Flies are ectotherms, meaning their internal body temperature is regulated almost entirely by the external environment. Temperature is a primary factor determining their survival, geographical range, and activity level. Their physiological processes function only within a relatively narrow thermal window. Understanding these thermal limits is essential for controlling the populations of common domestic pests, such as house flies and fruit flies.
Upper Lethal Temperature Thresholds
The thermal limit for immediate survival in most adult flies exists near or above 45°C (113°F). This is the Critical Thermal Maximum (CTmax), the temperature at which the insect loses coordinated movement and enters a state of heat knockdown. For common fruit flies (Drosophila), this loss of motor function can occur at 37.5°C (99.5°F) if exposure is prolonged for an hour.
True lethality, known as the Lethal Temperature (LT), is reached quickly once the internal temperature climbs above 47°C (116.6°F). Exposure to temperatures between 50°C and 55°C (122°F to 131°F) causes near-instantaneous death. The physiological cause of death is rapid protein denaturation, where the structure of enzymes and cellular components breaks down. This damage is coupled with rapid desiccation, as high heat forces uncontrolled water loss, leading to fatal internal imbalances.
Lower Lethal Temperature Thresholds
Flies face two distinct threats when exposed to cold: freezing and chilling injury. The temperature at which body fluids spontaneously freeze is the supercooling point (SCP), which can be as low as -20°C (-4°F) for some fruit flies. Most adult flies die long before freezing, often succumbing to chilling injury at temperatures slightly above freezing, such as 0°C to 5°C (32°F to 41°F).
Chilling injury is cumulative damage caused by metabolic imbalances that disrupt ion homeostasis in the nervous system. Exposure to -5°C (23°F) for just two hours is lethal for many adult fruit flies. At slightly warmer temperatures, such as 6°C (43°F), death is often caused by desiccation, as the fly enters a chill coma and is unable to seek moisture.
The ability of different life stages to tolerate cold varies significantly. While adult fruit flies die quickly at sub-zero temperatures, the eggs of some species have a much lower supercooling point, sometimes below -25°C (-13°F). This lower threshold allows the species to overwinter in protected, dormant stages, surviving temperatures lethal to the adult flies.
Factors Influencing Thermal Tolerance
Lethal temperatures determined in a laboratory setting are not absolute constants, as thermal tolerance is flexible. The life stage is a major determinant; eggs and pupae often exhibit different resistance levels than mobile larvae and adults. For instance, the pupal stage of some species can withstand temperatures that kill the adult, though prolonged exposure remains fatal.
Prior thermal exposure, known as acclimation, modifies a fly’s limits. Flies exposed to mild, non-lethal cold temperatures, such as 5°C (41°F), can rapidly cold-harden, significantly increasing survival time when later exposed to sub-zero temperatures. Conversely, prior exposure to non-lethal heat can boost heat tolerance for a short period.
Environmental humidity plays a complex role in thermal stress. Higher humidity can increase the lethality of mild cold stress by reducing a fly’s ability to minimize fatal water loss, a common cause of death near freezing. However, high humidity can slightly reduce the lethality of extreme heat, as evaporative cooling from the fly’s body surface sheds excess heat.
Applying Thermal Knowledge to Pest Management
Understanding the specific upper and lower thermal limits of flies provides a direct, non-chemical approach to pest control. Heat treatments are commonly used in the food industry and forestry, applying temperatures between 50°C and 60°C (122°F and 140°F) to infested structures or commodities for rapid disinfestation. Since temperatures above 47°C are lethal to most species, sustained heat is an effective quarantine tool, destroying all life stages.
Cold treatments are applied to food items or clothing that cannot be treated with heat. Freezing contaminated materials below -10°C (14°F) for a sustained period is effective because it forces the fly’s internal temperature past its lower lethal limit. However, heat does not penetrate dense materials like flour or grain easily, and flies can actively seek micro-shelters to avoid extreme conditions.