In animal dormancy, diapause is the delay in development in response to regular and recurring periods of adverse environmental conditions. It is a physiological state with very specific initiating and inhibiting conditions. The mechanism is a means of surviving predictable, unfavorable environmental conditions, such as temperature extremes, drought, or reduced food availability. Diapause is observed in all the life stages of arthropods, especially insects.
Activity levels of diapausing stages can vary considerably among species. Diapause may occur in a completely immobile stage, such as the pupae and eggs, or it may occur in very active stages that undergo extensive migrations, such as the adult monarch butterfly, Danaus plexippus. In cases where the insect remains active, feeding is reduced and reproductive development is slowed or halted.
Embryonic diapause, a somewhat similar phenomenon, occurs in over 130 species of mammals, possibly even in humans, and in the embryos of many of the oviparous species of fish in the order Cyprinodontiformes.
Phases of insect diapause
Diapause in insects is a dynamic process consisting of several distinct phases. While diapause varies considerably from one taxon of insects to another, these phases can be characterized by particular sets of metabolic processes and responsiveness of the insect to certain environmental stimuli. For example, Sepsis cynipsea flies primarily use temperature to determine when to enter diapause. Similarly, Chrysoperla plorabunda lacewings regulate their reproductive cycle using daylight length, with adults entering reproductive diapause when there are less than 12-13 hours of daylight. Diapause can occur during any stage of development in arthropods, but each species exhibits diapause in specific phases of development. Reduced oxygen consumption is typical as is reduced movement and feeding. In Polistes exclamans, a social wasp, only the queen is said to be able to undergo diapause.
Comparison of diapause periods
The sensitive stage is the period when stimulus must occur to trigger diapause in the organism. Examples of sensitive stage/diapause periods in various insects:<br/>
{| class="wikitable"
|-
| Scientific name || Common name || Sensitive stage || Diapause
|-
|Diatraea grandiosella || Southwestern corn borer || early larval || late larval
|-
|Sarcophaga crassipalpis || Flesh fly || early larval || pupa
|-
|Sarcophaga argyrostoma || Flesh fly || mid to late larval || pupa
|-
|Manduca sexta || Tobacco hornworm || late embryonic (egg) to late larval || pupa
|-
|Leptinotarsa decemlineata || Colorado potato beetle || early adult || late adult
|-
|Bombyx mori || Silkworm || late embryonic (egg) to early larval || embryonic
|-
|Lymantria dispar || Spongy moth || late embryonic || late embryonic
|-
|Danaus plexippus || Monarch butterfly || early adulthood || adulthood
|-
|Acronicta rumicis
|Knott grass moth
|mid larval
|mid larval
|-
|Cydia pomonella
|Codling moth
|early to mid larval
|mid larval
|-
|Gynaephora groenlandica
|Arctic woolly bear moth
|mid larval
|mid larval
|-
|Cuterebra fontinella || Mouse botfly || mid larval || pupa
|-
|Nothobranchius furzeri
|turquoise killifish
|egg
|egg
|}
Induction
The induction phase occurs at a genetically predetermined stage of life, and occurs well in advance of the environmental stress. During this phase, insects are responsive to external cues called token stimuli, which trigger the switch from direct development pathways to diapause pathways. Token stimuli can consist of changes in photoperiod, thermoperiod, or allelochemicals from food plants. These stimuli are not in themselves favourable or unfavourable to development, but they herald an impending change in environmental conditions. Diapausing puparia of the flesh fly, Sarcophaga crassipalpis, increase the amount of cuticular hydrocarbons lining the puparium, effectively reducing the ability of water to cross the cuticle.
Initiation
Photoperiod is the most important stimulus initiating diapause. Insects may also undergo behavioural changes and begin to aggregate, migrate, or search for suitable overwintering sites.thumb|Overwintering [[monarch butterflies in diapause clustering on oyamel trees. One tree is completely covered in butterflies. These butterflies were located on a preserve outside of Angangueo, Michoacán, Mexico]]
Maintenance
During the maintenance phase, insects experience lowered metabolism and developmental arrest is maintained. Termination may occur at the height of unfavourable conditions, such as in the middle of winter. Over time, depth of diapause slowly decreases until direct development can resume, if conditions are favourable. Termination can also occur in specific time frames linked to reproductive periods, such as in the beetle Colaphellus bowringi: diapause ends for spring-reproducing beetles between late February and early April and for autumn-reproducing beetles between mid August and early October.
Post-diapause quiescence
Diapause frequently ends prior to the end of unfavourable conditions and is followed by a state of quiescence from which the insect can arouse and begin direct development, should conditions change to become more favourable. Depending on the season in which diapause occurs, either short or long days can act as token stimuli. Insects may also respond to changing day length as well as relative day length. Temperature may also act as a regulating factor, either by inducing diapause or, more commonly, by modifying the response of the insect to photoperiod. Food availability and quality may also help regulate diapause. In the desert locust, Schistocerca gregaria, a plant hormone called gibberellin stimulates reproductive development. During the dry season, when their food plants are in senescence and lacking gibberellin, the locusts remain immature and their reproductive tracts do not develop.
Neuroendocrine
The neuroendocrine system of insects consists primarily of neurosecretory cells in the brain, the corpora cardiaca, corpora allata and the prothoracic glands.
Prothoracicotropic hormone stimulates the prothoracic glands to produce ecdysteroids that are required to promote development. Adult diapause is often associated with the absence of JH, while larval diapause is often associated with its presence.
In adults, absence of JH causes degeneration of flight muscles and atrophy or cessation of development of reproductive tissues, and halts mating behaviour. The presence of JH in larvae may prevent moulting to the next larval instar, though successive stationary moults may still occur. In the corn borer, Diatraea gradiosella, JH is required for the accumulation by the fat body of a storage protein that is associated with diapause.
Diapause hormone regulates embryonic diapause in the eggs of the silkworm moth, Bombyx mori. DH is released from the subesophageal ganglion of the mother and triggers trehalase production by the ovaries. This generates high levels of glycogen in the eggs, which is converted into the polyhydric alcohols glycerol and sorbitol. Sorbitol directly inhibits the development of the embryos. Glycerol and sorbitol are reconverted into glycogen at the termination of diapause.
Tropical diapause
Diapause in the tropics is often initiated in response to biotic rather than abiotic components. For example, food in the form of vertebrate carcasses may be more abundant following dry seasons, or oviposition sites in the form of fallen trees may be more available following rainy seasons. Also, diapause may serve to synchronize mating seasons or reduce competition, rather than to avoid unfavourable climatic conditions.
Diapause in the tropics poses several challenges to insects that are not faced in temperate zones.
See also
- Eburia quadrigeminata, the species with the longest reported diapause among insects (up to 40 years).
- Polygonia c-album, whose larvae exhibit density-dependent polymorphism where one of two morphs is a diapausing phase.
- Nebria brevicollis, a carabid beetle who exhibits diapause behavior due to low food resource availability.