Diapause in Insects

Diapause is a period of suspended or arrested development during an insect's life cycle. Insect diapause is usually triggered by environmental cues, like changes in daylight, temperature, or food availability. Diapause may occur in any life cycle stage—embryonic, larval, pupal, or adult—depending on the insect species.

Insects inhabit every continent on Earth, from the frozen Antarctic to the balmy tropics. They live on mountaintops, in deserts, and even in the oceans. They survive frigid winters and summer droughts. Many insects survive such extreme environmental conditions through diapause. When things get tough, they take a break.

Diapause is a predetermined period of dormancy, meaning it is genetically programmed and involves adaptive physiological changes. Environmental cues aren't the cause of diapause, but they may control when diapause begins and ends. Quiescence, in contrast, is a period of slowed development that is triggered directly by environmental conditions, and that ends when favorable conditions return.

Types of Diapause

Diapause can be either obligatory or facultative:

• Insects with obligatory diapause will undergo this period of arrested development at the predetermined point in their life cycle, regardless of the environmental conditions. Diapause occurs in every generation. Obligatory diapause is most often associated with univoltine insects, meaning insects that have one generation per year.
• Insects with facultative diapause undergo a period of suspended development only when conditions require it for survival. Facultative diapause is found in most insects and is associated with bivoltine (two generations per year) or multivoltine insects (more than two generations per year).

Additionally, some insects undergo reproductive diapause, which is a suspension of reproductive functions in adult insects. The best example of reproductive diapause is the monarch butterfly in North America. The migrant generation of late summer and fall goes into a state of reproductive diapause in preparation for the long journey to Mexico.

Environmental Factors

Diapause in insects is induced or terminated in response to environmental cues. These cues may include changes in the length of daylight, temperature, food quality and availability, moisture, pH, and other factors. No single cue solely determines the start or end of diapause. Their combined influence, along with programmed genetic factors, controls diapause.

• Photoperiod: A photoperiod is the alternating phases of light and dark in the day. Seasonal changes to the photoperiod (such as shorter days as winter approaches) cue the start or end of diapause for many insects. Photoperiod is the most important.
• Temperature: Along with photoperiod, changes in temperature (such as an extreme cold spell) can influence the start or end of diapause. The thermoperiod, alternating phases of cooler and warmer temperatures, also influences diapause. Some insects require specific thermal cues to end the diapause phase. For example, the woolly bear caterpillar must endure a period of chilling to trigger the end of diapause and continuation of the life cycle.
• Food: As the growing season ends, the diminishing quality of their food sources may help trigger a diapause phase in an insect species. As potato plants and other hosts turn brown and dry, for example, Colorado potato beetle adults enter a state of diapause.

 Sources

• Capinera, John L., (ed.) Encyclopedia of Entomology. 2nd edition, Springer, 2008, New York.
• Gilbert, Scott F. Developmental Biology. 10th edition, Sinauer Associates, 2013, Oxford, UK.
• Gullan, P.J., and Cranston, P.S. The Insects: An Outline of Entomology. Wiley, 2004, Hoboken, N.J.
• Johnson, Norman F., and Triplehorn, Charles A. Borror and DeLong's Introduction to the Study of Insects. 7th edition, Thomson Brooks/Cole, 2005, Belmont, Calif.
• Khanna, D.R. Biology of Arthropoda. Discovery Publishing, 2004, New Delhi.