Rates of change

It is widely appreciated that the rate at which insects cool has a profound influence on their ability to survive low temperatures (Miller 1978; Baust and Rojas 1985; Shimada and Riihimaa 1990; Ramlov 2000; Sinclair 2001a), although the supercooling point (SCP) or crystallization temperature (Section 5.3.1) apparently remains largely unaffected by cooling rate (Salt 1966). For example,

Miller (1978) demonstrated that even small deviations (<0.1°C min-1) from the 'optimal' cooling rate (0.32°C min-1) have a large influence on survival in Upis ceramboides (Coleoptera, Tenebrionidae). Likewise, Shimada and Riihimaa (1990) showed that in the freeze-tolerant Chymomyza costata (Diptera, Drosophilidae), altering the cooling rate from 0.1 to 1°Cmin—1 substantially reduces short-term (1 h) survival of temperatures below —10°C (Fig. 5.3). Cooling rate not only affects low temperature survival in Drosophila melanogaster, but also has a marked effect on critical thermal minimum (CTmin), a measure of knockdown temperature (Kelty and Lee 1999). Obviously, the use of slower cooling rates in experiments is more ecologically relevant: this is a result of the rapid cold hardening (Section 5.2.3) that takes place as a consequence, and which undoubtedly enhances survival in the field (Kelty and Lee 2001).

This theme of ecological relevance has been raised, inter alia, by Baust and Rojas (1985), Bale (1987) and Sinclair (2001a), with Baust and Rojas (1985) pointing out that the cooling and subsequent warming (or thawing) rates of an insect's

Test temperature (°C)

Figure 5.3 Influence of cooling rate on survival of the freeze-tolerant drosophilid fly Chymomyza costata at low temperatures. Black bars 0.rCmin_1, grey bars 0.5°Cmin~1, open bars 1°C min_1.

Source: Data from table 1 in Shimada and Riihimaa (1990).

Test temperature (°C)

Figure 5.3 Influence of cooling rate on survival of the freeze-tolerant drosophilid fly Chymomyza costata at low temperatures. Black bars 0.rCmin_1, grey bars 0.5°Cmin~1, open bars 1°C min_1.

Source: Data from table 1 in Shimada and Riihimaa (1990).

microhabitat are most relevant to determining their response to cold. Suboptimal experimental conditions provide a measure of response of an individual to these conditions, rather than to the field environment. Thus, it might come as something of a surprise that the large majority of investigations of insect cold hardiness have adopted cooling rates of approximately 1°Cmin-1 (Block 1990). A largely uniform protocol for experiments was initially adopted to standardize results (Salt 1966), and has clearly been instrumental in facilitating a sound understanding of the physiological and biochemical responses of insects to low temperatures. However, if the responses of insects to their surroundings are to be comprehended within the context of their environmental setting, and compared across regions (Section 5.4), then ecologically relevant rates might be more appropriate in subsequent investigations. Determining these rates and other relevant microclimatic parameters is more important than simply investigating the maximum and minimum temperatures likely to be encountered by a species in its usual environment (Feder et al. 1997a; Sinclair 2001b).

Although the rate of heating is also known to have an effect on mortality owing to high temperatures (Lutterschmidt and Hutchison 1997), probably as a consequence of exposure time (Feder et al. 1997a), the effects of heating rate on high temperature survival have enjoyed much less attention. Most studies expose insects directly to the assay temperature with equilibration taking place in less than a minute (e.g. Krebs and Loeschcke 1995a; Hoffmann et al. 1997). Rather, the focus of methodological investigations for high temperature treatments has generally been changes in the outcome of experiments adopting different assay techniques.

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