Chilling and Diapause

Diapause is a phase of arrested growth or metamorphosis that allows individuals to synchronise their development with environmental conditions. It is primarily triggered by chilling (Cragg & Cole 1952; Denlinger 1978; Bell 1994; Anderson and VanLaerhoven 1996) and shorter day lengths experienced by the larvae after hatching or by their mother before oviposition (Saunders 1987). When diapause occurs, it is usually late in the final instar of calliphorids and in the pupal phase of sarcophagids (Denlinger 1978; Greenberg and Kunich 2002); data for carcass beetles is lacking. It is genetically regulated and more prevalent in autumn in populations from higher latitudes, affecting all or only part of a cohort (Saunders and Cymborowski 2003), so it is not a pervasive source of bias. Malnourished maggots can evade diapause (Saunders 1987). Diapause may last months and lead to serious underestimates of a PMImin.

Cold spells near the developmental threshold temperature (D0, or base temperature) that do not trigger diapause may still disrupt development in different ways in different species (Johl and Anderson 1996; Myskowiak and Doums 2002; Ames and Turner 2003), which is a concern if live larvae are chilled, for instance in winter weather, through mortuary refrigeration (Huntington et al. 2007), or in an attempt to stop growth during transit to a forensic entomologist. Disruption of development by up to 56 h (907 h°C) have been measured experimentally in Protophormia terraenovae (Robineau-Desvoidy) (Myskowiak and Doums 2002), and leading to both under- or overestimates of PMImin, depending on which life stage is chilled. In one investigation, an inappropriate assumption of chilling led to an estimate of PMImin that predated the last time the person was seen alive by a month (Ames and Turner 2003).

(Greenberg and Kunich 2002) advise that, 'luckily for the entomologist, diapaus-ing larvae and pupae are rarely the key forensic indicators.' When it is suspected to have occurred (e.g. when PMI exceeds PMI ), alternative sources of evidence v ° min max'

should be sought. It is also recommended that, at least until more is known about the response of life stage of each species, live insects should not be chilled after collection to stop development while they are in transit to the laboratory (Myskowiak and Doums 2002). The effects of chilling appear to be highly repeatable ((Myskowiak and Doums 2002); Ames and Turner 2003), so that once suitable data are available, reliable corrections to estimates are a possibility. It is likely that precision will be much lower than usual.

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