Hornets are large social wasps whose activity levels are governed almost entirely by the temperature of their surroundings. As ectotherms, hornets cannot generate enough internal heat to maintain a constant body temperature independent of the environment. Their metabolism and physical capabilities, particularly flight, slow down dramatically as the ambient air temperature drops, eventually leading to complete inactivity. This dependence on external conditions dictates when hornets can forage, defend their nest, and continue the colony’s growth.
The Threshold of Flight Cessation
The practical answer to when hornets become inactive lies in the temperature at which their flight muscles cease to function effectively. This functional threshold is generally around \(10^{circ}text{C}\) (\(50^{circ}text{F}\)) or slightly below. Below this point, hornets become sluggish, and their ability to fly, forage, and pose any threat is severely compromised.
In studies observing hornet activity outside nests, researchers have noted that virtually no hornets are seen flying when the external temperature falls below \(10^{circ}text{C}\). This temperature does not signify death but rather a state of forced immobility where the insect can no longer generate the necessary power for sustained flight. The large flight muscles in the thorax must be maintained at a much higher temperature than the air to operate, often requiring a thoracic temperature exceeding \(30^{circ}text{C}\).
When the air is too cold, the hornet expends too much energy trying to pre-warm its flight muscles, making any foraging trip inefficient or impossible. A sudden cold snap, even above freezing, can bring all visible hornet activity to a halt. Its ability to engage in any colony-sustaining behavior is completely suspended, rendering the insect inactive.
Winter Survival and Lethal Cold
The temporary inactivity caused by cool weather is distinct from the ultimate fate of the colony when sustained cold arrives. At the end of the season, the entire hornet colony, including all workers, drones, and the old queen, is destined to perish once temperatures remain consistently low. The paper nest itself provides only minimal insulation and is not built to survive the winter.
The only member of the colony adapted for long-term survival is the newly mated queen, known as a gyne. Before the cold sets in, she leaves the nest to seek a secure, sheltered location, such as under tree bark, in a rotten log, or within a structure’s crevices. Once settled, she enters a state of metabolic slowdown called diapause.
For the unprotected worker hornets left behind, lethal temperatures are reached when freezing conditions persist. Temperatures that drop below roughly \(7^{circ}text{C}\) (\(45^{circ}text{F}\)) for five to seven days or longer can be fatal. At this point, the water in their bodily fluids can freeze, causing cellular damage and eventually leading to death.
How Hornets Maintain Internal Temperature
Hornets use specific biological mechanisms to extend their active season and initiate flight even when the ambient temperature is cool. They employ a strategy called endothermy, which involves generating heat internally through muscle activity. This is accomplished by rapidly vibrating their large thoracic flight muscles in a process often described as “shivering”.
This pre-flight warm-up is necessary because the muscles must reach a specific operational temperature, which can be in the range of \(30^{circ}text{C}\) to \(40^{circ}text{C}\), before they can power flight. By decoupling the contraction from the wings, the hornet can rapidly elevate its thoracic temperature above the surrounding air, allowing it to take off earlier in the morning or remain active later in the evening than a purely ectothermic insect.
Inside the nest, hornets use a collective form of thermoregulation, particularly when raising brood. The workers cluster together and utilize the same muscle vibration to maintain a stable core temperature within the nest, even when the air outside is cold. This collective heat generation helps to ensure the proper development of the young, but this behavior is not sufficient to save the entire colony from the sustained, deep cold of winter.