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INVERTEBRATE COLD HARDINESS

In the Storey lab we study the mechanisms of cold hardiness used by invertebrate animals including terrestrial insects and mollusks that live in the marine intertidal zone. Freeze tolerant marine mollusks can also live without oxygen for long times so for information on both freezing and anoxia tolerance of these animals, go to our Anoxia webpage.

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Eurosta solidaginis

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Epiblema scudderiana

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Littorina littorea

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Geukensia demissus

 

COLD HARDY INSECTS

In the Storey lab we work on two species of cold hardy insects that live in stem galls on goldenrod.

Both spend the winter as mature larvae within galls but they have opposite strategies for surviving the winter. Larvae of Eurosta solidaginis, the gall fly, are freeze tolerant. They freeze solid when outdoor temperatures fall below about -8C but happily survive with as much as 65% of their body water turned to ice. Larvae of Epiblema scudderiana, the gall moth, use the freeze avoidance strategy, packing their bodies with antifreezes so that they can stay liquid down to nearly -40C.

Below are photos of both species and their galls. This is followed by information on the Life Histories of the two species and an explanation of the different insect options for Surviving the Winter.

Visit NatureNorthZine for classroom experiments that you can do with freeze tolerant insects.

Visit the Solidago gall homepage at Bucknell University for lots more info on the ecology and life histories of Eurosta, its predators, its host plant, and many stunning photos.

 

Picture 1. Examples of ball galls containing Eurosta solidaginis larvae and elliptical galls containing Epiblema scudderiana larvae. Galls are found on the stems of goldenrod.

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Picture 2. Top: Eurosta solidaginis, the goldenrod gall fly, overwinters as a third instar larva in the central cavity of ball galls. Bottom: Epiblema scudderiana, the goldenrod gall moth, overwinters as a final instar caterpillar in elliptical galls.

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Picture 3. Closer look at a Eurosta larva in its gall. The larva eats and grows all summer in the central cavity. Just before settling down for the winter, the larva eats a tunnel to the surface of the gall, leaving only the skin of the gall in place. Then it returns to the central cavity and enters diapause (a form of dormancy) for the winter.

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Picture 4. Closer look at Epiblema in its gall. The larva eats and grows in the central cavity all summer but retreats downward into the hollowed out stem for the winter where it also settles into diapause.

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Picture 4. Eurosta solidaginis. The larva winters in the central cavity but in this photo is partly in the tunnel leading to the gall surface. Black mouth parts are seen on the left.

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Picture 6. Epiblema scudderiana in its winter position tucked into the hollow stem below the main gall cavity.

 

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Picture 7. There is only 1 Eurosta larva in each gall but double galls on one stem occur quite often -- the most we have ever found on one stem is 4!!

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Picture 8. Double Eurosta ball galls in late October after frost has killed the plant.

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Picture 9. Double Epiblema galls are less common. Here the goldenrod plant is still green in mid-September.

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Picture 10. Galls are well camouflaged on the dying plants in late October.

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Picture 11. The normal exit hole (left) made by the fly escaping in the spring is very neat but chickadees tear open the gall (right) to eat the high calorie larvae, a favorite winter food.

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Picture 12. Downy woodpeckers start to attack the galls in late October as soon as the larvae have finished their tunnels. They leave much neater holes than the chickadees.

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Picture 13. In late April, the Eurosta larvae pupate. Epiblema pupate a little later.

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Picture 14. Adult fly Eurosta solidaginis

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Picture 15. Adult fly

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Picture 16. Adult fly on JM Storey Description: DSCN2779-sm

Picture 17. The adult Epiblema moth is about 1 cm long.

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 Picture 18. Adult Epiblema moth on a gall showing the exit hole. aDescription: DSCN2918-sm

 Picture 19. Exit holes at the top of elliptical galls lDescription: DSCN2905-sm

 

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August field showing full size green galls on blooming goldenrod, Solidago sp.

In the background, brown galls on dead stems are the remains of last year's crop.

Picture 21.

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August field showing two of the main wildflowers blooming in late summer - goldenrod and purple asters.

The ball galls shine in the sunlight and are easy to spot.

 

Picture 22. Mature larvae of the hermit flower beetle, Osmoderma eremicola (Knoch)(Coleoptera, Scarabaeidae).

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These giant larvae (4-5 cm) live in the heartwood of decaying deciduous trees in eastern Canada and the USA. They take 3 years to reach maturity and are freeze tolerant each winter. We have only done one study on this species because collecting specimens is very difficult. Denis Joanisse gathered these while chopping firewood for his family.

  

LIFE HISTORIES of GOLDENROD GALL INSECTS

Eurosta solidaginis (Fitch) (Diptera, Tephritidae) is the larva of the goldenrod gall fly. Eggs are laid in the growing tips of goldenrod plants in the spring and when they hatch the larvae bore into the center of the stem and start to eat. Secretions of the larvae mimic plant hormones and cause the plant to form a ball gall around them and to stock the inside of the gall with cells that are high in nutrients which the larvae eat. Third instar larvae reach maximum size by early autumn. They bore a tunnel out to near the surface of the gall (leaving just the epithelium layer) and then settle back into the center of the gall to spend the winter. Downy woodpeckers and chickadees will tap on galls until they find this tunnel and then dive in the get the juicy larva which is a high fat winter treat for them. Once settled in the center of the gall, the larvae respond to autumn cues (shorter days, cooler temperatures, senescence of the plant) by preparing for winter. They accumulate 2 cryoprotectants, glycerol and sorbitol, and increase the supercooling point of their body fluids by adding ice nucleators that stimulate freezing of the larvae whenever temperature drops below about -8 to -10C. The larvae survive freezing and can endure the conversion of up to about 65 % of their total body water in the extracellular ice. They endure multiple freeze/thaw cycles over the winter and can survive to at least -30C in southern Canada. In late March and April, the larvae begin to break down their cryoprotectants and get ready to pupate in late April. After 2-3 weeks, the adults hatch, walk up the tunnel, push their way through the surface skin and then set off to start the cycle again.

Epiblema scudderiana (Clemens) (Lepidoptera, Olethreutidae) is the caterpillar of the goldenrod gall moth. Its life cycle is much the same as that of Eurosta. It lives inside an elliptical gall on the stem of goldenrod. The gall is much more camouflaged than that of Eurosta with wood-grain like scars on the outside but frequently the presence of the gall makes the plant above it split out into more bushy appearance so you can often find the galls by looking at the appearance of the plant. The final instar larva overwinters. It moves out of the main gall cavity and down into the hollowed out stem below where it fits snugly and vertically in a head up position. The caterpillar lines the interior of the stem and gall with silk which probably helps to act as a barrier to water or ice penetration. Epiblema's strategy for cold hardiness is freeze avoidance and by mid-winter the larvae in the Ottawa area can supercool to -38C. To keep from freezing they accumulate high concentrations of glycerol, as much as 18% of their total body mass, which prevents their body water from freezing by the same mechanism as the addition of ethylene glycol to an automobile radiator. The larvae probably also produce antifreeze proteins, as do other freeze-avoiding insects.

Eurosta solidaginis can be attacked by parasites and predators in their galls. Two species of wasps lay their eggs in the galls and the wasp larvae eat the gall fly larvae. One species (Eurytoma obtusiventris) lays its egg right inside the gall fly larva and eats it from the inside; it causes the gall fly larva to form a pupa in the autumn (instead of the spring) and then winters inside the pupal case. The other wasp larva (Eurytoma gigantea) eats up the whole gall fly and takes its place in the gall cavity. This wasp larva is also cream-colored, like the gall fly larva, but is a bit smaller and has distinctly pointed ends. Typically, too, once the gall fly larva is dead, the interior cavity of the gall degenerates and goes brown and partly hollow compared with the white and solid interior (except for the central cavity that just fits the larva) if the gall fly larva is alive. The third is a beetle larva (Mordellestina unicolor) which is described as an accidental predator but again eats the gall fly larva from the outside. The beetle larva looks like a small caterpillar with distinct legs.

Epiblema scudderiana also have parasites but we know less about these. However, when we took larvae out of galls in the winter and left them at room temperature in hopes that they would pupate, they often did not pupate. Instead, several small worm-like larvae would come out of each Epiblema.

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Picture 14. For more info on "Parasites and predators" see the Solidago gall homepage

Eurosta solidaginis and its parasites

Top: Eurosta - 2nd and 3rd instars

Left: Eurytoma gigantea, wasp larva which eats Eurosta

Right: Eurytoma obtusiventris, wasp larva which makes Eurosta pupate prematurely

Bottom: Mordellistena convicta, beetle larva that sometimes eats Eurosta

Photo: John Baust

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From left to right: (a) Eurosta solidaginis 3rd instar larva, (b) Eurosta are triggered to pupate in the autumn when parasitized by Eurytoma obtusiventris, (c) Eurytoma gigantea, and (d) Mordellistena convicta.

 

Photo: Jan Storey

 

SURVIVING THE WINTER

For an easy-to-read general article about Animal Cold Hardiness, also see PDF

 TEMPERATURES

To start with, lets define temperatures because scientists always use the Celcius scale. On the Celsius scale, 0C is the freezing point of water and 100C is the boiling point of water. This compares with the Fahrenheit scale where 32C is the freezing point of water and 212C is the boiling point.

A home refrigerator is generally at 4-5C and your home freezer is usually between -15 to -20C.

 

STRATEGIES OF INSECT COLD HARDINESS

For insects there are 5 main ways of dealing with winter temperatures that fall below the freezing point of water (below 0C).

1.     "heat up the house"

This in only done by honeybees. Bees generate heat by shivering and with thousands of bees in a colony and a huge supply of stored food (honey) they can afford to huddle and shiver all winter and keep the colony at a comfortably warm temperature that is a lot higher than the outdoor air temperature.

2.     "get out of town"

Some insects migrate to get away from cold temperatures; for example, Monarch butterflies fly to Mexico.

Other insects find places to hide for the winter where they will never experience subzero (below 0C) temperatures. Some go underground far enough so that they are below the frostline and some spend the winter in ponds and streams in an aquatic life stage. For example, dragonflies spend the winter as aquatic larvae and then transform into the adult flying form in the spring/summer.

3.     Spend the winter as a simple life stage -- usually an egg.

Eggs are easier to protect than is a multicellular larva or adult insect and many insects are present only as the egg stage during the winter. Some insect eggs can survive extensive dehydration and spend the winter in a very dry form. If you have little or no free water in your system, ice crystals can't form and you can't be harmed by freezing. Other insects pack their eggs with cryoprotectants such as glycerol or sorbitol. This creates such a syrupy solution that water in the egg can't freeze. Eggs can also reduce their metabolism to near zero and many can be in such deep dormancy that they can survive not only over a whole winter, but sometimes for many years.

4.     Freeze avoidance

Many insects happily remain liquid even at temperatures that are well below 0C. We call this strategy "freeze avoidance". We always think that water freezes at 0C but in reality, both plain water itself and the water in an animal's body can often be cooled to much lower temperatures before ice forms. Microscopic droplets of very pure water can be cooled to -40C before they freeze. Water "normally" freezes at 0C because the growth of ice crystals is "seeded" by the presence of some particle or surface that helps to line up the water molecules into the crystal shape. Once an initial microscopic nucleus of ice is formed, more and more water molecules quickly join the crystal and high speed crystal growth is triggered. The trick, therefore, is to keep your body fluids from coming into contact with ice crystals themselves or with molecules or particles that can act as nucleators to seed crystal growth.

Insects in general are good at this trick of "supercooling" - staying liquid when their body temperature is below zero. This is because of the small body size of most insects (the smaller the better for supercooling) and the strong, waxy cuticle that coats the insect's body which provide excellent waterproofing and also prevents external ice from coming into contact with body fluids. Even in summer, most insect won't freeze until at least -5C or even lower. In the winter, many insects activate adaptations that push supercooling even farther. Insects that spend the winter near the surface of the ground (e.g. on the forest floor or in the soil under your lawn) where they will be covered by a blanket of snow, can generally supercool to -15 or -20C without freezing. Insects that spend the winter high up in trees or in other exposed sites can often supercool to -40C.

The main adaptations that increase an insect's ability to supercool and avoid freezing are: (a) make antifreeze proteins, and (b) make cryoprotectants. Antifreeze proteins function by binding to microscopic ice crystals as they start to form in the insect's body fluids. Binding prevents the crystal from growing any larger. Most insects that spend the winter in insulated places such as under the snow, only have to add antifreeze proteins to their body fluids to get all the protection that they need. This is because its almost always a lot warmer under the snow than above it. Lots of insects and other animals can stay active under the snow at temperatures that rarely fall below -5 to -8C even though air temperatures above the snow may be -20 or -30C.

Insects that live in very exposed sites also produce cryoprotectants. In insects the most common cryoprotectant is glycerol which is a 3-carbon polyhydric alcohol. Glycerol is a longer version of ethylene glycol, the 2-carbon polyhydric alcohol that we use as antifreeze in the radiator and windshield washer fluid of cars. Several other types of polyols as well as some kinds of sugars are also used as cryoprotectants by some species. The freezing point of water decreases in proportion to the concentration of dissolved substances in it so the more cryoprotectant that you can pack into your body fluids, the lower the temperature at which you will freeze. Pack in enough and the insect won't freeze at the normal winter temperatures that it encounters. The key advantages of glycerol as a cryoprotectant are that it is highly soluble in water, is not toxic to metabolism, and can be easily synthesized by the insect. The concentrations of glycerol can be so high in some insect species in winter that as much as 20-25% of the insect's total body mass is glycerol! Glycerol also has another action and that is that it is very good at binding to water and, for insects that live in exposed sites such as in trees, this helps the animals to avoid death due to desiccation over the long winter months when they are exposed to dry, cold air.

 5. Freeze tolerance

Two of the disadvantages of the freeze avoidance strategy that we talked about above are (1) if your antifreeze is only good to -20C and temperature drops to -25C, you freeze and die, and (2) you can only stay supercooled if you can avoid contact with ice crystals -- even if your antifreeze is good to -20C, if you touch ice crystals your body fluids will be seeded and you will freeze and die. So, the freeze avoidance strategy is a gamble -- insects gamble that temperatures won't fall too low and that they are sufficiently protected so that they won't come in contact with external ice. For example, many insect pick dry spots to hibernate in such as under the bark of trees and many spin a waterproof cocoon around themselves or have a very thick wax cuticle on their body to block contact with ice.

Other types of insects have "given up" on the idea of avoiding freezing and figured out instead how to accept freezing yet not die. It's a much more difficult strategy to use because freezing requires many complex adaptations. You might be interested to know, however, that the methods used by insects to protect their tissues during freezing are the same ones that are used in medicine to freeze and preserve many types of cells and tissues to save them for transplant. Things like blood, sperm, embryos, skin, heart valves and corneas can be frozen, stored for many years, and then thawed and transplanted.

So, to survive freezing, here's what an insect has to do.

1.     Ice is allowed to grow only outside of cells, never inside the cells. So ice will form within the insect's body, surrounding all the internal tissues, and ice will grow through all the blood vessels of the insect. So, the animal is never completely frozen because the insides of its cells are always liquid. Only about 65% of the insect's total body water freezes -- the rest stays liquid inside cells. If ice grows inside of cells it kills them because the crystals cause way too much damage to the inside structure of the cells.

2.     Most insects are naturally good at supercooling but if an animal begins to freeze after it has supercooled to a low temperatures (say -10C or lower), the speed of ice formation is so high that the insect may be killed or severely injured. So, freeze tolerant insects use nucleators to help them "seed" their body fluids so that they start to freeze somewhere between 0 and -10C. Sometimes they do this by letting their bodies be seeded through contact with external ice in their environment and sometimes they produce special proteins (called ice nucleating proteins) that trigger ice formation. You might be interested to know that the ice nucleating proteins that are produced by some kinds of bacteria are used by ski resorts to "seed" freezing when making artificial snow.

3.     Freeze tolerant insects also have antifreeze proteins. Isn't that odd??? Why would they need a protein that stops ice crystals from growing? The reason is that small ice crystals can be harmless but big ones can do some serious damage. When something is frozen for a long time, the ice undergoes a strange process called recrystallization. Small ice crystals reform themselves into larger and larger crystals and eventually the crystals could get so big that they harm the delicate tissues of the frozen animal. So antifreeze proteins are used to keep small crystals small. The best example of recrystallization that you can see is in the ice cream in your home freezer. It is all smooth and creamy when you first open it but if you leave it in the freezer for a long time, you will find that big feathery ice crystals have separated from the ice cream.

4.     To keep the inside of cells liquid, freeze tolerant insects pack them up with glycerol or other cryoprotectants. So, instead of preventing your entire body from freezing, like freeze avoiding insects do, the insects that survive freezing only focus on protecting the insides of their cells with cryoprotectants.

5.     When ice crystals grow outside of cells (in what is called extracellular spaces), they set up a stress on cells that sucks water out of the cells so that the amount of water inside cells decreases and the cells shrink in size. The high cryoprotectants inside the cells help to hold onto the intracellular water and keep the cells from shrinking too much. Other protective molecules are also made strengthen the membranes that surround the cells so that the membranes do not break either when the cells shrink during freezing or when they swell again during thawing.

6.     When the extracellular water of an animal freezes, this includes its blood. Frozen blood cannot deliver nutrients to cells so all of the tissues of the animal have to stay alive during the freeze by relying only on the fuels that are inside each cell and without the oxygen that is normally delivered from the outside. Oxygen is delivered by the blood in mammals (like us) but in insects oxygen is delivered to each tissue by tiny tubes called trachea that lead inwards from small holes (called spiracles) that are all over the insect's body. When an insect freezes, oxygen delivery stops for 2 reasons: (a) the muscles that help to keep air flow moving through the trachea freeze, and (b) the extracellular fluid at the end of the trachea freezes. So a frozen animal has to live without oxygen for the whole time that it is frozen and this can be a challenge. Animals that can survive freezing have developed adaptations that help them live longer without oxygen.

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