In animal dormancy, diapause is the delay in development in response to regular and recurring periods of adverse environmental conditions.[The Insects; Structure and Function, 4th Edition. R.F. Chapman, Cambridge University Press, 1998. , p 403.][Tauber, M.J., Tauber, C.A., Masaki, S. (1986) Seasonal Adaptations of Insects. Oxford University Press] 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 , especially .
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,
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.
[Blanckenhorn, W. U. (1998). Altitudinal differentiation in the diapause response of two species of dung flies. Ecological Entomology 23, 1-8.] 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.
[The Insects; Structure and Function, 4th Edition. R.F. Chapman, Cambridge University Press, 1998. , pp. 403–404] 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:
[The Insects; Structure and Function, 4th Edition. R.F. Chapman, Cambridge University Press, 1998. , p 404.]
|
| Diapause |
late larval[Bulletin of Entomological Research (1976), 66:75–79 Cambridge University Press, Copyright © Cambridge University Press 1976, Diapause of the southwestern corn borer, Diatraea grandiosella Dyar (Lepidoptera, Pyralidae): effects of a juvenile hormone mimic: G. M. Chippendalea1 and C.-M. Yina1a1, Department of Entomology, University of Missouri, Columbia, Missouri 65201, U.S.A.] |
| pupa |
| pupa |
| pupa |
| late adult |
| embryonic |
| late embryonic |
| adulthood |
| Acronicta rumicis | Knott grass moth | mid larval | mid larval |
| Codling moth | Codling moth | early to mid larval | mid larval |
| Gynaephora groenlandica | Arctic woolly bear moth | mid larval | mid larval[Makarova, O. L.; Sviridov, A. V.; Klepikov, M. A. (2013-04-01). "Lepidoptera (Insecta) of polar deserts". Entomological Review. 93 (2): 225–239.. ISSN 0013-8738.] |
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.
This sensitive stage may occur within the lifetime of the diapausing individual, or in preceding generations, particularly in egg diapause.
[Huffaker, C.B. and Gutierrez, A.P., Eds. 1999. Ecological Entomology. John Wiley & Sons, Inc.] 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.
Preparation
The preparation phase usually follows the induction phase, though insects may go directly from induction to initiation without a preparation phase.
During this phase, insects accumulate and store molecules such as
lipids,
proteins, and
carbohydrates. These molecules are used to maintain the insect throughout diapause and to provide fuel for development following diapause termination. Composition of the
cuticle may be altered by changing
hydrocarbon composition and by adding
lipids to reduce water loss, making the organism resistant to desiccation.
Diapausing
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.
[ The initiation phase begins when morphological development ceases.]
Maintenance
During the maintenance phase, insects experience lowered metabolism and developmental arrest is maintained.
Termination
In insects that undergo obligate diapause, termination may occur spontaneously, without any external stimuli.
Post-diapause quiescence
Diapause frequently ends prior to the end of unfavourable conditions and is followed by a state of from which the insect can arouse and begin direct development, should conditions change to become more favourable.
Regulation
Diapause in insects is regulated at several levels. Environmental stimuli interact with genetic pre-programming to affect signalling, endocrine pathways, and, eventually, metabolic and enzymatic changes.
Environmental
Environmental regulators of diapause generally display a characteristic season. In temperate regions, photoperiod is the most reliable cues of seasonal change.
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.
The corpora allata is responsible for the production of juvenile hormone (JH). In the bean bug, Riptortus pedestris, clusters of neurons on the protocerebrum called the pars lateralis maintain reproductive diapause by inhibiting JH production by the corpora allata.
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.
Diapause hormone regulates diapause in the eggs of the silkworm moth, Bombyx mori.
Tropical diapause
Diapause in the tropics is often initiated in response to biotic component rather than abiotic components.
Diapause in the tropics poses several challenges to insects that are not faced in temperate zones.
Aggregations are common among diapausing tropical insects, especially in the orders Coleoptera, Lepidoptera, and Hemiptera.
See also
Embryonic diapause
-
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.
