Insect winter ecology describes 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. Unlike those animals, which can generate their own heat internally (endothermic), insects must rely on external sources to provide their heat (ectothermic). 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 threatens 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. Two broad strategies for winter survival have evolved within Insecta as solutions to their inability to generate significant heat metabolically. Migration is a complete avoidance of the temperatures that pose a threat. An alternative to migration is weathering the cold temperatures present in its normal habitat. Insect cold tolerance is generally separated into two strategies, freeze avoidance and freeze tolerance.
Migration
Migration of insects differs from migration of birds. Bird migration is a two-way, round-trip movement of each individual, whereas this is not usually the case with insects. As a consequence of the (typically) short lifespan of insects, adult insects who have completed one leg of the trip may be replaced by a member of the next generation on the return voyage. As a result, invertebrate biologists redefine migration for this group of organisms 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 This definition allows for mass insect movements to be considered as migration. Perhaps the best known insect migration is that of the monarch butterfly. The monarch in North America migrates from as far north as Canada southward to Mexico and Southern California annually from about August to October. The population east of the Rocky Mountains overwinters in Michoacán, Mexico, and the western population overwinters in various sites in central coastal California, notably in Pacific Grove and Santa Cruz. The round trip journey is typically around 3,600 km in length. The longest one-way flight on record for monarchs is 3,009 km from Ontario, Canada to San Luis Potosí, 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 lowered 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 order to maintain a suitable temperature. This strategy, similar to huddling in small mammals, makes use of body heat from all the organisms and lowers heat loss. Another common winter migrant insect, found in much of North America, South America, and the Caribbean, is the green darner. Migration patterns in this dragonfly species are much less studied than those of monarchs. Green darners leave their northern ranges in September and migrate south. Studies have noted a seasonal influx of green darners to southern Florida, which indicates migratory behavior. Little has been done with tracking of the green darner, and reasons for migration are not fully understood since there are both resident and migrant populations. The common cue for migration southward in this species is the onset of winter.
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. The general strategy adopted by insects differs between the Northern Hemisphere and the Southern Hemisphere. In temperate regions of the northern hemisphere where cold temperatures are expected seasonally and are usually for long periods of time, the main strategy is freeze avoidance. In temperate regions of the southern hemisphere, where seasonal cold temperatures are not as extreme or long lasting, freeze tolerance is more common. However, in the Arctic, where freezing occurs seasonally, and for extended periods (>9 months), freeze tolerance also predominates.
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.
… excerpt ends here. Continue reading the full article.
