The black garden ant (Lasius niger), also known as the common black ant, is a formicine ant, the type species of the genus Lasius, which is found across Europe and in some parts of North America, and Asia. The European species was split into two species; L. niger, which is found in open areas; and L. platythorax, which is found in forest habitats. It is monogynous, meaning colonies contain a single queen.

Lasius niger colonies normally range from 4,000 to 7,000 workers, but can reach 40,000 in rare cases. An L. niger queen can live for up to 29 years, the longest recorded lifespan for any eusocial insect. Lasius niger queens in the early stages of founding can have two to three other queens in the nest. They will tolerate each other until the first workers come, then it is most likely they will fight until one queen remains. Under laboratory conditions, workers can live at least four years.^{[better source needed]}

Lasius niger is host to the silver-studded blue butterfly Plebejus argus; it is also host to a number of temporary social parasites of the Lasius mixtus group, including Lasius mixtus and Lasius umbratus.

Ants mate on the wing, so "flying ants" are alates (reproductive individuals), which include males and gynes (virgin queens). The mating (or nuptial) flights of Lasius niger usually occur around June to September throughout the species' range; in North America, flights usually occur during autumn, whereas in Europe they generally take place during the hot summer months of July and August. Flights can contain thousands of winged males and females.

Disparities between local weather conditions can cause nuptial flights to be out of phase amongst widespread populations of L. niger. During long-lasting, hot summers, flights can take place simultaneously across the country, but overcast weather with local patches of sunshine results in a far less synchronized emergence of alates.^{[citation needed]}

Once the queens have mated, they will land and discard their wings and begin to find a suitable place to dig a tunnel. Meanwhile, males generally only live for a day or two after the mating flights and will then die.^{[citation needed]}

After removing her wings, a queen will move quickly to find moist ground, then start digging a tunnel. Once the tunnel has been completed, the queen will block the entrance and retreat to the bottom. Subsequently, she will dig out a small chamber. This will serve as the claustral chamber of the new colony. Generally, a queen will begin to lay eggs immediately after the construction of the chamber, and the eggs will develop to imagines in 8–10 weeks. Until the eggs hatch and the larvae grow to maturity, a Lasius niger queen will not eat, relying on the protein of her wing muscles to be broken down and digested. In some cases, a Lasius niger queen may eat some of her own eggs in order to survive.^{[citation needed]}

The first worker ants that emerge are very small compared to later generations. At this point, the workers immediately begin to expand the nest and care for the queen and brood; they eventually remove the seal from the entrance to the nest and begin to forage above ground. This is a critical time for the colony as they need to gather food quickly to support future growth and particularly to feed the starved queen, who would have lost around 50% of her body weight. From this point on, the queen's egg laying output will increase significantly, now becoming her sole function. The later generations of worker ants will be bigger, stronger and more aggressive because there is more nutrition for them at the larval stage. The initial brood being fed only by the scarce resources available to a queen will be much smaller than the brood supported by a team of foraging and nursing workers. If workers are able to find food, the early colony will see an exponential rise in population. After several years, once the colony is well established, the queen will lay eggs that will become queens and males. Black ants often make large nests with extensive tunnel connections.^{[citation needed]}