Temperature often limits the distribution of an insect species, but it can also influence its success in the occupied range. This is because insects are ectotherms, meaning their internal body temperature changes with their environment because they do not generate heat. Therefore, insects are directly affected by microclimates caused by daily or seasonal temperature fluctuations in their habitats.
How do insects adapt to harsh environmental conditions?
Like hibernation in other animals, insects may go dormant in response to adverse conditions. Dormancy is an inactive state characterized by depressed metabolic activity and arrested development. This may take many forms depending on intensity and duration of dormancy. Quiescence is a short period of dormancy that is directly induced by adverse conditions and can be quickly reversible when favorable conditions return. Diapause is a hormonally regulated process that is genetically determined to occur during a certain life stage. Like quiescence, diapause is a response to environmental cues, but it is triggered in advance of the adverse conditions occurring to allow time for profound physiological changes to occur. Initiating and terminating diapause is a gradual response to token stimuli, such as photoperiod, temperature, food quality or availability, maternal factors (diet, age, etc.), moisture, and others. Reliable indicators of winter diapause in temperate climates include photoperiod and temperature. Diapause typically lasts for months but can occur for weeks or greater than a year, depending on the species.
Typically, insects build up energy reserves and move to a protected overwintering site (warmer climates, soil, litter/debris, structures such as cocoons and galls) in preparation for diapause. Entering diapause does not ensure survival. Insects quickly assume a temperature close to that of their environment, leaving the water in their body vulnerable to freezing. Diapause and cold hardiness are not always linked; cold hardiness is sometimes achieved after diapause initiation. Physiological mechanisms involved with successful overwintering vary by insect species and are not fully understood. Here, we provide examples of some primary mechanisms that have been described for different strategies, but this is not an exhaustive list.
Freeze-tolerant insects can survive freezing by producing ice-nucleating and heat-shock proteins, increasing abundance of aquaporins, and accumulating cryoprotectants. Most freeze-tolerant insects freeze at relatively high temperatures to avoid the rapid formation of ice crystals that can cause injury. In freeze-tolerant species, no correlation exists between the supercooling point (SCP; the point at which water freezes) and winter temperatures. However, repeated freezing can lower the SCP and increase cryoprotectant concentrations. Typical SCPs for freeze-tolerant insects are below -40°F (Figure 1).
Freeze avoidance is the most common adaptation to cold temperatures. Freeze-susceptible insects lower their SCP by producing antifreeze and heat-shock proteins and accumulating cryoprotectants to avoid freezing. They do not produce ice-nucleating agents but instead rid their bodies of ice nucleators such as food particles or microbes in the digestive tract. In freeze-susceptible species, lower SCPs are correlated with more severe winter temperatures. Mortality can still occur in freeze-susceptible insects if temperatures go below the SCP. Typical SCPs for freeze-susceptible insects are between -4°F and -40°F (Figure 1).
Rapid cold hardening:
Diapause requires a more prolonged response to cold temperatures, but insects can adapt on very short time scales as well. Rapid cold hardening (RCH) allows for almost instantaneous cold tolerance for brief exposures (minutes to hours) to non-lethal temperatures, particularly when the insect is not yet in a cold-hardy state. Survival from RCH improves tolerance to more severe temperatures later.
Because the SCP of most overwintering insects is well below the lowest winter temperature, the biggest threat to survival is the cumulative impact of non-freezing chill injury, which is a function of temperature and duration of exposure. This type of injury occurs above the freezing point but below normal developmental thresholds when insects are typically in a chill-coma. Most insects enter a reversible chill-coma at or below 50°F. Injury can be reduced or reversed if a cold period is interrupted by brief warm temperatures (as short as 5 minutes). This reduced mortality is not necessarily due to reduced chill injury, but rather a repair of chill injury during the warm spells.
When mortality does not occur, exposure to cold temperatures has sublethal effects such as reduced growth, development, and reproductive potential. Fluctuating temperatures can allow development outside of the normal critical limits; however, development is typically delayed because direct cold injuries need to be repaired. If the lowest temperature does not cause injury, development may actually be accelerated. Fluctuating temperatures may reduce fecundity if stressful temperatures are experienced. Repeated cold exposures can increase survival and lower supercooling points relative to sustained warm or cold temperatures. Repeated freeze-thaw cycles have more mixed results; survival is generally decreased but depends on species and increases with increasing recovery time.
Overwinter – to survive the winter months in an arrested state of development, typically in a location that protects the insect from cold winter weather. Examples: migration, inside buildings, under rocks or tree bark, in leaf litter or soil.
Dormancy – the temporary suspension of development, growth, and physical activity during a part of an insect’s life cycle.
Quiescence – a short period of dormancy directly induced by adverse conditions that can be quickly reversible when favorable conditions return.
Diapause – a hormonally regulated state of metabolic activity that is genetically determined to occur during a certain stage. Growth and development are reduced, and insects have increased resistance to extreme conditions and reduced activity or altered behavior. Facultative diapause occurs in response to environmental cues, while obligatory diapause occurs during each generation regardless of environmental cues.
Freeze tolerance – the ability to tolerate frozen tissue by limiting the presence or location of ice in the body.
Freeze avoidance – the evasion of freezing by lowering the point at which water freezes in the body.
Supercooling – when water cools below the freezing point without changing to ice. If no particle is present to allow crystallization, water can cool to -36.5°F without freezing.
Supercooling point – the point at which a supercooled solution freezes.
Chilling – cooling without freezing, usually above 32°F.
Rapid cold hardening – a response to cold temperature that allows insects to survive short exposures by quickly decreasing their lethal temperature.
Cryoprotectants – examples include sugar alcohols such as glycerol, sorbitol, and inositol, trehalose, proline, and glucose.
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