insects Review Post-Mortem Interval Estimation Based on Insect Evidence: Current Challenges Szymon Matuszewski 1,2 1 Laboratory of Criminalistics, Adam Mickiewicz University, Swi˛etyMarcin´ 90, 61-809 Pozna´n,Poland; [email protected] 2 Wielkopolska Centre for Advanced Technologies, Adam Mickiewicz University, Uniwersytetu Pozna´nskiego 10, 61-614 Pozna´n, Poland Simple Summary: The post-mortem interval of human cadavers may be estimated based on insect evidence. In order to identify scientific challenges that pertain to these estimations, I review forensic entomology literature and conclude that research on the development and succession of carrion insects, thermogenesis on cadavers and the accuracy of PMI estimates are of primary importance to advance this field. Abstract: During death investigations insects are used mostly to estimate the post-mortem interval (PMI). These estimates are only as good as they are close to the true PMI. Therefore, the major chal- lenge for forensic entomology is to reduce the estimation inaccuracy. Here, I review literature in this field to identify research areas that may contribute to the increase in the accuracy of PMI estimation. I conclude that research on the development and succession of carrion insects, thermogenesis in aggregations of their larvae and error rates of the PMI estimation protocols should be prioritized. Challenges of educational and promotional nature are discussed as well, particularly in relation to the collection of insect evidence. Citation: Matuszewski, S. Post-Mortem Interval Estimation Keywords: forensic entomology; carrion insects; development; succession; validation Based on Insect Evidence: Current Challenges. Insects 2021, 12, 314. https://doi.org/10.3390/ insects12040314 1. Introduction Academic Editor: Daniel Martín-Vega Carrion insects living in human cadavers can be highly useful for the estimation of the post-mortem interval (PMI) [1,2]. Methods for PMI estimation based on insect evidence Received: 28 February 2021 are developed, validated, improved and applied by forensic entomologists. This field is Accepted: 29 March 2021 growing with a constant increase in the number of scientific publications and countries where Published: 1 April 2021 entomology-based estimation of PMI is regularly used in death investigations [3,4]. As a maturing field, forensic entomology contains several weaknesses and under-researched areas. Publisher’s Note: MDPI stays neutral These challenges are the focus of this article. with regard to jurisdictional claims in A PMI estimate is only as good as it is close to the true PMI. The accuracy of estimation published maps and institutional affil- is most important, particularly for the end users of insect evidence. Therefore, the major iations. general challenge for the field is to reduce the estimation inaccuracy. Its sources are related to both the collection and analysis of insect evidence (Figure1). I divided this paper into sections devoted to the collection of insect evidence, research on insect development and succession, reconstructing temperature conditions, analysis of challenging evidence and Copyright: © 2021 by the author. validation of the protocols for PMI estimation. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Insects 2021, 12, 314. https://doi.org/10.3390/insects12040314 https://www.mdpi.com/journal/insects Insects 2021, 12, 314 2 of 21 Insects 2021, 12, x 2 of 22 FigureFigure 1. Sources 1. Sources of of inaccuracy inaccuracy in the the estimation estimation of the ofthe post-mortem post-mortem interval interval (PMI) based (PMI) on based insect on evidence. insect evidence.Green circles Green represent the sources, their size represents importance of the sources. Pictures of insects were made by Anna Mądra- circles represent the sources, their size represents importance of the sources. Pictures of insects were made by Anna Bielewicz (Poznań, Poland). M ˛adra-Bielewicz(Pozna´n,Poland). 2. Collection of Insect Evidence 2. Collection of Insect Evidence Errors in the collection of insect evidence are certainly among the most important Errors in the collection of insect evidence are certainly among the most important sources of the inaccuracy in PMI estimation. Death scene samples frequently misrepresent sourcescadaver of entomofauna. the inaccuracy However, in PMI estimation.it is difficult Deathto discern scene how samples bad these frequently samples misrepresent usually cadaverare and entomofauna. what the consequences However, of sampling it is difficult erro tors are discern for the how estimation bad these of PMI. samples In most usually arecases and insects what theare consequencescollected by law of enforcement sampling errors officers are or for medical the estimation examiners, of and PMI. rarely In most casesby entomologists. insects are collected In a recent by case, law insects enforcement were sampled officers by or police medical officers examiners, with the andmedi- rarely bycal entomologists. examiner and independently In a recent case, by entomolo insectsgists, were which sampled enabled—in by police this officers paper—the with the medicalcomparison examiner of samples and independently taken by non-experts by entomologists, and experts [5]. which The sample enabled—in taken by this non- paper— theexperts comparison was distinctly of samples less diverse taken and by non-experts did not contain and insect experts evidence, [5]. The based sample on which taken by non-expertsPMI has finally was been distinctly estimated less (see diverse Tables and1 and did 2 in not[5]). containIf PMI was insect estimated evidence, in this basedcase on whichusing PMIonly hasthe non-expert finally been sample, estimated no meaningful (see Tables maximum1 and2 in PMI [ 5would]). If PMIbe derived, was estimated alt- inhough this case the minimum using only PMI the would non-expert be similar sample, to the noone meaningful estimated based maximum on the expert PMI wouldsam- be derived,ple (unpublished although data). the minimum Another kind PMI of would error bein the similar collection to the of one evidence estimated is thebased error of on the expertpreservation. sample Insects (unpublished may be preserved data). Another improperly, kind for of example error in using the collectionan unsuitable of evidencepre- isservative the error or of a preservation.leaking container Insects [6,7]. maySuch beerrors preserved may limit improperly, the scope of for possible example analyses using an and in extreme cases may even destroy the evidence. Although there are no surveys of unsuitable preservative or a leaking container [6,7]. Such errors may limit the scope of errors in the collection of insect evidence, I think that most experts share the opinion that possible analyses and in extreme cases may even destroy the evidence. Although there insect samples frequently misrepresent cadaver entomofauna or are preserved improp- areerly. no We surveys should of therefore errors in discuss the collection whether our of insectguidelines evidence, for the Icollection think that of mostevidence experts shareare truly the opinionfit-for-purpose. that insect samples frequently misrepresent cadaver entomofauna or are preservedGuidelines improperly. for the collection We should of insect therefore evidence discussstate that whether death scene our samples guidelines should for the collectionaccurately of represent evidence cadaver are truly entomofauna, fit-for-purpose. i.e., all life stages of each important species thatGuidelines inhabit a cadaver for the should collection be represent of insected evidence in the sample state [8–10]. that death Cadaver scene entomofauna samples should accuratelymay be very represent diverse cadaver and abundant, entomofauna, consisting i.e., allof many life stages life stages of each from important many species, species that inhabitsome in a cadaver very large should numbers. be represented However, to in estimate the sample PMI [only8–10 a]. small Cadaver part entomofaunaof it is necessary. may be very diverse and abundant, consisting of many life stages from many species, some in very large numbers. However, to estimate PMI only a small part of it is necessary. Usually, we choose the most developmentally advanced life stage of the most successionally advanced species, and even if the PMI estimate is based on a larger number of taxa, this is usually no more than two or three [11]. Therefore, in most cases a representative sample is redundant, Insects 2021, 12, 314 3 of 21 and for this reason we should reshape guidelines for the collection of insect evidence and abandon our commitment to the true representativeness of death scene samples. I believe it is possible to develop guidelines that are user-friendly, quick to implement and that yield more fit-for-purpose samples, i.e., the most developmentally and successionally advanced insects only (Table1). Insects are usually collected by the law enforcement officers with basic skills in entomology, whereas guidelines for the collection of insect evidence are usually addressed to entomologists. Therefore, we should provide guidelines for non- entomologists that specify what insect evidence they should look for and where it can be found, with pictures of the evidence and related preservation protocols. Table 1. A sketch of guidelines for the collection of insect evidence on a death scene. To