Kansas School Naturalist

Carpenter Ants
by John H. Klotz, Laurel D. Hansen, Byron L. Reid and Stephen A. Klotz

FEEDING HABITS

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.

Figure 7. Aphid colonies are tended by carpenter ants for honeydew.

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.

OPTIMIZING FEEDING

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.

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 observedin a hardwood forest in Indiana between two colonies ofCamponotus 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.

Figure 8. Carpenter ants on occasion go to war over disputed territory. Here two colonies form a battle line on a fallen log.

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.