Evaluation of Development Datasets for Hermetia illucens (L.) (Diptera: Stratiomyidae) for Estimating the Time of Placement of Human and Swine Remains in Texas, USA

Abstract

A basic tenet of forensic entomology is development data of an insect can be used to predict the time of colonization (TOC) by insect specimens collected from remains, and this prediction is related to the time of death and/or time of placement (TOP). However, few datasets have been evaluated to determine their accuracy or precision. The black soldier fly, Hermetia illucens (L.) (Diptera: Stratiomyidae) is recognized as an insect of forensic importance. This study examined the accuracy and precision of several development datasets for the black soldier fly by estimating the TOP of five sets of human and three sets of swine remains in San Marcos and College Station, TX, respectively. Data generated from this study indicate only one of these datasets consistently (time-to-prepupae 52%; time-to-eclosion 75%) produced TOP estimations that occurred within a day of the actual TOP of the remains. It is unknown if the precolonization interval (PreCI) of this species is long, but it has been observed that the species can colonize within 6 d after death. This assumption remains untested by validation studies. Accounting for this PreCI improved accuracy for the time-to-prepupae group, but reduced accuracy in the time-to-eclosion group. The findings presented here highlight a need for detailed, forensic-based development data for the black soldier fly that can reliably and accurately be used in casework. Finally, this study outlines the need for a basic understanding of the timing of resource utilization (i.e., duration of the PreCI) for forensically relevant taxa so that reasonable corrections may be made to TOC as related to minimum postmortem interval (mPMI) estimates.

Not by the Book: Observations of Delayed Oviposition and Re-Colonization of Human Remains by Blow Flies

“…The TOC estimations were used to estimate the TOP for each human donor in this study. A TOC estimation was considered accurate if it bracketed the TOP for the respective donor [ 19 , 64 ]. As TOC estimations rely on the developmental duration of each immature life stage, such estimations are limited to the maximum possible development time recorded for a given species under certain environmental conditions.…”

“…The TOC estimation can then be used by investigators to infer a portion of the PMI, known as the minimum PMI (PMI MIN or mPMI) [ 5 ]. Though the PMI and PMI MIN are sometimes used interchangeably, the most significant difference is that the PMI begins at the TOD, whereas the PMI MIN begins at the TOC [ 19 ]. Though it is possible that the PMI MIN = PMI (i.e., the TOC = TOD) under some circumstances, it is often impossible to know if this is true in forensic casework [ 20 ].…”

“…Though it is possible that the PMI MIN = PMI (i.e., the TOC = TOD) under some circumstances, it is often impossible to know if this is true in forensic casework [ 20 ]. Thorough comparisons and visual illustrations of these definitions have previously been reviewed [ 8 , 19 ]. Similar to the succession method, the TOC method is also not immune to error, as it is only useful when sufficient biological information is known for a given species.…”

“…Similar to the succession method, the TOC method is also not immune to error, as it is only useful when sufficient biological information is known for a given species. Multiple factors can contribute to the inaccuracy of age estimations, such as the true temperature experienced by larvae feeding on remains [ 21 , 22 , 23 , 24 , 25 ] (including neglecting the species-specific thermoregulatory abilities of larvae [ 26 , 27 ]), additional intrinsic and environmental factors that impact larval development rate (e.g., tissue type, density, and genetics [ 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 ]), and even the developmental datasets used to produce an estimation range [ 19 , 36 ]. As such, forensic entomologists are urged to be transparent about the assumptions being made regarding the generation of entomological estimations in forensic casework [ 8 , 37 ].…”