The red imported fire ant (Solenopsis invicta) is an aggressive invasive species known for its painful, venomous sting and ability to rapidly colonize new environments. These ants construct large, visible mounds and pose a significant threat to agriculture, wildlife, and human health. A common question is whether they are primarily active at night. Their surface activity is highly flexible, adapting based on environmental conditions rather than a strict internal clock. The timing of when fire ants emerge to forage is governed almost entirely by temperature, making their activity cycle dynamic throughout the day and year.
Activity Cycle: Timing and Temperature Dependence
Fire ants are not strictly nocturnal or diurnal; their surface activity is defined by a narrow, tolerable temperature range. Maximum foraging activity occurs when soil and ambient air temperatures are between 25°C and 35°C (77°F to 95°F). Outside of this optimal window, their ability to move and forage decreases substantially, though some minimal activity can be observed between 10°C and 50°C.
This dependence on temperature means their active hours shift significantly with the seasons and time of day. During cooler seasons, such as spring and fall, the warmest part of the day is often optimal, making the ants primarily diurnal. In the peak of summer, when daytime temperatures routinely exceed 35°C, the ants become crepuscular, shifting activity to the cooler hours of dawn and dusk.
This temperature-driven behavior allows them to avoid extreme heat. In regions with extremely high daytime temperatures, fire ants may become active at night to find suitable conditions for surface movement. Therefore, observing them foraging at night is a heat-avoidance strategy, not a fundamental nocturnal preference.
Specific Foraging Behavior
When conditions are suitable for surface activity, fire ant workers emerge from the mound to systematically search for resources within their territory. They are omnivorous, consuming a wide range of foods, including dead insects, small vertebrates, seeds, and plant materials. Adult workers, however, can only ingest liquids, so they primarily seek oils and proteins to carry back to the nest.
Upon finding a substantial food source, the initial scout ant returns to the colony while depositing a chemical trail. This trail is composed of a recruitment pheromone, which is used to organize the retrieval of the discovered food. The pheromone acts as a guide, rapidly recruiting hundreds of nestmates to the food location to swarm and process the resource.
The ants also exhibit aggressive surface behavior, often swarming and stinging to defend their food or their mound from disturbance. The efficiency of their recruitment system allows them to quickly dominate a resource and overwhelm many competitors.
Colony Behavior During Inactive Hours
When surface temperatures fall outside the tolerable range, the fire ant colony retreats deeper into the subterranean nest structure. This retreat is a sophisticated thermoregulatory strategy, where the ants actively move the queen, workers, and brood to maintain stable and optimal living conditions. The mound itself plays a significant role in this process, helping to buffer temperature extremes.
The workers constantly track the temperature gradient within the mound to ensure the brood is kept in a zone near 30°C to 32°C (86°F to 90°F), which is ideal for growth. During very cold periods, the colony moves downward, sometimes to depths of 40 centimeters or more, where the soil temperature is more stable. Conversely, during intensely hot days, they move deeper to avoid the extreme heat that builds up near the surface.
This temperature tracking is a direct response to thermal cues in the soil. The ability of the colony to move its entire structure within the soil column provides a survival benefit, allowing them to continue development even when surface activity is completely halted.