Tapinoma sessile is a species of ant that goes by the common names odorous house ant, stink ant, and coconut ant. Their colonies are polydomous (consist of multiple nests) and have a characteristic dominance hierarchy system. Like many social insects, T. sessile employs complex foraging strategies, allocates food depending on environmental conditions, and engages in competition with several other insect species.
These are both indoor and outdoor ants. They eat honeydew, which is made by aphids and scale insects, and other sugary foods. Due to the vast variety of food they eat and their commonality, they are a common household ant.
This is a eusocial species, meaning it lives in a nest with other individuals of the same species. This is characterized by a distinct caste systems, care of younger generations but the older generations, and a small reproductive caste that does all the breeding.  The T. Sessile has a system with a single queen and many workers, ranging from hundreds to thousands. 
T. sessile ranges in color from brown to black, and varies in length from 1/16 to 1/8 inches (1.5–3.2 mm). Their antennae have 12 segments.
Little is known about the lifespan of the ant, though it has been shown that queens live at least 8 months (and probably much longer), workers at least a few months (and show every indication of living as long as queens), while males appear to live only approximately a week.
This odorous house ant is tough; injured workers have been observed to continue living and working with little hindrance. Some queens with crushed abdomens still lay eggs, and there are documented instances of T. sessile queens surviving without food or water for over two months. They also appear highly tolerant to heat and cold. These ants are difficult to remove from the home. When killed, these ants leave an odorous smell which leads to their nickname "Stink Ant."
The gaster portion of the abdomen sits directly on top of the petiole in the abdomen of this ant species.  A comparison of the diagram of a normal ant body and the side view of the T. sessile shows how the gaster sits atop the petiole.
This also leads to a very small petiole and to the gaster being pointed downward. The anal pore then open ventrally (toward the abdomen) instead of distally. 
This species of ant demonstrates a dominance hierarchy system, consisting of a queen and subordinate workers. The colonies themselves vary in size from a few hundred to tens of thousands of individuals. The bigger colonies usually have multiple queens. The queens lay the eggs and then incubate between 11-26 days. The larva stage then lasts between 13-29 days, and the pre-pupal and pupal stages last between 10-24 days. 
In this species, foragers collect food that is around the nesting area and bring it back to the colony to share with the other ants. The T. sessile has polydomous colonies, meaning that it has multiple nests. Because it is polydomous, the T. sessile is very good at foraging for food where there is a great variance in the distribution of resources. Instead of always going back to the nest to deliver food, they move workers, queens, and the brood to be closer to the food, so that they can reduce food transport costs. This is called dispersed central-place foraging.  It was also found that the nest half-life for the ant was about 12.9 days. 
Buczkowski and Bennett also studied the patterns of food moving within a nest. They labeled sucrose with Immunoglobin G (IgG) proteins, and then identified them using an enzyme-linked immunosorbent assay (ELISA) to track the movement of food. They were able to find that food was spread through trophallaxis, which is when one animal regurgitates food to another. Despite this trophallactic spread of food, the workers kept most of the sucrose. They also found that some queens received more food than others, suggesting a dominance hierarchy even within the queens. They also found that the nests were located in a system of trails, and that their distribution depended on where food was found and the distance between these patches of food. 
It is also found that the rate of trophallactic feeding depends on the number of ants per nest, and the quality of food available. When the number of donors is kept constant, but the number of total individuals in increased, more individuals test positive for the food marker. This indicates that more individuals are eating, but the amount they eat is less. If the number of donors was doubled, and the size of the overall population increased, the number of individuals receiving food more than doubled, again indicating that the number of individuals fed increased, but that the per capita amount of food consumed decreased.  The ants prefer sugar and protein food sources, over lipids, and this preference did not change seasonally. When specific sugar sources were studied the ants preferred sucrose over other sugars, such as fructose and glucose. 
When searching for food, primary orientation is when ants are exploring a new terrain without the guidance of odor trails. Secondary orientation is when terrain is explored, and there are pre-existing odor trails which ants use to orient themselves. When the T. sessile ants are orientating themselves for the first time they often rely on the topography to orientate themselves. The major types of elements they rely on are bilaterally elevated, bilaterally depressed, unilaterally elevated, and unilaterally depressed. They uses these types of surfaces to orient along, and lay the first odor trails, which can then be followed in the future, the food source, by other ants. 
It was also found that the polydomy, having multiple colonies to have access to multiple food sources is seasonal in this ant species. The colony will overwinter in a single nest, and then during the spring and summer when resources are more abundant they will form multiple nests. This allows them to better utilize food sources that might be spread out. Then during the winter they will return to the same nest location. Seasonal polydomy is pretty rare, and only found in 10% of all polydomous species.  Although season polydomy is rare and not found in many ant species there are many ant species, including T. sessile, which move even within a season. Immigration is common and allows them to forage for better food. 
Seasonal activity patterns of the ants were also studied, and corresponding to the seasonal polydomy, it was observed that the ants displayed the most activity between March and September and displayed almost no activity from October to December. Daily activity patterns were also studied. In March T. sessile foraged during the day, but in April that pattern changed and the ant began to forage during both day and night. Throughout most of the summer the ant has low levels of activity during all twenty-four hours. 
Competition with other ant species
Competition between species is often classified as exploitation or interference. Exploitation involves finding and using limited resources before they can be used by other species, while interference is the act of preventing others from getting resources by more direct force or aggression. When it comes to these behaviors, a species is considered dominant if it initiates an attack and subordinate if it avoided other species. In comparison with eight other ant species, the T. sessile was more subordinate on the dominant to subordinate scale. The ant does not show a large propensity for attack, preferring to use chemical secretions instead of biting. 
When the T. sessile, a subordinate species, was in the presence of dominant ant species such as C. ferrugineus, P. imparis, Lasius alienus, and F. subsericea, they reduced the amount of time that was spent foraging. This was tested with the use of bait, and when the subordinate species, such as T. sessile, encountered a dominant species they would leave the bait. It would then make sense that the subordinate species would forage at a different time than dominant species, so that they could avoid confrontation, but there is sizable overlap in foraging period on a daily and seasonal basis. Because the T. sissile forages at the same time as domain species, but does not want to interfere, they must have excellent exploitative abilities to survive. 
One of the invasive species that T. sessile has had to contend with is the Linepithema humile, or Argentine ant. Studies of its interactions with L. humile has helped researchers better understand the aggression of T. sessile. T. sessile ants do not fight with their nest mates, and it was found that they only fought collectively in six of forty interactions. This often caused T. sessile to lose in interactions with other ants, such as the L. humile, even when they have more individuals. This is because species like the L. humile work together while the T. sessile does not. The T. sessile is however more likely to win in one-on-one interactions because they have effective chemical defenses. 
This species is a scavenger/predator ant that will eat most household foods, especially those that contain sugar, as well as other insects. Indoors they will colonize near heat sources or in insulation. In hot and dry situations, nests have been found in house plants and even in the lids of toilets. Outdoors they tend to colonize under rocks and exposed soil. They appear, however, to form colonies virtually anywhere, in a variety of conditions.
In experiments where T. sessile workers were confined in an area without a queen, egg-laying (by the workers) was observed, though the workers destroyed any prepupa that emerged from the eggs.
They appear to be more likely to invade homes after rain (which washes away the honeydew they collect).
Odorous house ants appear to be highly tolerant of other ants, with compound nests consisting of multiple ant species (including T. sessile) having been observed.
Predators and parasites
Wheeler (1916) mentions Bothriomyrmex dimmocki as a potential parasite of odorous house ant colonies (suggesting that B. dimmocki queens invade and replace T. sessile queens).
Isobrachium myrmecophilum (a small wasp) appears to parasitize odorous house ants.
Some birds and toads will also eat odorous house ants on occasion.
Sessile translates to "sitting" which probably refers to the gaster sitting directly on top of the petiole in the abdomen of the ant species.  The common names "odorous house ant" and "coconut ant" come from the odor the ants produce when crushed, which is very similar to the pungent odor of a coconut or turpentine.
These ants are not hard to control, and most ant killers will solve problems, especially if controlled as soon as the problem is noticed. At this point, they could be put under control in just a few days. However, the longer someone waits, the larger the population is and the longer it will take to control the situation, possibly a few weeks. Standing water should be eliminated, as odorous house ants are attracted to moisture. Plants should be trimmed back so they cannot be used to get inside. Cracks, holes and joints should be sealed with polyurethane foam or caulk, especially those that are near the ground. Firewood, rocks and other materials should not be stored next to a home because it encourages nest building. People should be on the lookout for these ants in late winter and early spring (particularly after rain), as this is when they most commonly appear.
|Wikimedia Commons has media related to Tapinoma sessile.|
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- Odorous House Ants Fact Sheet National Pest Management Association data highlighting habits, habitats and threats