Caterpillars in Winter: Do They Hibernate or Stay Active?

Winter cold can be harsh on any animal, but more so on the tiny and delicate caterpillars. Despite this, these invertebrates survive the harsh snow and freezing winter temperatures to triumphantly grace the spring and summer weather as beautiful butterflies. 

Caterpillars hibernate to counter the harsh winter cold and don’t stay active. Before winter sets in, caterpillars sense signs of extreme weather and begin to prepare for it. When weather becomes warmer, they slowly “wake up” from hibernation.

This article will tell you interesting details about caterpillar hibernation in winter by answering 4 important questions: 

• Why do caterpillars hibernate in winter? 
• How do caterpillars avoid freezing?
• Where and for how long do caterpillars hibernate?
• Which are the stages of hibernation?

Let’s find out just what the heck these little critters do…

Why Caterpillars Hibernate in Winter

Hibernation, also known as diapause, is not a normal stage in the growth process of caterpillars, but rather a response to environmental changes. It is a coping mechanism, a survival response to harsh environmental conditions.

While butterflies (which is what caterpillars develop into in spring and summer) can survive the warm spring breeze and the heat of summer, the winter cold is extremely harsh on caterpillars. To survive, caterpillars hibernate to avoid freezing from the extreme cold, which would otherwise compromise their survival. 

But there is another reason caterpillars hibernate – winter has little food for caterpillars since most of them are herbivores, and the foliage from trees is shed in fall.

To survive the food shortage, caterpillars eat in bulk to store up food for the hibernation period. It would also be difficult for caterpillars to go in search of food while temperatures are at freezing point. All of this means that caterpillars have to activate antifreeze mechanisms during hibernation.

How Caterpillars Hibernate to Counter Freezing

Because caterpillars cannot naturally adapt to extreme cold, hibernation is crucial for them to survive the winter, which they do using two mechanisms: freeze tolerance and freeze avoidance. Both mechanisms refer to the antifreeze process that caterpillars activate.

Basically, they either deal with the cold, or keep away from it totally.

Freeze tolerance

Caterpillars that adopt this mechanism synthesize their energy and food supply in autumn and initiate freezing when temperatures are around -4°C (24°F) up to -10°C (50°F). By the time winter hits hardest, the water in the caterpillar’s body has all been moved to the outer cells to create a protective coating that insulates the internal cells from freezing. If the internal cells freeze, the caterpillar’s survival is compromised. 

Freeze avoidance

Caterpillars using freeze avoidance eliminate body substances that would freeze, such as the gut content. These caterpillars do binge feeding before winter sets in so that their body cells have stored enough sugars and nutrients to survive during the hibernation period, which can last for the longest. 

Hastened maturity

In some cases, caterpillars in colder climates may opt to evade hibernation and hasten maturity into the adult stage. This “decision” sometimes results in the seasonal morphing or mutated forms of a species that do not fit in the previous or the consequent normal stages of the insect. 

Caterpillars from some species also reach maturity before winter and have to find a place to hibernate as adult butterflies. Examples of these are the Mourning Cloaks, the Milbert’s Tortoiseshell, and the White Admiral caterpillars.

So where exactly do hibernating caterpillars hide, and for how long?

Where and for How Long do Caterpillars Hibernate?

Caterpillars can choose different locations to hibernate depending on species differences. 

Some hibernate by burrowing under leaf-litter at the base of plants while others build cocoons that wrap them all through. And others still will tuck into log cracks, hollow trees, underneath tree barks, under rubble, in rock cracks or roof corners of unheated old buildings. Staying in low temperatures is crucial for preserving the stored up energy and nutrients throughout the hibernating period.

Usually, caterpillars have seasonal hibernation that lasts as long as the cold winter season (around 5/6 months in most cases). By the end of the last month, when days get warmer, caterpillars awaken to mate and produce eggs for the next generation of butterflies.

However, some species can hibernate for longer periods when prolonged extreme temperatures are perceived. Some studies have indicated that caterpillars can undergo diapause for up to two years to alleviate climate uncertainty. Nonetheless, longer periods of hibernation come with risks of predation, maladies, parasitism, and depletion of the stored energy, all possible threats to the caterpillar’s subsistence beyond hibernation.

Wherever and for whatever period caterpillars hibernate, the hibernation period is a long process with precise phases.

Stages of Hibernation in Caterpillars

The response activated by caterpillars to environmental signs of the approaching winter happens in 3 successive stages.

Stage 1: Pre-diapause stage

During this stage, caterpillars begin to detect environmental cues on the approaching winter. This could be the change in temperature or the number of daylight hours. Fewer daylight hours announce the approach of winter and hence the immediate preparation for hibernation.

Pre-diapause imposes intense food consumption for energy, and nutrients storage and development may be slowed in hibernating caterpillars or hastened in hibernating butterflies.  

Stage 2: Diapause stage

By the end of the pre-diapause period, caterpillars move around in search of a suitable hibernation spot. Feeding and nutrients storage may still continue but stops once the caterpillar settles for a microhabitat. 

In the diapause stage, all feeding is stopped, and any metabolic processes are halted. The caterpillar ceases to grow and is in hibernation until the environmental signs of a warm climate are perceived. This means warmer days for a period long enough to assure the hibernating insect that winter is not going to return after a few days. Longer days are also an environmental sign that it is time to wake up from diapause.

Stage 3: Post-diapause stage

Once the caterpillar reads an assurance of the onset of spring and the approaching summer, accelerated resumption of normal metabolic and growth processes is ignited and sustained. This happens until the normal state is resumed.  

If extreme cold winter set in after a period of warmer days, the caterpillars risk freezing since they do not have the time to go through the entire process of preparation for another period of diapause.  

Favorable post-diapause climates facilitate mating, the laying of eggs for the new larva, and the eventual maturation of caterpillars into butterflies.  

Wrapping It Up (Kind of Like a Cocoon?)

Caterpillars hibernate in winter and do not usually stay active.

Scientists use the term diapause to describe hibernation in insects.

Hibernation in caterpillars and other insects is activated by the onset of environmental signs for the approaching winter. These could mean lower temperatures or shorter days.

Caterpillars and other insects survive extreme winter by directing body fluids to the outer cells of their body to create insulation against freezing the inner cells (freeze tolerance) or by getting rid of body substances that would freeze such as the gut content (freeze avoidance).

Some caterpillars hasten growth to hibernate as morphs or full-grown butterflies. 

Caterpillars hibernate by burrowing under leaves, hiding in log cracks and under loose tree barks, under rubble, or in corners of houses that are not heated. 

The hibernation period lasts for 5/6 months from the end fall through winter to the onset of spring. Some caterpillars may hibernate for up to 2 years to evade seasonal uncertainties. 

Diapause happens in three stages: pre-diapause, diapause, and post-diapause stages.

When you notice the beautiful butterflies adorning the pleasant spring weather, then you know that hibernation for caterpillars has come to an end.

References

Gill, H. K., Goyal, G., & Chahil, G. (2017). Insect diapause: a review. J. Agric. Sci. Technol, 7, 454-473. 

Bale, J. S., & Hayward, S. A. L. (2010). Insect overwintering in a changing climate. Journal of Experimental Biology, 213(6), 980-994. 

Nylin, S. (2013). Induction of diapause and seasonal morphs in butterflies and other insects: knowns, unknowns and the challenge of integration. Physiological entomology, 38(2), 96-104. 

Salman, M. H., Bonsignore, C. P., El Fels, A. E. A., Giomi, F., Hodar, J. A., Laparie, M., … & Battisti, A. (2019). Winter temperature predicts prolonged diapause in pine processionary moth species across their geographic range. PeerJ, 7, e6530. 

Koštál, V. (2006). Eco-physiological phases of insect diapause. Journal of insect physiology, 52(2), 113-127.