Flies are ectotherms, meaning their internal body temperature is dictated by their surroundings, which makes them highly sensitive to thermal extremes. They possess a defined physiological range where life functions can occur, but outside of this window, their systems begin to fail. Lethal temperatures vary based on the duration of exposure and the fly species, but death is ultimately caused by the breakdown of cellular processes. Understanding the specific temperatures that induce this failure provides the foundation for predicting fly distribution and developing effective pest control strategies.
Lethal Cold Temperatures
The low-temperature limit is a spectrum of damaging effects, ranging from non-freezing injury to freezing death. Many common fly species, like the fruit fly, are considered “chill-susceptible” and die before their body fluids freeze, often succumbing to chilling injury. This death can occur from prolonged exposure to temperatures near freezing, such as 0°C (32°F), where metabolic systems fail to function correctly.
The Supercooling Point (SCP) is the temperature at which ice crystals form in the insect’s body, resulting in immediate death for most species. For adult flies, the SCP is often well below -5°C (23°F), but death rarely requires reaching this point. Exposure to temperatures slightly above freezing, around 6°C (43°F), can also be lethal over time due to desiccation, as flies lose moisture they cannot replenish at lower metabolic rates.
Lethal Heat Temperatures
The upper temperature limit is reached faster than the lower limit because high temperatures cause rapid biological damage. Heat primarily kills flies by denaturing proteins and causing metabolic failure as essential enzymes cease to function. The Critical Thermal Maximum (CTMax) is the temperature at which the fly loses motor control and enters a state of heat coma or “knockdown.”
While CTMax is not immediate death, true lethal temperatures are only a few degrees higher. For many common species, 100% mortality is observed after 24 hours of exposure to temperatures between 44.5°C and 45.0°C (112°F to 113°F). Temperatures exceeding 47°C (116°F) are rapidly fatal to most adult flies within minutes, as the rate of damage overwhelms their ability to repair cellular structures.
Key Factors Altering Thermal Tolerance
Acclimation and Hardening
The lethal temperature thresholds are not fixed numbers and can be significantly shifted by a fly’s life history and environment. One factor is acclimation, or “hardening,” where a fly’s previous exposure to non-lethal temperatures alters its tolerance. For instance, flies exposed to a mild cold period may develop increased cold tolerance by altering cell membrane composition or accumulating cryoprotectants. Developmental acclimation to higher temperatures can similarly increase heat resistance. However, this hardening is not always symmetrical; cold acclimation may improve cold survival but have little effect on heat tolerance.
Humidity and Desiccation
The amount of water vapor in the air, or humidity, plays a large role in heat survival. Low humidity dramatically increases the rate of desiccation, causing death faster than the heat itself. Flies reared in dry conditions often have a lower lethal heat threshold because their primary cause of death shifts from heat stroke to rapid water loss.
Life Stage
A fly’s life stage also influences its thermal limits. Eggs and pupae often show greater resistance to temperature extremes than adult flies. This difference is partly due to the immobility of these stages, which allows them to withstand stress in a protective casing.
Practical Application for Pest Management
Cold Treatments
Understanding the thermal limits of flies provides powerful, non-chemical methods for pest control. For cold treatment, simply lowering the ambient temperature is usually not sufficient to kill flies quickly, but it will inhibit their development and reproduction. A standard refrigerator temperature of about 4°C (40°F) will slow or stop the development of fruit fly eggs and larvae, but the insects may revive if warmed up. To ensure rapid death, freezing temperatures are necessary, which requires dropping the temperature well below the Supercooling Point. Placing infested items in a freezer at temperatures consistently below -18°C (0°F) for several days is a reliable method for killing all life stages.
Heat Treatments
Heat treatment is generally more rapid and is used extensively in commercial settings for disinfestation of commodities. High-temperature air or water treatments are effective because they quickly exceed the fly’s Critical Thermal Maximum (CTMax), leading to systemic failure. Immersion in hot water at approximately 46°C (115°F) for about 20 minutes is sufficient to kill fruit fly larvae in produce. For structural pest control, sustaining a temperature of 56°C (133°F) for 30 minutes is a common standard for killing insects in wood packaging material, demonstrating the efficiency of high heat as a rapid kill agent.