Insect Strategies for Winter Antifreeze
Some Escape Freezing, Others Freeze But Use Damage Control
Nov 2, 2008
Frigid winter temperatures freeze cells. When this happens, particles in body fluids serve as nuclei around which ice crystals form. If ice crystals grow without intervention, they cut cell membranes, destroying cells and tissues. Most organisms that survive winter produce antifreeze molecules and eliminate wastes from the body (to reduce the number of nuclei available to initiate ice formation). They use one of two strategies to withstand freezing temperatures: freeze solid, or prevent freezing from occurring. Both strategies require production of antifreeze molecules, but the manners in which these molecules function are vastly different.
Antifreeze Molecules
The most common antifreeze molecules are glucose and glycerol. When found in high concentration, glucose can reduce the freezing point by almost two degrees Centigrade (C). Many insects manufacture large quantities of glycerol. Tiaga.net indicates in some overwintering Arctic carabid beetles, glycerol forms as much as thirty percent of the body weight and protects them down to minus fifty degrees C. Less widely used is propylene glycol (the less toxic “green” antifreeze now available for use in automobiles) that can lower the freezing point almost five degrees centigrade.
Many organisms produce antifreeze proteins (AFPs) to lower the temperature at which freezing disrupts their cells. In these animals, the blood is supercooled below freezing and their bodies remain liquid in the temperature ranges they are normally exposed to.
Freezing Strategies
Insects that do freeze have large body cavities. As water freezes in the body cavity, more is extracted from the cells. This allows water to freeze outside the cells where it can’t damage them. Glucose plays an important role in animals that freeze in this way. The removal of water from the cells during freezing concentrates the glucose inside the cells, and allows the cell contents to turn to slush before freezing solid. The slush prevents the formation of large ice crystals that normally rupture the cells. When the animals thaw, the process is reversed, the cells rehydrate, and the organism resumes activity once temperatures warm up. Some insects can withstand freezing at temperatures approaching minus twenty degrees C.
Prevention of Freezing
Supercooling provides the greatest tolerance to freezing temperatures. Whereas the Arctic carabid beetles that produce glycerol and larvae of the spruce bark beetle, Dendroides canadensis, that produce AFPs are capable of withstanding minus fifty degrees C, adults of the spruce bark beetle utilize the strategy of freezing and die if exposed above the snow at minus ten degrees C.
Antifreeze Proteins
Insects, fish, amphibians, plants, and bacteria produce antifreeze proteins only during the winter. Different organisms produce proteins with different structures and protection abilities. Some organisms (plants and fish) are only protected down to minus three or four degrees C, while others can withstand temperatures below minus ten degrees C.
The surface of AFPs binds to water as it freezes and prevents sharp ice crystals from forming. Thus, the liquid in the cells of organisms containing AFPs remains liquid at temperatures far below freezing and these organisms may continue to undergo metabolism even in the dead of winter. Glycerol; metabolic intermediates such as citric acid, succinic acid, malic acid, and sorbitol; the amino acids aspartate, glutamate, and alanine; and ammonium salts interact synergistically with these proteins, lowering freezing points.
Circadian Rhythms Regulate Antifreeze Production
Kathleen Horwath and John Duman, in their article “Involvement of the circadian system in photoperiodic regulation of insect antifreeze proteins,” (Journal of Experimental Zoology, Vol 219, pp 267-270), found that spruce bark beetle larvae maintained on a short (eight hours light and sixteen hours dark) photoperiod, exhibited increased AFPs, whereas larvae “maintained on a long (16L/8D) photoperiod did not.”
Their experiments indicate circadian photoperiod triggers the onset of antifreeze production in preparation for winter. Other authors have found photoperiod also triggers increases of glucose, glycerol, and other molecules in various organisms that freeze or supercool in the winter.
