Flies, particularly the common house fly (Musca domestica), are ectotherms, meaning their body temperature is directly controlled by the surrounding environment. Temperature is one of the most significant factors governing their survival, distribution, and population size. The difference between an optimal temperature range and a lethal one can be a matter of only a few degrees, highlighting the narrow thermal window in which these insects thrive. Understanding these specific thermal limits—both high and low—is relevant to controlling fly populations.
The Upper Thermal Limit: Lethal High Temperatures
Adult house flies begin to experience distress and cellular functions become compromised when the ambient temperature approaches \(113^\circ\text{F}\) (\(45^\circ\text{C}\)). This marks the beginning of the upper thermal mortality threshold, requiring prolonged exposure time for death. Mortality speed increases dramatically as heat rises, causing the irreversible denaturing of biological proteins.
A rapid kill is achieved when temperatures reach \(120^\circ\text{F}\) (\(49^\circ\text{C}\)) or higher, causing irreparable cellular damage. Exposure to \(122^\circ\text{F}\) (\(50^\circ\text{C}\)) can result in the death of most adult flies within three to five minutes. Temperatures reaching \(140^\circ\text{F}\) (\(60^\circ\text{C}\)) are effective for near-instantaneous elimination across all life stages.
Chronic exposure to temperatures far below the acute lethal point can be detrimental to adult survival. Survival rates are significantly reduced when sustained temperatures hover around \(97^\circ\text{F}\) (\(36.1^\circ\text{C}\)). These high but non-lethal temperatures adversely affect the flies’ metabolism and physiological processes, leading to a much shorter lifespan. High temperatures are often leveraged in pest control through heat treatments designed to rapidly raise the temperature to the point of protein breakdown.
The Lower Thermal Limit: Cold Tolerance and Freezing
Flies are freeze-intolerant; the formation of ice crystals within their body fluids is lethal. The supercooling point, where body fluids spontaneously freeze, is typically around \(-10.1^\circ\text{C}\) to \(-12.7^\circ\text{C}\) (\(13.8^\circ\text{F}\) to \(9.1^\circ\text{F}\)) for adult house flies. Reaching this threshold causes instant death due to physical and osmotic damage from internal ice.
Death can occur above the supercooling point through chilling injury from prolonged non-freezing cold exposure. At \(48.2^\circ\text{F}\) (\(9^\circ\text{C}\)), an adult house fly becomes immobilized and unable to fly. Sustained cold conditions around \(60.1^\circ\text{F}\) (\(15.6^\circ\text{C}\)) inhibit physiological function, leading to low survival.
Flies surviving winter seek sheltered microclimates above the freezing threshold. They overwinter in sites like animal confinement quarters, where temperatures stay above approximately \(23^\circ\text{F}\) (\(-5^\circ\text{C}\)). Flies can produce cryoprotectants like glycol to lower their supercooling point and increase tolerance to sub-zero temperatures during hibernation.
Temperature Effects on Immature Stages (Eggs, Larvae, and Pupae)
The thermal tolerance of immature life stages—eggs, larvae, and pupae—is often more resilient to temperature extremes than adults. Optimal development occurs within a narrow range, typically between \(84^\circ\text{F}\) and \(90^\circ\text{F}\) (\(28.8^\circ\text{C}\) and \(32.2^\circ\text{C}\)). Development is inhibited at temperatures as high as \(97^\circ\text{F}\) (\(36.1^\circ\text{C}\)) or as low as \(60.1^\circ\text{F}\) (\(15.6^\circ\text{C}\)).
Larvae, living in moist substrates like manure, attempt to escape when the temperature reaches \(115^\circ\text{F}\) (\(46^\circ\text{C}\)). Complete larval mortality is achieved when the substrate temperature reaches \(120^\circ\text{F}\) (\(49^\circ\text{C}\)) or higher. The pupal stage offers resistance but requires about 30 minutes at \(122^\circ\text{F}\) (\(50^\circ\text{C}\)) for complete mortality. This heat principle is used in composting, where temperatures often reach \(140^\circ\text{F}\) (\(60^\circ\text{C}\)), sterilizing the material.
Immature stages also face challenges at the lower end of the thermal spectrum, where cold temperatures halt development. While embryos can survive a few days at \(41^\circ\text{F}\) (\(5^\circ\text{C}\)), prolonged exposure reduces survival and adult vitality. Younger embryos are vulnerable to chilling injury, with one-hour-old eggs unable to survive a single day at \(41^\circ\text{F}\) (\(5^\circ\text{C}\)).