Insect winter ecology

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 generate their own heat internally (endothermic), insects must rely on external sources to provide their heat (ectothermic). Thus, insects sticking around in the winter, must tolerate freezing or rely on their surroundings to provide enough heat 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.

Survival strategies
Two major strategies for winter survival have evolved in the Class Insecta due to their inability to generate significant heat metabolically. The first, migration, is a complete avoidance of the temperatures that pose a threat. If an insect cannot migrate, then it must stay and deal with the cold temperatures in one of two ways. This cold hardiness is separated into two categories, freeze avoidance and freeze tolerance.

Migration
Migration in insects is different than in birds. Bird migration is a two-way, round-trip movement of each individual, whereas this is not usually the case with insects. The short lifespan of insects compared to birds means that the adult that made one leg of the trip will be replaced by a member of the next generation on the return voyage. As a result, invertebrate biologists have redefined migration for this group of organisms as consisting of three parts:


 * 1) A persistent, straight line movement away from the natal area
 * 2) Distinctive pre- and post-movement behaviors
 * 3) 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 know 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 3600 km in length. The longest one-way flight on record for monarchs is an astonishing 3009 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 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.

Freeze avoidance
Insects that do not migrate from regions with cold winters rely on either avoiding freezing or tolerating it. Freeze avoidance is the evasion of both intracellular and extracellular freezing. Due to their small size, insects are limited in the amount of water they are able to carry within their bodies, resulting in an ability to supercool quite easily. Supercooling is the process by which water cools below its freezing point without changing phase into a solid, due to the lack of a nucleation source. Water requires a particle such as dust in order to crystallize. If no source is introduced, water can cool down to -38.1°C without freezing. One method of freeze avoidance is the selection of a dry hibernation site in which no ice nucleation from an external source can occur. In addition, insects typically evacuate their gut before hibernating in order to remove any substance upon which ice may nucleate. In addition to physical preparation for winter, many insects also alter their biomchemistry. For example, some insects synthesize polyols and sugars, which reduce freezing temperature of the body. Glycerol is the most common cryoprotectant, although sorbitol, mannitol, and ethylene glycol may also be found. In addition to stopping ice formation, these polyols also help the insect to retain water and limit dehydration. Some insects also produce “thermal hysteresis proteins” that lower supercooling and freezing points and block ice formation by attaching onto the surface of forming ice crystals, preventing growth. The Arctic willow gall insect, for example, uses high levels of glycerol (20% of it’s body weight) to maintain supercooling to as low as -66°C. The wingless midge is able to survive the incredibly low temperatures of Antarctica in similar ways, and is the only true insect there.

Freeze tolerance
Insects that don’t migrate and are subject to conditions in which freezing is inevitable must be able to tolerate frozen tissue. This adaptation is dependent on limiting the presence or location of ice in the body. Freezing of water intracellularly is dangerous due to the rupture of cell membranes by ice crystals. Insects protect their body against this structural damage by permitting the nucleation of ice only in extracellular spaces. These insects can remain frozen and survive post-thaw at temperatures as low as – 50°C, though they begin to freeze at temperatures between -5º and -10°C. Ice nucleating agents (INA’s) are chemicals used by insects to promote the freezing of fluids extracellularly rather than intracellularly. In addition to INAs, freeze tolerant insects use polyols like those seen in freeze avoidant species. . Freeze tolerant insects may also use thermal hysteresis proteins to limit the growth of extracellular ice and prevent intracellular freezing

Locations of hibernating insects
Insects are well hidden in winter, but there are several locations in which they can reliably be found. Ladybugs practice communal hibernation by stacking one on top of one another on stumps and under rocks to share heat and buffer themselves against winter temperatures. The female grasshopper (family Tettigoniidae &#91;long-horned&#93;), in an attempt to keep her eggs safe through the winter, tunnels into the soil and deposits her eggs as deep as possible in the ground. Many other insects, including various butterflies and moths also overwinter in soil in the egg stage. Some adult beetles hibernate underground during winter; many flies overwinter in the soil as pupae. Other methods of hibernation include the inhabitance of bark, where insects nest more toward the southern side of the tree for heat provided by the sun. Cocoons, galls, and parasitism are also common methods of hibernation.

Aquatic insects
Insects that live under the water have different strategies for dealing with freezing than do terrestrial insects. Many insect species survive winter not as adults on land, but as larvae underneath the surface of the water. Under the water many benthic invertebrates will experience some subfreezing temperatures, especially in small streams. Aquatic insects have developed freeze tolerance much like their terrestrial counterparts. However, freeze avoidance is not an option for aquatic insects as the presence of ice in their surroundings may cause ice nucleation in their tissues. Aquatic insects have supercooling points typically around – 3º to – 7°C. In addition to using freeze tolerance, many aquatic insects migrate deeper into the water body where the temperatures are higher than at the surface. Insects such as stoneflies, mayflies, caddisflies, and dragonflies are common overwintering aquatic insects. The dance fly larvae have the lowest reported supercooling point for an aquatic insect at – 22°C.