Insect overwintering is a fascinating process involving many physiological, epidemiological, bi¬ochemical and behavioral changes. The study of the overwintering process can offer an insight into the development of insects, as well as helps us predict the patterns of disease epidemic and crop destruction caused by some species. Insect winter ecology entails the overwinter survival strategies of insects, which are in many respects more similar to those of plants than to many other animals, such as mammals and birds. This is because unlike those animals, which can gen¬erate their own heat internally (endothermic), insects must rely on external sources to provide their heat (ectothermic).thus; insects have evolved a number of strategies to deal with the rigours of winter temperatures in places where they would otherwise not survive.
There are many places where an insect can overwinter. They include, but arent limited to, under rocks, in or under fallen trees or limbs, in leaf litter, under dead plants or grasses, in burrows, in holes dug into the side of a tree, under tree bark, or in the “folds” of tree bark. Typically though, we, as humans, usually see them in winter when they get into our houses or buildings. Although it is common for insects to get into a home to overwinter, most of the times we never know they are there. They get in spaces of windows or doors, in between walls, in attics, in cellars or base¬ments, under porches or decks, in storage sheds, or just about anywhere they can get out of the weather. Once they get inside they will sometimes maneuver to a warmer section of the house and, in doing so, can expose themselves out in the open where we can see them.
Three major strategies for winter survival have been evolved in the Class Insect due to their inability to generate significant heat metabolically. Migration is a complete avoidance of the temperatures that pose a threat. Migration in insects is different than in birds. Bird migra¬tion is a two-way, round-trip movement of each individual while in that of insects is one way round trips movement. Migration for this group of organisms as consisting of three parts first per¬sistent, straight line movement away from the natal area, 2nd Distinctive pre- and post-move¬ment behaviors and 3rd Re-allocation of energy within the body associated with the movement. Perhaps the best known insect migration is that of the monarch butterfly. It migrates from Canada to Mexico in August to September. The round trip journey is typically around 3,600 km in length. The longest one-way flight on record for monarchs is an astonishing 3,009 km from Canada to Mexico. They use the direction of sunlight and magnetic cues to orient themselves during migration. The monarch requires significant energy to make such a long flight, which is provided by fat reserves. When they reach their overwintering sites, they begin a period of low¬ered metabolic rate. Nectar from flowers procured at the overwintering site provides energy for the northward migration. To limit their energy use, monarchs congregate in large clusters in or¬der to maintain a suitable temperature.
Another common winter migrant insects the Green Darner in September and migrate south. Reasons for migration are not fully understood since there are both resident and migrant populations. The common signal for migration southward in this spe¬cies is the onset of winter. If an insect cannot migrate, then it must stay and deal with the cold temperatures in one of two ways including avoid freezing lethal freezing occurs when insects are exposed to temperatures below the melting point (MP) of their body fluids. So insects unable to migration should adopt certain mechanisms to avoid freezing of intra and extracellular fluids. The stage of development at which an insect over-winters varies across species, but can occur at any point of the life cycle like egg, pupa, larva, and adult.
Super cooling is the process by which water cools below its freezing point without changing phase into a solid, due to the lack of a nu¬cleation source. Water requires a particle such as dust in order to crystallize and if no source of nucleation is introduced, water can cool down to -42°C without freezing. Removal of ice-nucle¬ating material from the gut can be achieved by cessation in feeding, clearing the gut and remov-ing lipoprotein ice nucleates (LPINs) from the heamolymph and in some species, by the shed¬ding of the mid-gut during molting.
Freeze avoidance involves both physiological and biochemical mechanisms. The problem of climate variability is a factor, especially for insects that use chemicals to stave off the cold or control internal ice crystallization. These insects use glycerols (antifreeze), primarily, to stop ice from forming at the cellular level, as this would destroy tissue and lead to death. The problem is that when the temperature rises enough to bring them out of diapause then their body essentially thinks its spring and they re-animate and wander around searching for food, as opposed to stay-ing dormant the entire winter season, which is the norm. But if this warming trend only lasts a few days then turns cold again the insect will go back into diapause. This is very hard on insects if it happens several times during a winter.
Freeze tolerant organisms limit super cooling and initiate the freezing of their body fluids at rel¬atively high temperatures. Freeze tolerance is also more prevalent in insects from the Southern Hemisphere (reported in 85 per cent of species studied) than it is in insects from the Northern Hemi¬sphere (reported in 29 per cent of species studied) because sudden cold snaps occurred in southern hemisphere as compared to predictable seasonal changes in southern and northern hemisphere respectively.
Examples of freeze tolerant insects include the woolly bear, the flightless midge and the alpine cockroach. Chill tolerance of insect represents a vital temperature re¬sponse that markedly influences their ability to persist in cold environments. Susceptibility to chill tolerance is associated with the inability to maintain ion and water homeostasis which causes muscular dysfunction and ultimately chill-injury and death. However, little is known about the underlying physiological mechanisms of chill tolerance in insects.
The overall objec¬tive of this project is to identify the key physiological processes determining chill tolerance, to investigate how these processes are influenced by acclimation and other environmental factors and to investigate their evolutionary origin and molecular basis of chill tolerance. Truly cold hardy insects can either tolerate freezing of their body fluids or survive winter by maintaining their body fluids in a super-cooled state. Many temperate, sub-tropical and tropical insects in¬cluding bees, grasshoppers, mosquitoes and flies do, however, not possess any significant cold tolerance and are therefore referred to as chill susceptible/tolerant. The physiological corre¬lates of chill tolerance are poorly understood but seem to be related to an inability to maintain trans-membrane ion homeostasis at low temperatures. Dissipation of ion gradients and resting membrane potential cause a disruption of nervous and particularly muscular-excitability and as a consequence, the chilled insect loses its ability to maintain posture and enters a state termed chill coma.
The writers are associated with the Department of Plant Pathology, University of Agriculture Faisalabad, Pakistan. They can be reached at < Zeeshansattar2206@yahoo.com>