Benoît Vincent

Using estimated on-site ambient temperature has uncertain benefit when estimating postmortem interval.

The forensic entomologist uses weather station data as part of the calculation when estimating the postmortem interval (PMI). To reduce the potential inaccuracies of this method caused by the distance between the crime scene and the meteorological station, temperature correlation data from the site of the corpse may be used. This experiment simulated the impact of retrospective weather data correction using linear regression between seven stations and sites in three climatic exposure groups during three different seasons as part of the accumulated degree days calculation for three necrophagous species (Diptera: Calliphoridae). No consistent benefit in the use of correlation or the original data from the meteorological stations was observed. In nine cases out of 12, the data from the weather station network limited the risk of a deviation from reality. The forensic entomologist should be cautious when using this correlation model.

Effect of low temperature in the development cycle of Lucilia sericata (Meigen) (Diptera, Calliphoridae): implications for the minimum postmortem interval estimation

ABSTRACT Knowledge of necrophagous insects’ developmental data is necessary for the forensic entomologist to estimate a reliable minimum postmortem interval (PMImin). Among the most represented necrophagous species, Lucilia sericata (Diptera, Calliphoridae) is particularly interesting. It is regularly identified in samples, with a predominance in summer, and is commonly used by analysts of our entomology department (Institut de Recherche Criminelle de la Gendarmerie Nationale) to estimate the PMImin with the accumulated degree days (ADD) method. This method requires the mathematical lower thermal threshold to be known. This value dictates the quality of the applied ADD method but cannot be considered as fixed, especially when insect development occurs at temperatures close to the biological threshold. In such conditions, it is necessary to study the influence of such temperatures on development rate, as well as the consequences of estimating the period of first oviposition on cadavers, when using the ADD method. Seven replicate rearings were conducted at six different temperatures: 30 °C, 24 °C, 18 °C, 15 °C, 12 °C and 10 °C. Time of development and time of emergence were recorded. The effect of low temperature on the development cycle and the reliability of the ADD method under this entire temperature spectrum were studied using different linear regression models. Calculated durations of total insect time development and experimental rearing duration were then compared. A global linear model cannot be used on the whole temperature spectrum experienced by L. sericata without resulting in an overestimation at some temperatures. We found a combination of two linear regression models to be suitable for the estimation of the total development time, depending on the temperature experienced by L. sericata. This approach allowed us to obtain a variation lower than 2% at 12 °C and 10 °C between the calculated duration and experimental duration of development. In comparison, the results obtained with a global model show a variation higher than 3% at 12 °C and 10% at 10 °C.