Diapause is a period in the life cycle of insects in which metabolic activity is suspended thanks to the secretion of neurohormones . This period is triggered by environmental conditions that are generally unfavorable for the species, such as changes in temperature, light or the ability to find food. Diapause can occur at any time in the insect’s life cycle, be it embryo, larva or adult; this also depends on the species.
On earth there are millions and millions of insects; some thrive and adapt to warmer tropical climates, while thriving in freezing Antarctica. Deserts and oceans are also teeming with arthropods, and they survive sudden temperature changes like winters and droughts. Many of these insects manage to survive extreme environmental conditions thanks to diapause.
The diapause period is genetically determined, which implies adaptive physiological changes. Environmental conditions are not the direct cause of diapause; however, its influence can control when it begins and when it ends. On the other hand, the spontaneous inactivity of an insect in the face of unfavorable changes in the environment develops progressively, and is subject to environmental conditions. The inactivity ends just when the habitat returns to favorable conditions.
Types of diapause
There are two types of diapause: obligatory and facultative.
Insects that must undergo a mandatory diapause undergo this genetically predetermined period of lethargic development at some point in their life cycle regardless of environmental conditions. This type of diapause is closely related to univoltine insects, that is, insects that have only one generation per year.
On the other hand, insects with diapause go through this period only when environmental conditions are not suitable, so they need this state of inactivity to survive. This type of diapause is found in most insects and is associated with bivoltine and multivoltine insects; are those that have two generations annually, in the case of bivoltines, and more than two generations in the case of multivoltines.
There is also another type of diapause, reproductive in this case, in which reproductive functions are suspended in adult insects. One such example is the reproductive diapause of the Monarch butterfly. The generation that is supposed to migrate between late summer and early fall enters reproductive diapause to prepare for the long journey from the United States to Mexico.
Environmental factors
Environmental conditions play an important role in the onset and end of diapause. They can be changes in temperature, food available in the environment or its quality, including the duration of sunlight and humidity, but this is not decisive. These environmental cues, combined with the genetic programming of the species, control diapause.
The photoperiod, which refers to the alternating phases of light and darkness during the day, is one of the most important factors, since it marks the beginning or end of the diapause of many insects. An example of photoperiod changes might be getting shorter days as winter approaches.
Temperature changes, together with the photoperiod, influence the beginning or end of diapause. The thermoperiod refers to the alternations of temperatures from warmer to colder, and vice versa. There are species that depend on these temperature changes to complete their diapause process. An example is caterpillars that, at the end of winter, despite the fact that the days are longer again, need favorable thermoregulation to begin the life cycle.
In some insect species, declining feeding can trigger diapause, which occurs as the growing season draws to a close. An example of this is the adult beetles: as the plantations dry out, they go into diapause waiting for them to flower again.
References
Balcells, E. (2015). Insects: Morphology and physiology. Available at: https://digital.csic.es/bitstream/10261/118117/1/Balcells_insectos.pdf
Rebolledo, R. (1995). Study of the behavior of Trogoderma granarium Everts (Coleoptera: Dermestidae) in diapause due to the absence of food. Available at: https://www.miteco.gob.es/ministerio/pags/Biblioteca/Revistas/pdf_plagas/BSVP-21-03-319-327.pdf
