Insects in Winter
When the leaves begin to fall and winter starts to creep in, we're usually so consumed with our own needs that it's easy to forget all the animals and insects left outside to fend for themselves.
In the fall they disappear, and then in the spring, they appear again. But what exactly happens to all the insects during the cold, snowy, icy winter months?
No matter how much you might dislike insects, you have to give them credit for surviving such harsh conditions alone. We have houses, heaters, thick layers of clothing, and lots of things that make surviving winter much easier.
As we'll learn, insects have some survival tricks up their sleeves as well.
Insect Winter Ecology
Insect winter ecology is the study of how insects survive through the winter months. In a lot of ways, insects behave more like plants than animals, in that they can't generate their own heat internally.
Instead, insects need to either find an external source of heat, or be able to adapt themselves to live without it. That narrows overwintering insects down to two distinct classes: those that can survive being frozen (freeze-tolerant), and those that can't (freeze avoidant.)
Two Main Types of Overwintering Insects
For the most part, insects avoid freezing and instead act to combat the possibility of ice crystallization inside their bodies. However, a few species have adapted to be able to survive even after ice has crystallized their bodily fluids.
Freeze avoidant insects tend to occur more in the Northern Hemisphere. This is because weather patterns are more predictable, and winters generally last much longer.
Freeze tolerant insects, on the other hand, occur mostly in the Southern Hemisphere. It's believed these insects have adapted to the greater climate and weather variability of these regions, having to adapt to both extreme cold snaps and unseasonably warm weather.
In regions of the Southern Hemisphere that have predictable, long winters (like Antarctica), insects tend to be freeze-avoidant as well.
1. Freeze-Avoidant Insects
These insects will die if their bodily fluids freeze. Their methods for surviving the winter involve keeping their bodily fluids liquid (as opposed to frozen.) This is accomplished in a few different ways, such as supercooling and the use of agents like cryoprotectants and antifreeze. Most insects are freeze avoidant, but a few species can survive after their bodily fluids have frozen or begun to crystallize.
A few freeze-avoidant insects:
- Black ant (Lasius niger)
- Ladybug (Coccinellidae)
- Monarch butterfly (Danaus plexippus)
2. Freeze-Tolerant Insects
These insects can survive after ice has formed in their tissues and bodily fluids. They've adapted to be able to control where, when, and how ice forms in their bodies. For example, extracellular freezing is avoided by these insects, since this removes water from cells through osmosis. Instead, the production of ice-nucleating proteins allows these insects to direct what freezes, where, and when.
A few freeze-tolerant insects:
- Alpine cockroach (Celatoblatta quinquemaculata)
- Flightless midge (Belgica antarctica)
- Woolly bear moth (Pyrrharctia isabella)
Methods For Winter Survival
Different insects overwinter in different stages of development (larva, nymph, egg, pupae, or adult.) Each species has found the stage and method of overwintering that works best for them. Most insects use a combination of methods to survive the winter months.
Insect migration is defined a little more loosely than when other animals migrate. This is because the short lifespan of most insects renders them unable to return round-trip.
Instead, when they arrive at their destination they stay, while a member of the next generation makes the return trip, usually in the spring.
The monarch butterfly is famous for its annual migration, while a little lesser-known migrant is the green darner dragonfly.
In order to crystallize into ice, water needs a nucleating particle, such as a speck of dust. Some insects select dry hibernation sites that cannot form ice due to the absence of a nucleating particle.
To further ensure they don't freeze, these insects need to lower the freezing point of their bodily fluids. With no nucleation particle present, the bodily fluids can supercool to temperatures below their freezing point while still remaining a liquid.
In order to remove nucleation particles in their bodies prior to winter, these insects will stop eating and will clear the gut of all ice nucleators before winter hits.
The formation of ice crystals in extracellular fluid causes cells to lose water through osmosis. This can negatively affect cell structure and cause harm to the insect.
To combat this, many insects have high levels of solutes like glycerol in their bodies, composing up to 20% of the insect's mass. Glycerol is evenly distributed inside and outside the cells, decreasing ice formation outside the cells and dehydration within the cells.
While glycerol is by far the most common cryoprotectant, other polyols in this category include sorbitol, mannitol, and ethylene glycol.
Antifreeze proteins (AFPs) bind to and inhibit the growth and recrystallization of ice that would otherwise be fatal.
There are two types of AFPs in insects, Tenebrio and Dendroides, which are both in different insect families. Tenebrio is found in beetles, and Dendroides is found in butterflies and moths.
For ice crystals to grow, they must have a nucleus on which to begin formation. Insects with ice-nucleating proteins can regulate the formation of ice crystals in their bodies.
These insects avoid the supercooling that other insects use and instead initiate ice formation at higher temperatures. In this way, they can decrease the rate of ice growth and gradually adjust to the pressures caused by ice formation over time.
When the optimal temperature meets the ice-nucleating protein, freezing is initiated, and ice spreads throughout the insect's body.
Diapause is used as a means to survive predictable, recurring, unfavorable environmental conditions, such as temperature extremes (winter.) It's considered a physiological state of dormancy with very specific initiating and inhibiting conditions. It's controlled by day length, temperature, and the genetic makeup of the population.
This is a pre-programmed genetic (and hormonal) state of suspended or arrested development marked by a slowed metabolic state. The degree to which each type of insect is affected varies.
Some remain somewhat active, but feeding is reduced and reproductive development is slowed or halted. Others completely stop all growth and development until diapause ends.
This state of dormancy occurs after diapause. Unlike diapause, quiescence does not involve physiological changes.
This interim phase allows insects to withstand harsh conditions, but ready themselves for quick development as soon as conditions are favorable.
As mentioned before, different insects overwinter in different stages of development (larva, nymph, egg, pupae, or adult.) In addition, they "hide" in areas that ensure the likelihood of their survival in the spring. These include:
- Burrowing under the mud and into the soil
- Hiding inside decaying logs and under tree bark
- Hiding inside tree holes and under rocks
- Hiding inside plant stalks and stems
- Aggregating or clustering in sheltered areas
- Coming into your home/business for warmth
What You Can Do to Help
While not everyone appreciates the importance of insects in the big scheme of things, the fact is that without them we wouldn't be able to survive. Although sometimes it's difficult to imagine they're important (especially when they're biting you), they are a key part of the network of life on Earth.
The benefits of insects in your garden are too many to list, but suffice it to say they keep both animals and plants in equilibrium. From soil aeration to food for other animals, to pollination, a healthy planet has a vast array of healthy insects in it.
We have an insect hotel in our yard that has made a noticeable difference in the pollination levels in the garden. Anyone who likes to grow flowers or vegetables should have a bug hotel to ensure robust pollination in the spring and fall.
How to Build an Insect Hotel
It's easy to build a simple "bug hotel" or "insect hotel" with hollow bamboo, sticks, pine cones, straw, and some thicker sticks with holes drilled down the middle. Place in a wood frame and cover with metal mesh, as pictured above. Put the insect hotel out of direct sun and in a sheltered location.
Aside from making them, they're also easy to buy online and are pretty inexpensive. It doesn't have to be big. Mine is about 8" x 12" (20 cm x 30 cm) and cost about USD $15. The larger the garden/yard, the more insect hotels you'll need. But one is always better than none!
Sources and Further Reading
Blake, E. (2018, July 12). How Do Insects Survive the Winter? Not the Way We Thought. Retrieved November 2, 2018, from https://www.cbc.ca/news/canada/north/antifreeze-protein-study-insects-1.4743408
Brockbank, K., Campbell, L. H., Vu, H., & Duman, J. G. (2015, July 14). A Lesson from Nature for Controlling Ice During Organ Cryopreservation by Vitrification. Retrieved November 2, 2018, from https://www.sciencedirect.com/science/article/pii/S0011224015001339
Brueck, H. (2017, November 27). Bugs Survive the Winter Through a Trick Straight out of Science Fiction. Retrieved November 2, 2018, from https://www.businessinsider.com/how-insects-survive-the-winter-2017-11
McDonough, M. (2011, February 3). How Insects Survive the Long, Cold Winter. Retrieved November 2, 2018, from http://www1.udel.edu/udaily/2011/feb/insects-winter-020311.html
Panko, B. (2017, February 15). What Do Insects Do in Winter? Retrieved November 2, 2018, from https://www.smithsonianmag.com/science-nature/what-do-insects-do-winter-180962183/
Ring, R. A. (2003, March 21). Freezing-Tolerant Insects with Low Supercooling Points. Retrieved November 2, 2018, from https://www.sciencedirect.com/science/article/pii/0300962982902675
Spider, I. (2016, December 18). The Woolly Bear Caterpillar in Winter. Retrieved November 2, 2018, from https://infinitespider.com/the-woolly-bear-caterpillar-in-winter/
Storey, K. (2018). The Storey Lab: Cell and Molecular Responses to Stress. Retrieved November 2, 2018, from https://www.kenstoreylab.com/research/freeze-avoidance-2/
Where Do Insects Go in the Winter? (n.d.). Retrieved November 2, 2018, from https://www.si.edu/spotlight/buginfo/winter
© 2018 Kate P