Establishing Blow Fly Development and Sampling Procedures to Estimate Postmortem Intervals
Leon G Higley, Neal Haskell, Timothy Huntington, Amanda Roe
Abstract
Currently, the most reliable biological indicator of time since death in decomposition cases is blow fly development. Because insects do not maintain constant body temperatures, development is a function of time and temperature. Consequently, having accurate models for describing temperature and blow fly development is essential in producing accurate estimates of the post mortem interval (PMI). To date, data on blow fly development have been limited and comprehensive growth models lacking, our focus in this project was to develop datasets, models, and procedures to improve PMI estimates through measures of blow fly development. We used 66 sampling times in replicated laboratory studies to determine development requirements for each insect growth stage and transitions between stages. We developed improved degree day (linear) models and curvilinear models for Lucilia sericata and Phormia regina, and are working on data for additional species. We determined that stage transitions show normal distributions and can last from hours to days, depending on temperatures and stage. We noted that the third migratory stage does not show temperature-dependent development above 17.5° C in either L. sericata or P. regina. This stage also had the most variability in development time, but all stages showed substantial variability, despite having little genetic variation. Field and laboratory observations confirmed that blow fly feeding and stage transitions can occur at night and in darkness. Studies on anoxia tolerance with Calliphora vicina, Cochliomya macellaria, P. regina, and L. sericata showed limited tolerance to anoxia with LT50 times between 1-10 hours. Consequently, while larval movement in maggot masses could be associated with oxygen starvation, as LT50s are in the range of hours, rather than minutes, it is more likely that movement is associated with thermoregulation. Practical implications of these results include more precise and accurate methods for determining insect development and the associated PMI, including statistically valid confidence intervals. The recognition of the potential importance of the duration of stage transitions, normal distributions in transitions, and potential temperature- independent development of migratory third stage larvae will require changes in current recommendations and improvements in PMI estimates as changes are implemented.