Diapause in Insects - Meaning With Examples

Diapause is a strategy in which insects can suspend their development or growth. They do so by entering a phase of inactivity for a prolonged time in order to endure a season when survival is more difficult. Said inactivity can occur at any stage of the insect's lifecycle, whether during the embryonic, larval, pupal, or adult stages. However, it will depend on the species in question and its genetic determination.

Diapause occurs due to a change in the endocrine system. Certain glands stop secreting the hormones they would normally release. This is regulated by brain neurosecretory cells going into quiescence, i.e. dormancy or inactivity. In pupal and larval stages, this period is controlled by the hormone ecdysone. In adults, it is regulated by retention of the corpus allatum gland which produces juvenile hormones.

The process of diapause helps many animals to survive harsh conditions. It is both useful for insects going from warm areas to very cold environments and vice versa. The latter includes insects from temperate areas surviving a very hot summer. Insects, unlike many mammals, must rely heavily on this process to overcome challenging environmental conditions.

This process is typically frequent in insects, but it does not only occur in this group. It can also be observed in plants and in animals under the name of dormancy, but it does not occur in the adult stages of mammals. It is possible it occurs in the embryonic stage of humans, but further research is required to know for sure.

Learn more about arthropod survival with our article on how do insects breathe?

The series of processes that are activated by the arrival of adverse conditions can be divided into the following chronological phases:

1. Induction: for the diapause to be activated, the insect organism will perceive the changes that announce a nearby severe change in the environment via various stimuli. For example, they can capture the change in the duration of sunlight that marks the arrival of winter. They do not respond immediately to adverse conditions, but rather anticipate the signs leading up to them in order to prepare in advance. This anticipation is crucial for their survival.
   
   
2. Preparation: the insects begin to stock up on reserve material that will serve them during the inactivity phase. They accumulate lipids, carbohydrates, proteins and other essential molecules. In some cases this phase may be absent. However, when present, it significantly enhances their chances of enduring harsh conditions.
   
   
3. Initiation: marks the end of the preliminary stages as the organism stops physiologically. There may be changes in color, enzyme production or behavioral changes. These changes are often subtle yet vital for the insect's adaptation.
   
   
4. Maintenance: development is arrested and sensitivity to external factors decreases to prevent the diapause from ending. This phase ensures that the insect remains dormant until conditions are favorable again.
   
   
5. Termination: after a stimulus alerts the dormant insect or after a certain period of time, the diapause gradually comes to an end. This occurs a little before the unfavorable conditions end completely. It allows insects to gradually resume activity and take advantage of optimal conditions.
   
   
6. Quiescence: since diapause does not end along with severe conditions, this state of quiescence and rest functions as a transition. Their metabolism is not as slow as before. It is more of a lethargy from which the insect can wake up when the conditions are already optimal to resume their normal life. This phase acts as a buffer, ensuring a smooth transition back to normalcy.

We can find different types of diapause according to certain criteria:

According to the relationship with the environment

• Obligatory diapause: the environment does not play any role and it occurs forcibly at some stage of the insect's cycle. This type of diapause is often genetically predetermined.
  
  
• Facultative diapause: in this case it is activated by adverse environmental conditions. Such flexibility allows insects to adapt to unpredictable climatic changes.

According to the stage

• Developmental delay: present in the egg. This delay helps in synchronizing the hatching with favorable conditions.
  
  
• Yolk accumulation: in the egg. This accumulation serves as a nutritional reserve.
  
  
• Without feeding the organism: in the larva and in the adult. This adaptation minimizes energy expenditure.
  
  
• Stage extension: in the egg, pupa and larva. Extending these stages ensures survival through adverse periods.
  
  
• Delay in reproduction: in the adult. This postponement aligns reproduction with optimal conditions for offspring survival.
  
  
• Stasis or lack of movement: in the adult, larva or pupa. This stasis reduces energy consumption significantly.

Although quiescence is part of diapause, both are considered different processes:

• Quiescence is a process of torpor where the insect is not completely inactive, while diapause is stricter in the sense that there is a physiological stoppage forced by environmental conditions. Quiescence allows for a quicker response to favorable changes, while diapause ensures survival over longer adverse periods.
  
  
• Quiescence can be terminated at will by the animal, but diapause cannot. This distinction highlights the flexibility quiescence offers compared to the rigidity of diapause.

To finish, we indicate some examples of diapause in biology:

• Diapause in mosquitoes: despite looking forward to it, we know there are negative consequences of the arrival of summer. Mosquitoes are one of the worst for many people. If in winter we do not see these biting insects it is because they have gone through diapause. In this case, the state of inactivity can occur at any stage and is activated by the modification of sunlight. This adaptability allows mosquitoes to thrive seasonally despite adverse conditions.
  
  
• Diapause in corn earworm (Helicoverpa zea): they enter this phase when they are in the pupal state, induced by photoperiods of 10 hours. This precise timing ensures that the pupae remain dormant until conditions are suitable for emergence.
  
  
• Delayed fertilization in mammals: in this case, the spermatozoa accumulate so that they can be used later, when the environmental conditions are more convenient for the birth of the offspring. This strategy, although not exactly diapause, mirrors the timing advantages diapause provides in insects.
  
  
• Seasonal diapause in mammals: embryos can be retained within the mother. For example, those who were conceived in autumn and should be born in winter will do so until spring so that they do not have to go through this harsh season. Although not true diapause, this reproductive strategy ensures offspring survival.

There are various types of dormancy in nature, with diapause being only one of them. Although mammals do not go through diapause, there are periods of necessary dormancy when the seasons change. Learn more with our article on different types of hibernating animals.

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