Chapter

660 To appreciate the mode of action of these cryoprotectants, it is necessary to understand the process of freezing. When water is cooled the speed at which individual molecules move decreases, and the molecules aggregate. As cooling continues there is an increased probability that a number of aggregated molecules will become so oriented with respect to each other as to form a minute rigid latticework, that is, a crystal. Immediately this crystal (nucleator) is formed the rest of the water freezes rapidly as additional molecules bind to the solid frame now available to them. Freezing of a liquid does not always depend on the formation of a nucleator, but can be induced by foreign nucleating agents such as dust particles or, in the present context, particles of food in the gut or a rough surface such as that of the cuticle.

In freezing-susceptible species cold-hardiness is attained in a two-step process (Bale, 2002). In the first step behavioral and physiological activities occur that collectively reduce the insect's chance of freezing. These may include emptying the gut of food and overwintering as a non-feeding pupa, hibernating in dry locations, building structures that prevent contact with moisture, reducing body water content, and increasing fat content. Collectively, these processes may lower the supercooling point to -20° C. In the second step polyhydroxyls and antifreeze proteins are produced. These molecules not only increase the concentration of solutes in the body fluid so that the freezing point is depressed, but by their chemical nature they considerably improve the insect's supercooling capacity; that is, the body fluids remain liquid at temperatures much below their normal freezing point. Because of their hydroxyl groups, the cryoprotectants are capable of extensive hydrogen bonding with the water within the body. The binding of the water has two important effects with respect to supercooling. First, it greatly reduces the ability of the water molecules to aggregate and form a nucleating crystal, and second, even if an ice nucleus is formed, the rate at which freezing spreads through the body is greatly retarded because of the increased viscosity of the fluid.

A remarkable degree of supercooling can be achieved through the use of cryoprotectants. In the overwintering larva of the parasitic wasp Bracon cephi, for example, glycerol makes up 25% of the fresh body weight (representing a 5-Mole concentration) and lowers the supercooling point of the hemolymph to —47°C. Perhaps a disadvantage to the use of supercooling as a means of overwintering is that the probability of freezing occurring increases both with duration of exposure and with the degree of supercooling so that, for example, an insect might freeze in 1 minute at —19° C but survive for 1 month at —10° C. Thus, to ensure survival an insect must have the ability to remain supercooled at extreme temperatures for significant periods of time, even though the average temperatures to which it is exposed may be 10°C to 15°C higher. In other words, it may have to produce much more antifreeze in anticipation of those extremes than would be judged necessary on the basis of the average temperature.

The alternative method, employed by freezing-tolerant species, is to permit (be able to withstand) a limited amount of freezing within the body. Freezing must be restricted to the extracellular fluid, as intracellular freezing damages cells. Ice formation in the extracellular fluid, which is accompanied by release of heat (latent heat of fusion), will therefore reduce the rate at which the body's tissues cool as the ambient temperature falls. Thus, it will be to an insect's advantage to have a large volume ofhemolymph (and there is evidence that this is characteristic of pupae) and to be able to tolerate freezing of a large proportion of the water within it. Two subsidiary problems accompany the freezing-tolerant strategy: it is necessary (1) to prevent freezing from extending to the cell surfaces (and hence into the cells) and (2) to prevent damage to cells as a result of dehydration. As water in the

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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