Pharaoh ant

The pharaoh ant (*Monomorium pharaonis*) is a tiny, yellowish or light brown insect, roughly 2 millimeters long and almost see-through. It’s infamous as a major indoor pest, particularly troublesome in hospitals. This species, whose origins are unknown, has spread to nearly every corner of the globe, including Europe, the Americas, Australasia, and Southeast Asia, and is a serious problem in the United States, Australia, and Europe. Its common name may come from the idea that it was one of the plagues of ancient Egypt.

Each colony contains many queens, a trait called polygyny, which shapes how the ants interact and how the colony operates. This setup also lets the colony quickly split into smaller, separate groups, a process known as budding.

**Physical characteristics**

Worker pharaoh ants measure about 1.5 to 2.0 millimeters long. They range from light yellow to reddish brown, with a darker abdomen. Their stinger is non-functional and is used only to produce pheromones. The narrow waist between the thorax and abdomen, called the petiole, has two nodes, and the thorax lacks spines. Their eyesight is poor, with an average of 32 ommatidia. The antennae end in a distinct club made of three progressively longer segments. Males are roughly 3 millimeters long, black, and have wings but do not fly. Queens are dark red, 3.6 to 5.0 millimeters long, and initially have wings that they lose soon after mating; they also do not fly.

**Life cycle**

A pharaoh ant queen can lay hundreds of eggs over her lifetime. Early on, she typically lays 10 to 12 eggs per batch, but later batches contain only four to seven eggs. At 27°C (80°F) and 80% relative humidity, eggs hatch in five to seven days. The larval stage lasts 18 to 19 days, followed by a three-day pre-pupal period and a nine-day pupal period. It takes about four more days to produce reproductive females and males. From egg to sexual maturity, the process takes roughly 38 to 45 days, depending on temperature and humidity. In heated buildings, they breed year-round, and mating occurs inside the nest. Mature colonies contain several queens, winged males, workers, eggs, larvae, pre-pupae, and pupae.

**Colony proliferation**

Each colony produces reproductive individuals about twice a year, though lab-raised colonies can be manipulated to produce them at any time. Colonies spread through budding, where a group of queens, workers, and brood (eggs, larvae, and pupae) leaves the main nest for a new site. Pharaoh ants tend to prefer familiar nests over new ones when budding, suggesting they remember certain features of their living space. However, if a novel nest is clearly better, the colony may initially head toward the familiar one but will eventually choose the unfamiliar site. This decision-making minimizes the time spent without a nest while still picking the best option.

The number of available budding spots greatly affects how fragmented the colony becomes. Many bud nests lead to small fragments, showing the colony can control group size and caste ratios. Still, a minimum group size of about 469 individuals seems preferred. The amount of fragmentation doesn’t affect food distribution. After budding, the new nest units don’t compete for resources but cooperate instead, likely because they are genetically similar. Major disturbances to the central nest cause the colony to abandon it and flee to a bud nest. Nest units may exchange individuals after budding, which further explains their cooperative behavior. In Australia, *Monomorium* species are especially successful, even though a very aggressive ant family, *Iridomyrmex*, dominates through interference competition. *Iridomyrmex* ants quickly find food and block other ants from reaching it. But *Monomorium* species, despite being small and unaggressive, thrive in these areas thanks to an efficient foraging strategy and the use of venom alkaloids as repellent chemical signals. These behaviors let them rapidly take over and defend food sources.

**Foraging trail pheromones**

Pharaoh ants use three types of trail pheromones. One is a long-lasting attractive chemical that builds a trail network; it remains detectable even if the trail isn’t used for days. The ants stop activity at night and start each day around 8 am, yet parts of the trail network are identical day after day. The second pheromone is also attractive but decays to undetectable levels in minutes without reapplication. This helps mark unpredictable food sources, allowing the colony to quickly adapt to changes—individuals won’t waste time on a useless trail. The third pheromone is a repellent. Pharaoh ants were the first species found to use a negative trail pheromone. If an ant finds an unprofitable area with little food or significant danger, it releases this repellent, warning others and steering them elsewhere. While positive pheromones for good foraging sites are common in social insects, this negative pheromone is highly unusual.