The Use and Abuse of the Degree Day Concept in Forensic Entomology: Evaluation of Cochliomyia macellaria (Fabricius) (Diptera: Calliphoridae) Development Datasets

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Cuttiford, Louise Alison
forensic entomology; validation; evaluation; blow fly; degree day
thesis / dissertation description
The Degree Day method allows the temperature-dependent growth of poikilothermic organisms to be measured within their upper and lower thermal limits. It has been used in agricultural and entomological sciences for more than a century. Subsequently, it has been adopted, by means of the Accumulated Degree Hour (ADH) or Accumulated Degree Day (ADD) by the forensic entomology community as a way of predicting the growth of arthropods, most commonly in homicide cases. However, despite being used in casework, development data created using this model have rarely if ever been validated at the time of their making nor have they been subject to field evaluation using human remains. Forensic sciences have come under considerable scrutiny by the legal system in recent years and as such it is necessary to create more robust data and modeling systems for evidential analysis. The current study took samples at four different time points from 29 sets of human remains. These samples were used to evaluate the accuracy of two development datasets pertaining to the blow fly Cochliomyia macellaria (Fabricius) (Diptera: Calliphoridae) to predict the actual time of placement (TOP) of each set of remains. The phenotypes, stage (time to third instar), length and weight were used to predict TOP in each case and although stage performed the best overall (29 out of 80 cases), in real terms none of the development datasets performed at a level that might be considered accurate enough for evidential analysis in this study. A discussion of the caveats of the Degree Day model are presented, accompanied by suggestions for improvement in developing future development datasets and a call for better evaluation and validation of forensically-oriented entomological studies.