The time for which an insect is exposed to a given potentially lethal temperature has a considerable effect on tolerance, with extended exposure generally resulting in a decline in survival, though often in a complex fashion. The interactions between time and temperature have long been known (see Salt 1961; Asahina 1969; Cossins and Bowler 1987), and are often exploited in high temperature mortality assays to allow probit analyses of the time required for 50 per cent of a sample of insects to die (Berrigan 2000; Hercus et al. 2000). However, exposure time is also of considerable ecological relevance given that sublethal temperatures may often become lethal if exposure time is prolonged. For example, at subzero temperatures a liquid cooled beyond its melting point is metastable, and its probability of nucleation (freezing) is a function of temperature, solute concentration, volume, the presence of ice nuclei, and importantly, time (Salt 1961; Somme 1982; Zachariassen 1991a,b; Ramlov 2000). In consequence, the longer an animal remains below subzero temperatures, the greater its risk of freezing. Bale (1987, 1993) has championed the importance of exposure time showing that although the lethal temperatures of some freeze intolerant (Section 5.3.2) species may appear to be relatively low when assayed over a short period, prolonged exposure results in considerable mortality (see also the contributions of R.W. Salt discussed in Ring and Riegert 1991 and additional discussion in Somme 1982). Moreover, Bale (1993) has argued that exposure time is an essential component of insect cold hardiness classifications (Section 5.3.1), and in consequence has called for greater knowledge of the microclimates insects are likely to experience. In re-emphasizing the importance of exposure time, both in terms of pre-freezing mortality (or chilling injury) and mortality at subzero temperatures, Somme (1996) pointed out that information on survival of prolonged cold and its relevance to the field situation is comparatively scarce.
However, it is not only exposure time that has an influence on assessments of the survival of potentially lethal temperatures. Warming rates, following a low temperature exposure, and the recovery time after which survival assessments are made, are both crucial components of thermotolerance experimental protocols (Baust and Rojas 1985). The latter is of particular significance. Most assessments of survival in mortality assays are made 24 h after exposure (e.g. Krebs et al. 1998), and occasionally after longer periods (Klok and Chown 1997; Jenkins and Hoffmann 1999). Baust and Rojas (1985) point out that in some species, coordinated activity and apparent feeding may persist for longer than a week before the insects die (but see Jenkins and Hoffmann 1999). As a result, several authors have suggested that assessments of fecundity (and possibly fecundity of the F1 generation) are more appropriate measures of the insects' response to the experimental conditions (Baust and Rojas 1985; Bale 1987). The reasoning is that the effects of stress in one stage might not be immediately detectable, but might carry over to another stage, negatively influencing either development, fecundity, survival, or some combination thereof. From an evolutionary (and ecological) perspective, assessments of both survival and fecundity are important (see Endler 1986 for discussion), but in many cases one or the other of these assessments might be difficult or impossible to make. This is likely to be especially true for non-model species that are unwilling inhabitants of the laboratory, but which, nonetheless, constitute the vast majority of the insect fauna. In this instance, survival assessments will be the only straightforward way of assessing the response of the species concerned, and the time over which these assessments should be undertaken would depend substantially on the life history of the species and the stage that is being assessed. For most experiments, however, 24 h should be considered a minimum first assessment time.
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