Jean-Philippe Michaud

Predicting the visitation of carcasses by carrion-related insects under different rates of degree-day accumulation.

Common assumptions in forensic entomology are that insects visit and colonize carcasses following a predictable sequence, and that this succession varies among seasons. However, currently available evidence for insect succession on decomposing bodies is essentially descriptive and, to our knowledge, the fine-scale predictability of insect succession with respect to seasons has never been confirmed statistically. In this study, we test these assumptions through the sampling of carrion-related insects attracted to pig carcasses. The study was carried out during the summer and fall of 2006 in rural fields of New Brunswick, Canada. Of the five species of carrion-related insects with high enough occurrence on carcasses to allow modelling, three showed predictable occurrence with respect to degree-day accumulation and seasonal effects. This demonstrates that the occurrence probability of some carrion-related insects on carcasses can be estimated from meteorological records even across seasons with different rates of degree-day accumulation. As opposed to the prevailing idea that adult insects are not reliable for post-mortem interval estimation, the adults of some species exhibited a specific pattern of visitation that could be determined and used in forensic investigations. It is stressed, however, that the statistical predictability of species occurrence must be assessed before any species is considered as a post-mortem interval indicator.

A statistical approach based on accumulated degree-days to predict decomposition-related processes in forensic studies.

Abstract:u2002 Using pig carcasses exposed over 3u2003years in rural fields during spring, summer, and fall, we studied the relationship between decomposition stages and degree‐day accumulation (i) to verify the predictability of the decomposition stages used in forensic entomology to document carcass decomposition and (ii) to build a degree‐day accumulation model applicable to various decomposition‐related processes. Results indicate that the decomposition stages can be predicted with accuracy from temperature records and that a reliable degree‐day index can be developed to study decomposition‐related processes. The development of degree‐day indices opens new doors for researchers and allows for the application of inferential tools unaffected by climatic variability, as well as for the inclusion of statistics in a science that is primarily descriptive and in need of validation methods in courtroom proceedings.

Sampling flies or sampling flaws? Experimental design and inference strength in forensic entomology.

ABSTRACT n Forensic entomology is an inferential science because postmortem interval estimates are based on the extrapolation of results obtained in field or laboratory settings. Although enormous gains in scientific understanding and methodological practice have been made in forensic entomology over the last few decades, a majority of the field studies we reviewed do not meet the standards for inference, which are 1) adequate replication, 2) independence of experimental units, and 3) experimental conditions that capture a representative range of natural variability. Using a mock case-study approach, we identify design flaws in field and lab experiments and suggest methodological solutions for increasing inference strength that can inform future casework. Suggestions for improving data reporting in future field studies are also proposed.

Natural and anthropogenic changes in the insect fauna associated with carcasses in the North American Maritime lowlands.

The insect pool available for carrion visitation and colonisation varies with geographical areas, hence the need to build a comprehensive database wherever such data could be used in forensic investigations. However, most of the geographic records on carrion-related insects are from short-term seasonal studies. Here, we provide the year-round taxonomic composition for the dominant ecosystem of the Maritime lowland ecological region that borders the Gulf of St. Lawrence, Canada, and we examine how this composition is subjected to natural (seasonal) as well as anthropogenic changes. Fresh pig carcasses, used here as human surrogates, were exposed recurrently throughout the whole annual period when carrion-related insects are active in forests and adjacent areas subjected to human-induced land cover changes from agricultural practices. A total of 130 necrophageous and predacious insect species representing 2 orders, 18 families and 75 genera were recovered from carcasses. Abundant fly species were able to visit and/or colonise carcasses exposed in both forests and agricultural fields but the species involved varied throughout the year. Conversely, the complex of abundant coleopterans found on carcasses remained stable throughout the year but differed between forests and agricultural fields. Considering the seasonal and anthropogenic changes that were observed in the complex of carrion-related insects, we stressed that inference on the taxonomic composition in relation to minimum postmortem interval should be restricted to a specific habitat and time of the year. These results also have methodological implications, suggesting that the experimental designs of forensic studies in temperate areas require adjustments to permit robust estimations of minimum postmortem intervals from the insect fauna associated with carcasses.

Effect of Variable Rates of Daily Sampling of Fly Larvae on Decomposition and Carrion Insect Community Assembly: Implications for Forensic Entomology Field Study Protocols

ABSTRACT Experimental protocols in forensic entomology successional field studies generally involve daily sampling of insects to document temporal changes in species composition on animal carcasses. One challenge with that method has been to adjust the sampling intensity to obtain the best representation of the community present without affecting the said community. To this date, little is known about how such investigator perturbations affect decomposition-related processes. Here, we investigated how different levels of daily sampling of fly eggs and fly larvae affected, over time, carcass decomposition rate and the carrion insect community. Results indicated that a daily sampling of <5% of the egg and larvae volumes present on a carcass, a sampling intensity believed to be consistent with current accepted practices in successional field studies, had little effect overall. Higher sampling intensities, however, slowed down carcass decomposition, affected the abundance of certain carrion insects, and caused an increase in the volume of eggs laid by dipterans. This study suggests that the carrion insect community not only has a limited resilience to recurrent perturbations but that a daily sampling intensity equal to or <5% of the egg and larvae volumes appears adequate to ensure that the system is representative of unsampled conditions. Hence we propose that this threshold be accepted as best practice in future forensic entomology successional field studies.

Rewriting ecological succession history: did carrion ecologists get there first?

Ecological succession is arguably the most enduring contribution of plant ecologists and its origins have never been contested. However, we show that French entomologist Pierre Mégnin, while collaborating with medical examiners in the late 1800s, advanced the first formal definition and testable mechanism of ecological succession. This discovery gave birth to the twin disciplines of carrion ecology and forensic entomology. As a novel case of multiple independent discovery, we chronicle how the disciplines of plant and carrion ecology (including forensic entomology) accumulated strikingly similar parallel histories and contributions. In the 1900s, the two groups diverged in methodology and purpose, with carrion ecologists and forensic entomologists focusing mostly on case reports and observational studies instead of hypothesis testing. Momentum is currently growing, however, to develop the ecological framework of forensic entomology and advance carrion ecology theory. Researchers are recognizing the potential of carcasses as subjects for testing not only succession mechanisms (without assuming space-for-time substitution), but also aggregation and coexistence models, diversity-ecosystem function relationships, and the dynamics of pulsed resources. By comparing the contributions of plant and carrion ecologists, we hope to stimulate future crossover research that leads to a general theory of ecological succession.

Facilitation may not be an adequate mechanism of community succession on carrion

The facilitation model of ecological succession was advanced by plant ecologists in the late 1970s and was then introduced to carrion ecology in the late 1980s, without empirical evidence of its applicability. Ecologists in both disciplines proposed removing early colonists, in this case fly eggs and larvae, from the substrate to determine whether other species could still colonize, which to our knowledge has never been attempted. Here, we tested the facilitation model in a carrion system by removing fly eggs and larvae from carcasses that were exposed in agricultural fields and assigned to one of the following treatment levels of removal intensity: 0, <5, 50, and 100%. Subsequent patterns of colonisation did not provide support for the applicability of the facilitation model in carrion systems. Although results showed, in part, that the removal of fly eggs and larvae decreased the decomposition rate of carcasses, the removal did not prevent colonization by secondary colonizers. Finally, we discuss future studies and make recommendations as to how the facilitation model could be improved, firstly by being more specific about the scale where facilitation is believed to be occurring, secondly by clearly stating what environmental modification is believed to be involved, and thirdly by disentangling facilitation from priority effects.

Reply: On Throwing out the Baby with the Bathwater: A Reply to Wells

Dr. Wells letter to the Editor states that the postcolonization Interval (PCI) and period of insect activity (PIA), as defined in Tomberlin et al. (2011), were esoteric and impracticable concepts. Wells then stated that many researchers were incorrectly using the terms in recent articles and cited our article (Michaud et al. 2012) as an example, along with several others (Mondor et al. 2012, Tomberlin et al. 2012, Berg and Benbow 2013, Bygarski and LeBlanc 2013, Picard et al. 2013). Here, we comment on Wells assessment of PIA and PCI and we clarify why a citation to Tomberlin et al. (2011) appeared in the introduction of Michaud et al. (2012). We feel it is important to note that we are not involved in forensic entomology casework. Our views are from a research perspective.nnWells criticism of the PIA and PCI concepts is largely one of semantics. One …

On the mishandling of probabilities in Lamotte & Wells’ commentary on J.P. Michaud, G. Moreau, Predicting the visitation of carcasses by carrion-related insects under different rates of degree-day accumulation

LaMotte and Wells re-analyzed and criticized one of our articles in which we proposed a novel statistical test for predicting postmortem interval from insect succession data. Using simple mathematical examples, we demonstrate that LaMotte and Wells erred because their analyses are based on an erroneous interpretation of the nature of probabilities that disregards more than 300 years of scientific literature on probability combination. We also argue that the methods presented in our article, more specifically the use of degree-day-based logistic regression analysis to model succession, was a positive contribution to the fields of forensic entomology and carrion ecology, which LaMotte and Wells forgot to mention by instead focusing on issues that were either trivial or did not exist.