When entomological evidence crawls away: Phormia regina en masse larval dispersal

In criminal and civil legal investigations the forensic entomologist usually assists in providing an estimate of the postmortem interval, which closely couples with the time or period of insect activity. A minimum period of insect activity is often interpreted and estimated by dipteran larval developmental age of the oldest specimens collected at a crime scene and/or autopsy. In entomological evidence protocols investigators are recommended to search a 2-10 m perimeter area for the oldest larvae that may have begun to disperse away from the body for burial and pupation. In this study, we described a case of a large aggregate (> 90% larvae) en masse postfeeding dispersal of blow fly larvae away from replicate swine carcasses serving as models of human decomposition. Larval dispersal was evaluated for a spring and a summer trial, with en masse characteristics only occurring during the latter. This en masse dispersal occurred in five out of six replicate carcasses and masses moved from 2 to 26 m away. These data and observations suggest the importance of performing searches > 10 m from human remains for entomological evidence at crime scenes. By missing the oldest larvae at a crime scene, interpretation of entomological evidence can be compromised and erroneous. Based on these data and observations we recommend the crime scene investigators and researchers consider increasing the search radius around crime scene remains to increase the likelihood that the oldest larvae have been collected for analysis.