter ants stand out in forest ecosystems as the dominant insect. Their predation on other insects affects the population dynamics and distribution of the species they prey upon, and their nesting habits initiate the degradation process of tree cellulose (Figure 2) to a form usable by other plants and animals. Thus, in the ecological web of forest life, carpenter ants play a critical role in the biological control of forests insects and the recycling of organic and inorganic nutrients.
A winged ant begins the life cycle. Carpenter ant colonies are formed after reproductive adults (winged virgin queens and winged males) emerge from their nest of origin for mating flights, usually during the first warm days of spring. After mating, the male dies. Each inseminated queen selects a nesting site, often in a small cavity in a stump or long, or perhaps under bark in a standing tree. She then breaks off her wings and within a few days lays her first eggs. In two to three weeks, the eggs hatch into larvae that are fed by the queen, who mobilizes food stored in the flight muscles of her thorax and in the fat bodies of her abdomen. The queen does not leave the nest to hunt for food during this time. Instead, she remains to protect, feed, and raise the brood (Figure 3). At the end of the larval developmental period, the larvae pupate and emerge as minor workers, numbering some 10 to 25 individuals. Workers, which are all female, assume the functions of foraging, nest excavating, and brood rearing. In two years, a population of workers ranging in size from small minors (6 mm) to large majors (13 mm) will be present. The size of a worker is not genetically determined; rather it is dependent on environmental factors such as larval nutrition. Winged adults (reproductives) are produced in six to 10 year-old colonies when populations exceed 2000 workers. Mature, or parent colonies, establish satellite colonies nearby whenever there is a need for more territory, resources, or a drier, warmer nesting site for development of their larvae and pupae. The queen, workers, and small larvae are always present in the parent colony whereas the satellite colonies contain workers, larger larvae, and pupae. Except during winter diapause, workers travel between various satellites of the colony that are connected by well-defined trails (discussed later).
Figure 3. A newly mated queen carpenter ant with her first brood.
Populations of ant colonies can reach tremendous numbers. For example, over 50,000 workers have been found in colonies of Camponotus modoc (cover), a western carpenter ant. This is a relatively small social group in comparison with another carpenter ant found in the West, Camponotus vicinus, whose colonies may number over 100,000 workers. Part of the explanation for the vast difference in populations of colonies is the presence of multiple queens, a condition called polygyny, that is common in C. vicinus colonies. As many as 41 functional queens have been collected in a single C. vicinus colony. Most species of carpenter ants are monogynous (possessing only one queen) and as a consequence, the colonies are smaller and require years to reach maturity.
HOW CARPENTER ANTS FIND THEIR WAY AROUND
Existing in an area with several different colonies and avoiding aggressive encounters requires carpenter ants to be familiar with their home range. Since they are primarily nocturnal, they rely heavily on physical cues and chemical trails for orientation to and from the nest. Well-maintained physical trails and trunk lines of carpenter ants serve as roadways through vegetation and debris (Figure 4). These trails are reminiscent of the wide, cleared trails of leaf-cutting ants, common to Central and South America. In extreme northern latitudes, carpenter ant trails will often go underground following natural hollows, such as those left by decaying tree roots in the soil. These tunnels are usually 1.5 to 3.0 centimeters in diameter and may be as deep as one meter below the earth’s surface.
Figure 4. Carpenter ants maintain large trunk lines. One trunk line is shown below in a grassed area.
Chemical trails consist of hydrocarbons produced in the hindgut of the ant and deposited on the trail surface. These hydrocarbons are pheromones and are deposited by the ant when the tip of her abdomen is dragged on the substrate for short distances as she moves along the trail. Pheromones are odorous compounds produced by the ant to convey information. In the case of trail pheromones, the compounds guide ants to locations outside of the nest. Heavy deposits build up over time on heavily traveled trails forming “trunk trails” or main transportation arteries guiding foragers to resources. Resources include aphid colonies where ants collect honeydew, a favored food rich in sugars and sought by many different ant species.
Trail pheromones also recruit nestmates to newly discovered food resources. Based on the relatively large size of the carpenter ant’s olfactory lobes located in the brain, the sense of smell is clearly important. Smell serves the ants well in their nighttime activities. However, the individual forager eventually must leave the trunk trail to search for resources using both touch and sight, and to accommodate this, other orientation cues are used.
Figure 5. Carpenter ants forage at night on a oak tree using a “moon compass” for orientation.
“Structural guideline orientation” is one such important cue for foraging carpenter ants. Unlike the chemicals in odor tails, structural guidelines are tactile stimuli in the form of edges, grooves, or crestlines provided by tree bark, vines, branches, or roots on the forest floor. Carpenter ants follow elaborate detours along branches or sidewalks rather than go in straight lines. An ant’s movement is more efficient on smooth, uncluttered guidelines compared to movement along trails on the ground where turf and surface features impose numerous obstacles to the ant’s passage. The benefit of these structural detours is the shortening of overall trip time. Structural guidelines are the lowest level of cue found in investigations of carpenter ants’ orientation system. If placed in total darkness, ants are unable to negotiate shortcuts by using visual cues and resort to tactile orientation along structural guidelines. If total darkness is momentarily interrupted by an overhead view of the forest canopy or another visual cue, ants switch to another orientation method called landmark orientation. Landmarks include any visually conspicuous object such as a tree or shrub. Landmarks are memorized in detail and guide ants to and from the nest.
Figure 6. How carpenter ants orient in their environment. Left side: moon compass orientation as well as landmarks may be used. Right side: sun compass orientation and edges, such as the house or telephone line going into the house may be used. Pheromone hydrocarbons may be used as well, especially on well-traveled roadways as shown by the path.
Canopy orientation is one type of landmark orientation that carpenter ants use in temperate forests and under low light conditions of the night sky. Since carpenter ants nest within trees, the use of leaf canopy landmarks as cues may be an adaptation to increase the likelihood of ants returning to the nest tree after foraging.
Carpenter ants show a strong response to light at night. This suggests that the moon is also used by carpenter ants as a directional cue (Figure 6). Felix Santschi, a French entomologist, demonstrated sun compass orientation in desert ants in Africa in 1911. He reversed the direction of the sun using mirrors and showed clearly that desert ants do orient by means of the sun. A similar mirror experiment on moonlit nights gives similar results with carpenter ants. Foraging carpenter ants reverse their direction in response to a change in the apparent position of the moon caused by the introduction of mirrors.
Within the assemblage of orientation cues for carpenter ants, there is a built-in redundancy. Foraging ants actually rely on more than one orientation cue; for instance, a forager uses an odor trail as well as a light source to orient. As a consequence, the ants possess back-up cues with which they can orient in the absence of any one particular cue. This arrangement provides carpenter ants with the ability to forage in the woodlands in daylight and total darkness (Figure 7).
Figure 7. Aphid colonies are tended by carpenter ants for honeydew.
The redundancy in orientation cues is fortuitous as it allows foraging under most environmental conditions. To feed a colony of over 100,000 ants is a formidable task. From large, centrally located nests, foraging ants will fan out along trails leading to various destinations within the forest habitat. Carpenter ants are voracious predators of arthropods, such as flies, caterpillars, beetles, harvestmen (daddy long-legs), and spiders. Carpenter ants also collect honeydew from aphids and can often be observed tending them (Figure 7). Aphids are small plant-sucking, soft-bodied true bugs which excrete copious quantities of honeydew, which is rich in sugars. Many species of ants are attracted to aphid-infested trees, shrubs, and herbaceous plants. Husbandry of aphids by ants is usually viewed as detrimental to the host plants because the aphid population usually grows under the ants’ protection and aphids damage plant tissue. On the other hand, aphid husbandry maybe beneficial since the ants kill many plant eating insects that destroy the host plants.
The black carpenter ant has a distinct cycle of food preferences. During the spring and early summer, when brood production is high, the ants have a strong preference for proteins, which are fed to the developing larvae. For example, freshly diced mealworms are mobbed by workers from May through July but are less attractive when offered in August or September. Conversely, carpenter ants recruit slowly to simple sugar or honey baits in the spring, but any carbohydrate source is rapidly depleted from July through the end of colony activity at the time of approaching winter. Carbohydrates are used as an energy source by adults throughout the year, but the mass provisioning in the fall, before the onset of diapause, may contribute to overwintering survival.
Foraging theory suggests that animals conserve energy during foraging. One prediction of foraging theory is that, as the distance between the nest and a food source increases, foragers will become more selective in their diet. In economic terms, the ants must maximize caloric “revenue” to compensate for their increased “expenditures” incurred by foraging at greater distances from the nest. If given a choice of different concentrations of sucrose sugar on a feeding station, a colony of Camponotus pennsylvanicus will preferentially gather the higher concentration as the distance traveled to the feeding station increases. Therefore, carpenter ants follow in practice the foraging theory of maximizing energy gains through selectively feeding among different resources.
Figure 8. Carpenter ants on occasion go to war over disputed territory. Here two colonies form a battle line on a fallen log.
TERRITORIAL ANTS GO TO WAR
Carpenter ants are fiercely territorial and battle with unrelated members of their own species and with other ants. The causes of these territorial wars are unknown. We have witnessed a number of such conflicts between neighboring colonies of carpenter ants. One large-scale conflict of carpenter ants occurred in a dense pine forest in Idaho. A battle line 90 meters long was drawn between two neighboring colonies of Camponotus modoc, and fighting occurred on the ground and stumps. Part of the battle line included along 20 meters long lying on the ground upon which combatants were exposed on the crest of the log. The war was waged for two entire days and nights (Figure 8).
Another notable battle was observed in a hardwood forest in Indiana between two colonies of Camponotus pennsylvanicus. Upon cessation of combat, thousands of dead ants lay at the base of a tree in which one carpenter ant colony was nesting. This colony was attacked by migrating colony in search of a new nesting location. The previous night’s carnage was frozen in time, fallen soldiers locked in combat with their foes. Ants were disfigured, dismembered, decapitated, often disarticulated, and mandibles still gripped the legs of dead adversaries. Mortally wounded survivors could be seen moving about with abdomens severed or missing.
Ant battles are momentous and clearly exceptional behavior. Under normal conditions, carpenter ant colonies live side by side, coexisting even at high densities. For example, in a one acre plot in Indiana, six unrelated colonies with their satellites nested in 22 trees. These trees, and the nest sites and foraging resources they represented, were perfectly partitioned among these colonies. In Florida, a one acre tract of sandhill harbored 20 nests representing nine coexisting unrelated colonies and 11 satellite colonies.
To determine the relatedness and territory size of colonies, one relies on the aggressive behavior of carpenter ant workers toward non-nestmates. Aggression between non-nestmate workers ranges from mild encounters where the ants fence with their mandibles (back cover) to intense interactions where prolonged combat and death result. In order to test colony relatedness, worker ants from one tree are paired in a plastic tube with workers from a different tree (Figure 9). If here is no aggression between the ants, they are most certainly colony nestmates. If there is mild or intense aggression, the ants are non-nestmates from different unrelated colonies. These interactions between ants from different trees or nests can be used to draw accurate maps of the individual colony distributions, including satellite colonies.
Figure 9. An aggressive encounter between two carpenter ants from different colonies.
WHY ACTIVE AT NIGHT?
By working after sunset (nocturnal), carpenter ants are able to share resources with competing species of ants that work during the day (diurnal). For example, in Kansas the black carpenter ant, Camponotus pennsylvanicus, and a species of black field ant, Formica subsericea, live together in woodland habitats and forage for similar foods. During the daytime, the field ant collects aphid “honeydew” in the same trees which are used later in the evening by carpenter ants for the same purpose. Since carpenter ants are primarily nocturnal, and the field ant is diurnal, a clear division of daily rhythms occurs. Moreover, for unknown reasons, carpenter ants are able to sustain a higher traveling velocity than the field ant as temperatures drop, another adaptation to a nocturnal existence.
An additional advantage carpenter ants gain from a nocturnal existence is avoiding predators that rely on sight for spotting their prey. The few carpenter ant workers foraging during the day make large, conspicuous prey for birds such as robins, grackles, and starlings. Experimental set-ups placed in the field to study the ants must be designed to prevent birds from feeding on the exposed carpenter ants.
ECOLOGICAL VALUE OF CARPENTER ANTS
Foraging carpenter ants are a dominant force and vital link in the forest food web. The impact of carpenter ants on a forest ecosystem is immense. They play a key role by serving as the premier natural biocontrol agents of such forest defoliators as tent caterpillars and spruce budworms. However, since most species of carpenter ants are nocturnal, studies of predation are difficult to conduct and are therefore few in number.
Carpenter ants can also be considered an indicator species of the health of a forest. For example, pileated woodpeckers, Hylatomus pileatus, are rarely or never seen in forests without Camponotus modoc. This carpenter ant selects nest sites in decaying stumps and current monoculture techniques in our large forests do not allow for a variety of habitats such as decaying stumps. Thus, pileated woodpeckers are not seen in such managed forests. In the western United States, the pileated woodpecker, a crow-sized bird, will not survive the winter without access to trees, snags, or tree stumps containing colonies of the carpenter ant since carpenter ants constitute the bulk of the winter diet of these large woodpeckers (Figure 10).
|Figure 10. The pileated woodpecker depends upon the presence of carpenter ants as a protein source. These pileated woodpecker chicks await the arrival of one of their parents.|
|Figure 11. Many species of passerine or perching birds occasionally “ant,” often after new feather growth. Here is shown a bluejay (Cyanocitta cristata) engaging in passive anting with carpenter ants. Even poultry, such as the peafowl may engage in anting. In active anting, the birds place ants in their feathers with their beaks.|
On the other hand many species of birds undergo “anting,” a process where ants (including carpenter ants) are used for their excretions in the bird’s act of preening. The purpose of this behavior is unknown but some scientists speculate that it may serve an anti-ectoparasitic or antibiotic function (Figure 11).
What happens to the ants during the cold winter months? Parent colonies containing the queen, workers, winged reproductives, and larvae overwinter in a metabolic state termed diapause. In temperate regions, diapause is a period of dormancy during which the ants are in a state of “suspended animation.” The encasing wood of the colony’s residence provides the overwintering colony with insulation from cold temperatures. Carpenter ants also produce glycerol, a compound which acts as an antifreeze preventing destructive ice crystals from forming in their bodies.
In temperate regions, colonies break diapause from January to June (depending upon the latitude, elevation, and habitat), and the queen begins her first egg-laying period of the season, lasting for 7–10 days. The voracious appetites of the developing larvae trigger increased foraging activity. The most intense foraging of the season occurs when the workers are driven by increasing food requirement so the rapidly developing larvae. A second peak of activity occurs in June when the queen again lays eggs for a period of 7–10 days. The foraging activity period in the second peak is shorter and less intense, and the colony enters into diapause in September or October along with the late summer brood, which overwinters as larvae and completes development in February. Colonies are perennial and may exist for more than 20 years.
Species of Camponotus that live in forest environments and serve as important ecological components are also recognized as structural or nuisance pests in human habitations (Figure 12).
As is the case with many organisms, human activities have greatly influenced the distribution and abundance of carpenter ants. In the northern United States and the provinces of Canada, carpenter ants cause millions of dollars of damage to structures and to standing trees used for lumber. For instance, a minimum of 50,000 houses are treated each year by professional exterminators in the state of Washington for carpenter ants, and many more are treated by homeowners themselves. One example from Washington will illustrate the damage that can be done by Camponotus modoc. When an older home was being remodeled, the inner wallboards were completely removed. Most of the wall studs along a 20-foot wall were tunneled by carpenter ants. The most seriously damaged wood was so extensively tunneled that an 8-foot-long two-by-four weighed less than two pounds. This tunneling also extended into the attic joists so that the owner fell with one leg through the ceiling while he was showing the damage. A home near Grand Rapids, Minnesota, had sawdust piles to ten inches in height from carpenter ant excavations in the basement and attic.
All across suburban America, it is a common practice to build houses on forested lots without removing the trees. Unfortunately, nearly all forested lots contain one or more carpenter ant colonies, and the newly constructed house is frequently invaded by satellite colonies even before construction is completed. Thus, homeowners are not dealing with colonies that grow from the progeny of a single queen, but their houses suffer from the damage of 5,000 to 50,000 workers in satellite colonies that move into the structure. Since these houses and other man-made structures have optimal temperature and moisture conditions for the rearing of brood, they are ideal satellite nesting sites.
Notwithstanding the potential destructiveness of carpenter ants, their household invasion is sometimes a blessing in disguise, at least from the point of view of the scientist. One of the authors took advantage of a carpenter ant infestation to study ant behavior. The colony, whose nest was outside in a tree, followed a telephone wire into the house, through an electrical switch plate and along the edge of a kitchen counter to a loosely capped jar of honey. In place of the honey, a mixture of sugar in milk and diced mealworms were offered to the foraging ants. After several days of feeding on this rich resource, the ant colony migrated into the house and located its new nest site in some moist wood next to a dripping pipe underneath the sink. It was in this makeshift laboratory setting that carpenters ants reveals some of their special adaptations which make them so successful not only in their natural surroundings but in our home environments as well.
|Figure 12. Destruction by carpenter ants in a crawl space of a house located in Spokane, Washington.|
CONTRAST BETWEEN TERMITES AND CARPENTER ANTS
| Termites (above)
Wings of Equal Size
Wings of Unequal Size
Overall the benefits from the “premier forest predator” far outweigh the harm caused when humans and the carpenter ant meet on common ground. Future studies will surely uncover further secrets of carpenter ants behavior and biology as well as better management techniques. Living together with carpenter ants will continue to challenge both scientists and homeowners.
Bull, E. L., R. C. Beckwith and R. S. Holthausen. 1992. Arthropod Diet of Pileated Woodpeckers in Northeastern Oregon. Northwest Naturalist 73:42–45.
Ehrlich, P.R., D. S. Dobkin and D. Wheye. 1986. The Adaptive Significance of Anting. Auk 103: 835.
Hansen, L. D. and R. D. Akre. 1990. Biology of Carpenter Ants. Pages 274–280 in R. Vander Meer, K. Jaffe and A. Cedeno [eds.], Applied Myrmecology: A World Perspective. Westview, Boulder, CO.
Holldobler, B. and E. O. Wilson. 1990. The Ants. Belknap, Cambridge, MA.
Holldobler, B. and E. O. Wilson. 1994. Journey to the Ants. Belknap, Cambridge, MA.
Klotz, J. H. and B. L. Reid. 1992. The Use of Spatial Cues for Structural Guidelines in Tapinoma sessile andCamponotus pennsylvanicus. Journal of Insect Behavior 5: 71–82.
Klotz, J. H. and B. L. Reid. 1993. Nocturnal Orientation in the Black Carpenter Ant Camponotus pennsylvanicus (DeGeer) (Hymenoptera: Formicidae). Insectes Sociale 40: 95–106.
Back cover: Aggression between non-nestmate workers ranges from mild encounters where the ants fence with their mandibles (back cover) to intense interactions where prolonged combat and death result.
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