Insect winter ecology describes the overwinter survival strategies of insects. As summer turns to fall and temperatures drop, insects go through an abrupt change in lifestyle. Their activity and development largely cease, but they have strategies to survive the winter. Insect overwintering is frequently used to refer to a sort of hibernation undertaken by insects to survive the cold temperatures. Most insects have a single-generation life cycle, but there are some that require two or more years to fully develop. Insects can overwinter in any stage of their development unless they migrate. Many insects overwinter as adults, pupae, or eggs. This can be done inside buildings, under tree bark, or beneath fallen leaves or other plant matter on the ground. Unlike those animals, which can generate their own heat internally, insects must rely on external sources to provide their heat. Thus, insects persisting in winter weather must tolerate freezing or rely on other mechanisms to avoid freezing. Loss of enzymatic function and eventual freezing due to low temperatures daily threaten the livelihood of these organisms during winter. Not surprisingly, insects have evolved a number of strategies to deal with the rigors of winter temperatures in places where they would otherwise not survive.
Is it time to overwinter?
Insects show a range of responses to the approach of winter, including migration, diapause, polymorphism, and dormancy. Whatever response or set of responses they use, it is important that the developmental stage at which they overwinter is reached at the correct time of year. Insects prepare for winter in response to certain cues. These are, as you might expect abiotic, light and temperature being the two primary cues. Usually, the insect responds to a combination of the two to ensure that a sudden cold spell in the middle of summer does not send the insect into hibernation prematurely. Winter periods, especially in temperate regions, are characterized by decreasing temperatures and shortening day lengths. Photoperiod is, of course, the most reliable cue that an insect has to tell it when winter is on the way. Changes in day length follow a regular seasonal pattern, which, especially given the recent perturbations in weather patterns, is much more reliable than something like temperature or relative humidity. Aphids (Hemiptera: Aphidoidea), for example, respond to a decrease in day length together with a decrease in temperature. Once the insect has ascertained that winter is indeed on the way it has then to ensure that it will survive the winter.
Where to Overwinter?
A basic requisite is to find somewhere protected from the environment, although sometimes what an insect “sees” as protected is perhaps not the same thing that we think of as a nice cozy site to pass the winter. Some of the protective factors are not directly under the control of the insect, e.g., snow cover, which acts as a useful buffer against sudden temperature changes and exposure. Insects are, however, able to take advantage of these naturally occurring variables, even if not deliberately. For example, insects that pass the winter just below the soil surface are better protected (both from weather conditions and predation) when the soil is covered by a thick snow layer than when the ground is fully exposed. Some insects overwinter in litter layers while many others become thigmotactic as winter approaches (i.e., they develop a need to be in contact with each other or to be in confined spaces).
How do insects survive winter?
In some parts of the world winter temperatures can reach as low as -40°C and stay there for some considerable time – yet many insect species can survive these temperatures without shelter or fur coats! Although insects may not wear fur coats, they are able to take protective measures against severe weather conditions. Two broad strategies for winter survival have evolved within Insecta as solutions to their inability to generate significant heat metabolically. These include migration and cold tolerance.
It is a complete avoidance of the temperatures that pose a threat to the insect’s survival.
Migration for this group of organisms can be redefined in three parts:
- A persistent, straight line movement away from the natal area.
- Distinctive pre- and post-movement behaviors.
- Re-allocation of energy within the body associated with the movement
2. Cold Tolerance
Insects that do not migrate from regions with the onset of colder temperatures must devise strategies to either tolerate or avoid lethal freezing of intracellular and extracellular body fluids. Insects that survive subfreezing temperatures are generally classified as freeze-avoidant or freeze-tolerant.
Dangers of freezing
Intracellular ice formation usually causes cell death, even in freeze-tolerant species, due to physical stresses exerted as ice crystals expand. Ice formation in extracellular spaces increases the concentration of solutes in the extracellular fluid, resulting in the osmotic flow of water from intracellular spaces to extracellular spaces. Changes in solute concentration and dehydration can cause changes in enzyme activity and lead to the denaturation of proteins. If the temperature continues to decrease, the water that was drawn out of cells will also freeze, causing further cell shrinkage. Excessive cell shrinkage is dangerous because as ice forms outside the cell, the possible shapes that can be assumed by the cells are increasingly limited, causing damaging deformation. Finally, the expansion of ice within vessels and other spaces can cause physical damage to structures and tissues.
Insect cold tolerance is generally separated into two strategies, freeze avoidance and freeze tolerance
1. Freeze Avoidance
Freeze-avoidant insects cannot tolerate internal ice formation, so they avoid freezing by depressing the temperature at which their body fluids freeze. This is done through supercooling, the process by which a liquid cools below its freezing point without changing phase into a solid. The freezing process is usually initiated extracellularly in the gut, tissues, or hemolymph. In order to supercool to lower temperatures, freeze-avoidant insects will remove or inactivate ice-nucleating agents (INAs) such as food particles, dust particles, and bacteria, found in the gut or intracellular compartments of these organisms. Removal of ice-nucleating material from the gut can be achieved by cessation in feeding or clearing of the gut. Freezing can also be initiated by external contact with ice (inoculative freezing). Thus, some insects avoid freezing by selecting a dry hibernation site in which no ice nucleation from an external source can occur. Insects may also have a physical barrier such as a wax-coated cuticle that provides protection against external ice across the cuticle.
2. Freeze Tolerance
Freeze tolerance in insects refers to the ability of some species to survive ice formation within their tissues. Insects that have evolved freeze-tolerance strategies manage to avoid tissue damage by controlling where, when, and to what extent ice forms. In contrast to freeze-avoiding insects that are able to exist in cold conditions by supercooling, freeze-tolerant insects limit supercooling and initiate the freezing of their body fluids at relatively high temperatures. Freezing at higher temperatures is advantageous because the rate of ice formation is slower, allowing the insect time to adjust to the internal changes that result from ice formation. Most freeze-tolerant species restrict ice formation to extracellular spaces, as intracellular ice formation is usually lethal.
Time to stop?
Not surprisingly, this is done using the same cues as the decision to prepare for overwintering: photoperiod and temperature. It is not entirely as straightforward as it sounds. Although winter is, on average, cold in temperate regions, very warm spells in the winter are not uncommon. Thus, it would not be a good idea to emerge and then be covered in snow and ice. The emergence cues are usually a combination of photoperiod and temperature, although some insects have an internal clock that dictates an obligate time in diapause before they start to respond to rises in temperature or lengthening photoperiod. Interestingly enough, some insects require a period of cold before they can break diapause. Thus, there is a certain degree of variability in the responses seen, although it is not known why such variability exists.
This article has provided a basic overview of the main ecological and physiological strategies involved in insect overwintering. By understanding these strategies and the factors that influence them we are able to make predictions about insect populations and many pests forecasting schemes are now based on the overwintering stages of insects.