Ecological modeling helps to dissect local temporal variation in entomology-based PMI estimates

Post Mortem Interval (PMI) estimates are based on species-specific development data that are temperature-dependent, can be highly variable, and therefore a potential source of error. Currently, the precision of PMI estimates is unknown, as error can arise due to different sources of variation such as temperature fluctuations, phenotypic variation, and predation. Ecological models are an abstract, mathematical representation of an ecological system that can make predictions about the dynamics of the real system. To quantify the uncertainty associated with PMI estimates, and meet NRC requirements and the Daubert Standard for admission of evidence in court, we developed an ecological model that simulates the colonization of vertebrate remains by a blow fly species, to represent the uncertainty associated with local temperature variability, and to refine PMI estimates at local sites. The model is based on a Texas-specific development data set for a primary colonizer of carrion in the southern US, Cochliomyia macellaria (Fabricius) (Diptera: Calliphoridae). After a PMI estimate is calculated for each individual, the model calculates the maximum, minimum, and mean PMI, as well as the range and standard deviation, for the entire sample collected from the remains in Texas investigations. Our results suggest that PMI estimates would result in variation of approximately one to eighteen days in colder months and less than four days in warmer months. This study suggests that for this species in Texas, forensic entomologists should be more confident in PMI estimates for warmer months than colder months.