Fascinating World of Ants

Editor’s Note: The Mallis Handbook of Pest Control has been an essential educational resource for pest management professionals for nearly eight decades. Late last year, PCT’s parent company, GIE Media, published the eagerly anticipated 11th edition of this 1,300+page reference book, which features editorial contributions from some of the industry’s most respected university educators, independent consultants and urban entomologists. Order this book from the PCT Online store at bit.ly/3NIFmGt, or by calling 800/456-0707. In the following excerpt, Drs. Edward Vargo and Robert Puckett of Texas A&M University share the insights about colony formation and the feeding habits of ants. Visit here for information about the references mentioned here.

Ants are among the most fascinating creatures on earth, being a highly evolved and dominant insect group. They are one of the largest families of insects in terms of the sheer number of individuals and the number of different species. According to the Myrmecological News Blog, as of 2021, 13,379 ant species have been identified, with thousands more remaining to be discovered and described.

Despite the tremendous diversity of ant species, relatively few species have been classified as pests. Based on pest ant species listed in Klotz et al. (2008) and the nearly 1,000 ant species in North America (Fisher and Cover 2007), a generous approximation of 13% of the ant species in North America could be considered pests in the urban environment. This percentage includes species that may only occasionally invade structures or cause a problem in limited localities.

While the overall number of different pest ant species can be limited, the potential of having millions upon millions of a single ant species in a locality can present a significant challenge to the pest professional. With molecular and behavioral analyses, scientists have documented the existence of super-colonies of Argentine ants that span several counties in California (Tsutsui et al. 2000) and a mega-colony in Europe extending over several countries (Giraud et al. 2002).

Expansion of global commerce brings more opportunities for the introduction of new pest ants. Around the start of the new millennium, we have not only seen the spread of the red imported fire ant into new states in the U.S. (California), but into new countries (Mexico) and continents (Australia and Asia).

In addition, once innocuous exotic ants that were found in the U.S. decades ago now have exploding populations that have become serious problems in both natural and urban landscapes (Groden et al. 2005).

Factors triggering these population explosions are not completely known, but often these ants are not territorial, resulting in interconnected nest sites over large areas with many reproducing queens per colony. Once established, invasive ants can thrive and spread into surrounding areas. Eventually they become the dominant faunal species, displacing native ants as well as other native organisms (Figure 1).

THE ANT COLONY. All ants are eusocial, meaning that at least two generations live together cooperatively. Within their society, ants perform different functions. A group of individuals of similar age or body type that are specialized for carrying out a particular task is called a caste.

Ants have three chief castes: queens, males and workers. The queens are fertile, egg-producing females. They are winged until after the mating flight when they shed their wings. The males are also winged but die soon after mating. The workers are wingless, infertile females of various shapes and sizes. For example, soldiers are modified workers with enlarged heads and thickened or elongated mandibles for defending the colony.

A relatively small percentage of workers forage, and these typically are older ants (Hölldobler and Wilson 1990). Most of the workers remain in the nest, where they perform duties such as cleaning, excavating and caring for the brood (Figure 4). Pharaoh ant workers may live only nine to 10 weeks (Peacock et al. 1950), while the workers of some field ants can live longer than six years (Wheeler 1910). The worker caste can be divided into subcastes according to their specialty. In the black carpenter ant (Camponotus pennsylvanicus), for example, certain individuals in the foraging population perform specific tasks. The smallest workers guard aphids and transport some honeydew, but larger workers transport the majority of food (Fowler and Roberts 1980). Medium-sized workers are more responsible for colony defense and excavation than smaller or larger workers (Fowler 1985).

Big-headed ants have two size classes of workers (dimorphic): majors with large heads, and more numerous minors with small heads. Fire ants, carpenter ants, and leaf-cutting ants have workers that range in size along a continuum (polymorphic). The workers of most structural pest species are all approximately the same size (monomorphic). The size of an individual worker is determined by the quantity and quality of food that it receives as a larva.

The queen and female reproductives are the largest ants in the colony. Some species of pest ants have multiple queens in a colony (polygyne), such as Pharaoh ants, little black ants, Argentine ants, crazy ants, odorous house ants and thief ants. Harvester ants and pavement ants have only one queen per colony (monogyne). Most carpenter ants have only one queen, but C. vicinus is an exception (Akre et al. 1994a). A colony of red imported fire ants can be either monogyne or polygyne.

Some ants produce new queens asexually, i.e., from unfertilized eggs. In both the little fire ant and the longhorn crazy ant, queens develop from unfertilized eggs and are clones of their mothers, whereas workers develop from fertilized eggs. Curiously, males in both of these species are also produced clonally by males through a process in which the sperm nucleus eliminates and destroys the egg nucleus, leaving only the male chromosomes to produce an identical copy of the father (Fournier et al. 2005).

After mating, the queen stores enough sperm to last a lifetime. A Pharaoh ant queen typically lives about 200 days (Hölldobler and Wilson 1990), but carpenter ant queens can live for years (Figure 5). In Pharaoh ants, larvae that are sufficiently fed can become queens and replace older ones that die (Hölldobler and Wilson 1990). The sole purpose of the males is to mate. They are endowed with large eyes and a muscular thorax for the mating flight.

COLONY FORMATION. Ants establish colonies in a variety of ways. In some species, new queens remain in the nest and contribute to the growth of the parent colony. In a more typical case, winged male and female reproductives (alates) emerge from nests distributed over a large geographic region and fly to a congregation site, sometimes the same location every year. These mating flights are usually triggered by a weather cue. A conspicuous landmark, such as a bush, often marks their congregation site. When the alates reach their destination, they swarm on the ground (e.g., harvester ants), or in the air (e.g., red imported fire ants).

After mating, the newly fertilized queen searches for a suitable nest site. She removes her wings and digs a chamber in the soil or wood where she lays many eggs. Most serve as food for her or the faster-developing larvae. She also obtains nutrition from her fat reserves and nonfunctional wing muscles, which are broken down and resorbed.

After the first workers emerge, they care for the subsequent brood, expand the nest and begin foraging. Hereafter, the sole function of the queen is to lay eggs. After attaining considerable strength in numbers, the colony produces a new group of alates. The time from colony foundation to production of alates varies by species but usually takes two or more years.

In contrast, Pharaoh ants and Argentine ants lack mating flights (Passera 1994). Instead, mating takes place in or near the nest. Mating flights appear to be rare in the tawny crazy ant (Wang et al. 2016). In these ants, colony multiplication is by fission or budding. One or more queens and a group of workers, some carrying brood, split off from the colony and migrate to a new nest site. In Pharaoh ants, workers by themselves can form a successful colony by rearing their own queens from the brood they bring along with them (Wilson 1971). In these “tramp” ant species, no clear distinction exists between the colony and the population, with ants readily transferring from one nest to another along interconnecting trails.

FEEDING HABITS. Most ants are omnivores that feed on a wide variety of foods. Their diet varies, however, based on what the colony needs and what foods are available. With the onset of egg production and larval growth in spring and early summer, the colony requires a high amount of protein for its developing larvae (Abbott 1978). Foraging ants scavenge and prey upon arthropods to satisfy this requirement.

Later in the summer, as newly emerged adult ants add to the workforce, the colony’s dietary needs shift to carbohydrates, a primary source of energy (Abbott 1978). Partitioning of nutrients within the colony reflects the different dietary requirements of workers and larvae. Workers, who are unable to consume solids and lack proteases, feed solid protein to the larvae (Stradling 1987).

The quantity and quality of nitrogen in the proteins and amino acids they receive are key factors for their growth and development (Hagen et al. 1984). On the other hand, the workers, who need sugar to fuel their foraging activities, consume carbohydrates such as honeydew produced by Homopteran insects (Figure 8). Trophallaxis, which aids in the distribution of liquid food among workers and from workers to larvae and the queen, has long been known to be a fundamental behavior in the social life of ants. In trophallaxis, liquid food stored in a worker’s crop is regurgitated and shared with other ants. The crop, therefore, is called a “social stomach.” The recognition of the primary role that liquid foods play in ant nutrition and colony cohesion has stimulated research and development of liquid baits for ant control.

CONCLUSION. There is always more to learn when investigating the biology, behavior and control of pest ants. A working knowledge of the feeding habits, colony formation and population dynamics of ants is an excellent first step for developing effective, targeted strategies to manage and control infestations.