If you are a pest management professional practicing your craft anywhere in the South from coast to coast, you have surely been tested by one or more species of tramp ants. Argentine ants, crazy ants, pharaoh ants, ghost ants and white-footed ants are persistent, difficult to control pests, but why? What makes a species a “tramp ant,” and which characteristics do they have in common that makes them so pestilent?
The ant species characterized as “tramps” include those shown in Table 1. These ants are significant pests of structures, and they occur in many parts of the world. Other tramp ant species attack crops. Their common characteristics will briefly be discussed here. Not all tramp species will have all of these characteristics, but each exhibits a majority of them.
COMMON TRAMP ANT SPECIES
Argentine ant, Linepithema humile
Crazy ant,Paratrechina longicornis
Little fire ant, Wasmannia auropunctata
Ghost ant,Tapinoma melanocephalum
Odorous house ant,Tapinoma sessile
Pharaoh ant,Monomorium pharaonis
Big-headed ant,Pheidole megacephala
White-footed ant,Technomyrmex albipes
Related crazy ant species, Paratrechina bourbonica
Tramp species are highly adapted to living with humans, and in fact, often are only seen in significant populations where humans have disrupted the natural landscape by development. They get their name from their remarkable ability to move from place to place. Like a tramp steamer picks up new passengers wherever it goes, tramp ants manage to hitch rides to other locales and are generally carried, living within soil and belongings, to new locations around the globe and to new sites within countries. For example, in most areas, the pharaoh ant is only capable of moving to new structures with the help of people. In one case involving Argentine ants, a colony was discovered in the packing carton of an electron microscope shipped from Japan.
Multiple Queen Colonies. All of the common U.S. tramp ant species have multiple queen colonies. Numerous queens are a significant benefit in that they enable a colony to survive even catastrophic die-offs of workers — if at least one queen and a few workers survives. Multiple queens also assist in successful establishment of numerous satellite colonies that can survive on their own if separated from the other colonies. Such factors provide significant advantages for survival.
Multiple Colony Sites. The ability to establish subcolonies interconnected by trunk trails permits tramp species to survive most catastrophic events, such as a visit by a pest management professional. One or more of these subunits invariably survives the best inspection and treatment efforts. Additionally, if one subcolony is eliminated by a rival ant colony, flooding, or other reason, other subcolonies will quickly move back in to exploit the territory and its resources. In this way, for example, Argentine ants can quickly reinfest treated areas from adjacent, untreated properties, frustrating both homeowners and pest management professionals alike.
Multiple colony sites also inhibit the success of baiting programs. Most tramp species are difficult to control using baits; the pharaoh ant being an obvious exception. Even if one subcolony picks on a bait, how much stays with the subcolony and how much is shared with other subcolonies? Surely, workers are killed, but is the amount of bait consumed so diluted that colonies are barely hindered by the subsequent loss of workers? Surely, overcoming this issue is a significant factor in bait development for these pest species.
Unicolonial Behavior. Unicolonial behavior is a common but not universal trait among the tramp species. With this behavior, workers from different colonies are not aggressive toward one another. Argentine ants, for example, make great advantage of independent colonies merging together to increase their ability to exploit resources and ensure survival. Another tramp species, the odorous house ant, however, is not unicolonial, and workers from separate colonies are antagonistic to one another. During the peak summer period of activity, Argentine ant colonies will merge with each other to form huge, extended supercolonies that may stretch across several properties. This characteristic is also important to the white footed ant, pharaoh ant, and some species of big headed ants, particularly P. megacephala.
Interspecific Aggression. Interspecific aggression involves the competitiveness of a tramp species against other ant species in its territory. The Argentine ant is a fierce competitor, often driving other ant species completely out of its foraging range. In some areas where this species has been present for many years, native ant species are almost nonexistent.
Budding and Fission. Colony dispersal by budding and fission is of benefit to species dependent upon man for dispersal. Species that propagate by budding can form new colonies at any time instead of being restricted to mating flights at certain annual periods. This ensures that more new colonies can be produced in this manner, although the dispersal distance is greatly limited (pending involvement of people). Colony fission is a trait that is utilized successfully by pharaoh ants to survive. When treatments are applied to buildings infested by this ant, the colonies commonly split into two or more colonies to help ensure that one or more will survive.
Monomorphism. Most tramp species are monomorphic, having only one size of worker. The workers are typically small in size, rarely reaching more than ¼ inch. Monomorphic workers are rarely specialized and can assume any task within the colony. This is especially important because satellite colonies can often be quite small and new colonies started by budding require versatile workers.
Living in Close Association with Humans. People are essential to the success of tramp ant species. In most areas where such species are significant pests, they fail to have much success in thriving in undisturbed wild areas adjacent to developed communities. Development disrupts the natural environment and imperils native ant species which would be the primary competitors to tramp species. Human activities provide innumerable opportunities for shelter and food. In Southern California, for example, people supply an ample amount of moisture to their communities — probably the most important factor in the survival and ultimate success of the Argentine ant.
CONCLUSION. Tramp ants are our most difficult species to control in and around structures. Experience has proven this time and again. Although considerable research is being completed on pharaoh ants and Argentine ants, other species are rapidly exceeding these ants as pests in areas where they are endemic; for example, the ghost ant and the white footed ant in Florida and Hawaii. Much more needs to be understood about the common traits among such species so the keys to controlling them can be unlocked. The industry should make the effort to support research exploring the secrets of pest ants.
Stoy Hedges is director of training and technical services for Terminix International, Memphis, Tenn.
Sidebar: CHARACTERISTICS OF TRAMP ANT SPECIES1. Multiple queen colonies (polygyny)
2. Multiple colony sites (polydomous)
4. Nonaggressiveness between members of different colonies (unicolonial)
5. Interspecific aggression
6. Reproduction by budding
7. Living in close association with humans
8. Primarily dispersed by human activities