Avoid Multiple Freeze/Thaw Cycles: Woolly Bear Caterpillars

Recently, I was walking through a local park near Promega, when I spotted my first woolly bear of the season. As this furry brown and black caterpillar wandered along in front of me, I recalled the old wives’ tale about the width of their stripes being indicative of the upcoming winter fury. Spotting that little fellow in the sidewalk piqued my curiosity, and I decided to see what I could discover about my friend, the woolly bear.

The “woolly bear” is actually the freeze-tolerant final instar caterpillar of the common tiger moth Pyrrharctia isabella. These nondestructive caterpillars feed on corn, asters, birches, and sunflowers among other things. They leave their plants as third instar larvae then look for a cool, dark place, usually underneath leaf detritus to overwinter. They survive the freezing winter by producing “antifreeze” in the form of glycerol. Their super cooling point (lowest temperature they can reach without freezing) is –6° to –8°C. In the spring the hibernating caterpillars become active, eat for a few days and then each one will spin a silk cocoon from which an adult moth will emerge in about one month. From spring to fall there are usually three generations produced, and they are incredibly common in North America.

As one web site stated, every fall thousands of woolly worms are killed by bicycles and cars as they start the phase of “wandering” before their hibernation, but that may be the least of their problems. A recent study published in the Journal of Experimental Biology by Marshall and Sinclair points out that climate change and decreasing snow cover may be exposing woolly bears and other freeze-tolerant insects to repeated freeze/thaw cycles. While a fair amount of work has been done to determine how these caterpillars survive a single freeze and over winter, not much research has looked at how animals fair with multiple freeze/thaw cycles. What are the energetic costs? Is there mechanical tissue damage?

The authors of this study sought to answer some of those questions by looking at the sub-lethal effects of multiple freeze/thaw cycles on the caterpillars. They measured parameters like SCP, glycerol concentration, reproductive ability, and immune function to see if repeated freeze/thaws were more detrimental to overall fitness than a single freeze event. In their study they tested three hypotheses: (1) If there is no cost of freezing, fitness will not differ among individuals exposed to multiple freeze/thaw events and those exposed to a single event; (2) If there is a cost associated with freezing or thawing, repeated freezing will reduce fitness compared to sustained freezing and control experiments, and (3) If the lowered metabolic rate experienced by frozen animals increases fitness and overwinter energy use is a determinant of fitness, then caterpillars that experienced multiple freeze/thaw cycles and those experiencing sustained freezing would exhibit greater fitness than control animals.

To test these hypotheses, the researchers collected caterpillars from Ontario during the months of September and October over three years. Single caterpillars were placed plastic containers and feed pinto beans until feeding stopped (mid-November). Food was removed and incubator temperatures were set to follow weekly maximum and minimums for London, Ontario. Caterpillars were maintained at 0°C in constant darkness, and experiments conducted after at least three weeks under these conditions. There were some modifications to the conditions during the feeding periods during year 2 and 3 of the study. The caterpillars were divided into three groups: Control, maintained at 0°C; Multiple cycle, frozen for 5 events of 7 hours; and Sustained, frozen for one event of 35 hours. The freezing temperature varied with each year to reflect annual variability in the SCP.

The authors report on several parameters in this study. The difference in mortality among the treatment groups was not statistically significant for every year, but when data for all three years were pooled mortality was significantly higher for the multiple cycle group (29.7%) than for the sustained freeze (10.3%) or control (13.1%) groups. Survival was defined as exhibiting a curling response to stimulus 24 hours after removal from freeze treatment. They were unable to detect any trends in overwinter metabolic rates as measured by rates of CO2 production. Repeated freezing did increase glycerol concentration over control and sustained freeze groups, but again no significant changes were detected regarding protein content, triglyceride content or water content among the groups. Repeated freezing was associated with significantly increased tissue damage to Malpighian tubules and hemocytes compared to sustained freezing, but the viability of fat body cells did not seem to be affected.

Repeated freezing actually appeared to enhance the ability of the caterpillars to survive a challenge when injected with pathogenic fungal spores, which is an intriguing finding. The authors note that it is possible that the damage to the tissues in these animals may have stimulated a wound response of some sort, which would lead to increased immune function in these animals. However, determining whether this finding is truly significant and explaining it requires more extensive investigation.

Clearly there is increased mortality and tissue damage associated with repeated freezing that is not seen in sustained freeze events. As snow covers become less predicable and a climate warms, understanding how multiple freeze/thaw events affect the physiology of freeze-tolerant organisms poses a fascinating set of questions for biologists to ask.

One thing is sure, I will never look at a woolly worm in quite the same way again.

ResearchBlogging.org
Marshall, K. and Sinclair, B. (2011). The sub-lethal effects of repeated freezing in the woolly bear caterpillar Pyrrharctia isabella Journal of Experimental Biology, 214 (7), 1205-1212 DOI: 10.1242/jeb.054569

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Michele Arduengo

Senior Content Developer / Social Media Lead at Promega Corporation
Michele earned her B.A. in biology at Wesleyan College in Macon, GA, and her PhD through the BCDB Program at Emory University in Atlanta, GA. Michele manages the Promega Connections blog. She enjoys leisure reading, writing creative nonfiction and knitting, and the occasional cross-country skiing jaunt.

19 thoughts on “Avoid Multiple Freeze/Thaw Cycles: Woolly Bear Caterpillars

  1. Hi ya

    I am a head teacher in scotland, i just watched a program about are wee wooly bear’s, on BBC UK, and found them amazing, can this be done in humans ? how do they survive temps of -8 and live ? totally amazing and thanks for your web site :)

    Martin

    • Hi Martin,
      Woolly Bears can make their own “antifreeze” that gives them a little freeze protection. There are other insects that do the same thing, but you are right, they are amazing creatures. As for humans, I am unfamiliar with any work in that area (doesn’t mean it’s not being researched). Glad you enjoyed the blog.
      Michele

  2. Hi,I currently have eight wooly bears right now,and I want to know whats best for them while hibernating.
    Is it bad to have them all in one big container while hibernating?
    Is it bad to disturb,or touch them while they are hibernating?
    What does the ”Antifreeze”look like?Is it red?

    I have them all in an old mouse container that we used for my mice,I covered the bottom with soil and then propped up some twigs for them to climb on and added three leave clovers.I do not know if I am taking care of them right,and also,do they actually eat the clovers?

    • No, that wouldn’t be correct. The “antifreeze” keeps them from freezing solid; it prevents ice crystals from forming in cells for instance. If they did “freeze solid” they would not recover.

  3. Thanks for your speedy reply, Michele. I was doing some research for a little book I am writing. You have cleared up my confusion on this amazing little creature.

  4. Is there any scientific study that would determine that their hybernation capability could be of benefit to humans. Such as slowing down metabolism for surgery, or treatment of cancer at the cellular level, or using their “antifreeze for those who travel in subzero weather

    • I don’t know of any specific research to link directly to human science applications. It’s possible that there may be some biomimicry work going on to look at the natural “antifreeze” compounds that are used by many organisms that have to overwinter in harsh conditions. This site http://www.ecomii.com/building/biomimicry indicates that a fish antifreeze was used to develop an antifreeze for transplant organs, but they give no peer-reviewed citations to back up their claim. Certainly that is one type of application that could come out of this research.

  5. Hi I don’t know id this is going to let me post or not but I have a woolly bear caterpillar right now named Toby and he looks as if he died but I feel like maybe he might be hibernating. Do you think maybe you would have a picture of what they look like when they are? I am just scared that he might be dead. But I just want to be 100% sure about it.

    • Hi Julie Ann,
      Woolly Bear caterpillars do hibernate, but I do not have any pictures of hibernating caterpillars, nor have I actually kept one over the winter. However, I am going to “crowdsource” your question to other scientists and see if we can get an answer for you. Don’t throw Toby out, just cover him in warm leaves and keep him in the dark for the winter.
      Michele

    • Hi Julie Ann,
      I have a little more advice from a Twitter science contact. In addition to providing some nice leaves and detritus, put Toby in a garage or other cool space for the winter. If he goes “floppy” the prognosis isn’t too good. Good luck!
      Michele

  6. The soil I provided may be a little slight much. Not sure if I should spray him with water and if so what temperature should the water temp be? I am using a tall, used, wide, glass flower vase for ‘Wooly-Bully’s’ habitat. Was concerned about the possibility of cramping him in there with the green and dead leaves, carrot, apple slice, and 2 small oak limbs. He ate his fill and then dumped, (which I cleaned out later.) I have refreshed him with milkweed leaves, now he appears fed and exhausted! Just found this totally black wooly bear climbing inside and up on the draperies. (That was shocking! :) He has been resting and has burrowed under dry leaves I provided for him. Now he is all set seemingly, for a good emergence come spring time, (AS FAR AS SURVIVAL RATE IN THE WAY I AM CARING FOR HIM? I AM HOPEFUL. THNX FOR ALL THE HELPFUL READS. JUST NEED TO HIT ON A FEW MORE THINGS AND THEN I’M GOOD. IT IS MY FIRST TIME EXPERIMENTING WITH ‘WOOLY-BULLY’ (but he’s not actually….I think he’s a cute little creature!) I am enjoying this 1st time experiment.
    For his emergence next spring I have put his habitat inside an unlocked cat carrier. His domain lid happens to be a domed mesh which seems to give what he needs in terms of fresh air, (though I doubt drafts of air goes up and into that old wide, glass flower vase.) I hope that’s okay. Please comment on my approach to ‘wooly-bully’s’ happiness in 2-space habitat. (Question, should a towel or blanket go over the cat carrier to cover up breathe air holes?) This set-up is placed at the OPEN back porch wall and seems to be a perfect place to let him hibernate.

  7. The soil I provided him may have a bit more water than is needed. Not sure if I should spray him with water; if so should the water be tepid or cool?

    I am using a tall, used, wide, glass flower vase for ‘Wooly-Bully’s’ habitat. Was concerned about the possibility of cramping him in there with the green and dead leaves, carrot, apple slice, and 2 small oak limbs. He ate his fill and then dumped, (which I cleaned out later.) I have refreshed him with milkweed leaves, now he appears fed and exhausted!

    Just found this totally black wooly bear climbing inside and up on the draperies. (That was shocking! :) He has been resting and has burrowed under dry leaves I provided for him. Now he is all set seemingly, for a good emergence come spring time, (AS FAR AS SURVIVAL RATE IN THE WAY I AM CARING FOR HIM? I AM HOPEFUL. THNX FOR ALL THE HELPFUL READS. JUST NEED TO HIT ON A FEW MORE THINGS AND THEN I’M GOOD. IT IS MY FIRST TIME EXPERIMENTING WITH ‘WOOLY-BULLY’ (but he’s not actually….I think he’s a cute little creature!) I am enjoying this 1st time experiment.
    For his emergence next spring I have put his habitat inside an unlocked cat carrier. His domain lid happens to be a domed mesh which seems to give what he needs in terms of fresh air, (though I doubt drafts of air goes up and into that old wide, glass flower vase.) I hope that’s okay. Please comment on my approach to ‘wooly-bully’s’ happiness in 2-space habitat. (Question, should a towel or blanket go over the cat carrier to cover up breathe air holes?) This set-up is placed at the OPEN back porch wall and seems to be a perfect place to let him hibernate.

    • Well, I’m no wooly bear expert, but they both seem well cared for. I would keep them both in the dark and undisturbed so that they can hibernate through the winter in peace. If your porch will drop very far below freezing, make sure they are close to a wall, and covered with a towel or blanket. Good luck and let us know if they emerge successfully in the spring time! Michele

      • Thank you Michele for your answer! Here’s a little more! I reside in central-east Tennessee upon a 1200 ft. plateau with cool nights, (50’s), & warm days, (70’s. I will check on him thru the glass jar he’s placed in to see if he is still moving and crawling around. (BTW, is glass okay to use as well as plastic?) I am tempted to give Wooly one more cleaning and add more DRY dirt, (less wet for crawling down in), then want to add more greens and grass! (I didn’t think to place any in the jar.) Later I will post a poem I wrote for ‘Wooly the bully’ (lol)…ao for now, have a good day wherever you are! :) I will give back a report about him and will wait a response accordingly. (If he hasn’t moved I will not disturb him; if I see he has maybe there is a chance for one more cleaning and adding. Thanx for the good wishes for a hopeful emergence in spring time! Until then… I’ll keep checking back here.

        Bet

  8. He is totally black so it is possible he didn’t get enough water from being here inside the house (without my knowledge until he crawled up the drapes looking for a way out.) I wonder how he got in unless his parent laid her eggs there. Otherwise, as the folk lore goes, for black without any brown, there will be a short winter but very cold.

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