To estimate insect age, forensic entomologists must determine the amount of time a particular insect spends in each stage of development and apply this information to temperature data at a crime scene to determine a post-mortem interval estimation. Timing of various insect stages is determined in a laboratory at standardized temperatures so that ranges may be applied to the different temperature conditions encountered in the field. Post-mortem intervals are estimated by determining the accumulated degree days (ADD) or accumulated degree hours (ADH) acquired by each insect stage on the remains; this information is then used to calculate the overall degree days or degree hours accumulated during the insect’s entire life cycle and association with the carcass, representing the minimal amount of time the body has been available for colonization by insects. This estimation does not claim to represent an accurate time of death, but rather a minimal time of colonization. This is an important distinction that must be considered with the use of forensic evidence in courts of law.
Accumulated degree days can be determined by subtracting the minimum threshold from the average ambient temperature over a 24-hour period and multiplying the difference by the corresponding number of days. Accumulated degree hours, which are equally useful, are similarly determined by subtracting the minimum threshold from the ambient temperature over a 1-hour period and multiplying the difference by the corresponding number of hours. The term "minimum threshold" refers to the lowest temperature at which an insect species will develop; any temperatures below this threshold will result in delayed development and probable death of the insect in question.
It is important to note that similar calculations may be carried out to determine "accumulated degree minutes," "accumulated degree seconds," etc., though ADD and ADH are typically used in forensic analyses.
For S. bullata, the following temperature data has been determined in a laboratory setting. These calculations correspond to a standardized temperature of 27 °C.
|Stage||Time Spent in Stage (27 °C)|
|1st Instar Larvae||26 hrs|
|2nd Instar Larvae||18 hrs|
|3rd Instar Larvae||54 hrs|
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