Impact of Comingled Heterospecific Assemblages on Developmentally Based Estimates of the Post-Mortem Interval-A Study with Lucilia sericata (Meigen), Phormia regina (Meigen) and Calliphora vicina Robineau-Desvoidy (Diptera: Calliphoridae)

The utility of blow fly (Diptera: Calliphoridae) evidence from burned human remains

Burning of human remains is a common method to conceal or destroy evidence associated with homicides and illegal activities. However, data regarding blow fly colonization of burned remains are scarce, with all previously published empirical studies focusing only on non-human animals. It is critically important to obtain basic data on blow fly colonization patterns of burned human remains as such evidence may represent the only feasible method for PMI estimation in cases of burning. In this study, we thermally altered six human donors to a Crow-Glassman Scale Level 3 (CGS-3) and placed them at the Anthropology Research Facility at the University of Tennessee in Summer 2021, Spring 2022, and Summer 2022. Six unburned human donors were used as controls. Observations for insect activity began within 24 h of placement and continued twice weekly through decomposition. Age estimations were performed with immature blow flies to estimate the time of colonization (TOC), and accuracy was assessed against the time of placement for each donor. All burned donors examined in this study were colonized by blow flies. No significant difference in species composition was determined between treatments, though TOC estimations from burned donors were slightly (but significantly) less accurate than TOC estimations from unburned donors (80% vs. 83% accuracy; χ2 = 0.041, df = 1, P = 0.840). These results indicate that blow flies can successfully colonize human remains burned to CGS-3 and that accurate TOC estimations can still be generated from larval specimens. Though several limitations to this study exist (e.g., inconsistent donor BMI, lack of donor temperature data), our results underscore the utility of entomological evidence in cases of burned human remains.

Aggregation in an heterospecific population of blowfly larvae: social behaviour is impacted by species-specific thermal requirements and settlement order

Larvae of several blowfly species grow on carcasses and actively aggregate together. They face harsh developmental conditions resulting in a strong pressure to reduce development time: this is achieved either through thermoregulation or aggregation. We investigate how these two developmental strategies are modulated within heterospecific groups. In a first experiment, larvae of two species with different thermal requirements were deposited simultaneously on a thermal gradient. This resulted in the formation of two monospecific groups, each located at the species-specific thermal preferendum. However, when Calliphora vomitoria (Linnaeus) larvae were placed first, the later arriving Lucilia sericata (Meigen) larvae attracted the whole group to its own thermal preferendum. In the reverse experiment, half of the replicates resulted in single dense heterospecific groups observed at temperatures ranging from C. vomitoria to L. sericata preferendum. The other half of the replicates resulted in loose groups spread out on the thermal gradient. These results highlight the emergence of collective decisions ranging from thermal optimization to heterospecific aggregation at suboptimal temperatures. They demonstrate that species settlement order strongly affects self-organization processes and mixed-species group formation. We conclude that thermal optimization and heterospecific niche construction are two developmental strategies of carrion fly larvae. This article is part of the theme issue 'Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes'.

Density, Temperature, and Comingled Species Affect Fitness within Carrion Communities: Coexistence in Phormia regina and Lucilia sericata (Diptera: Calliphoridae)

Blow fly (Diptera: Calliphoridae) interactions vary between competition and facilitation. Female blow flies engage in aggregated egg-laying, resulting in larval feeding masses differing in density and species composition. Numerous species are abundant within the same season, and some oviposit near or directly on eggs of other species, modifying their oviposition location choice depending on the presence or absence of other species. The ability to coexist on carrion, a temporary resource, was successfully attributed to resource, spatial, and temporal heterogeneity. Despite these broad categorizations, the specific mechanisms of coexistence within blow fly communities require further investigation. This study investigates variation in temperature and larval density as potential mechanisms of coexistence between two forensically important blow fly species: Lucilia sericata Meigen and Phormia regina Meigen (Diptera: Calliphoridae). Larval density, species ratio mix, and ambient temperature during development were manipulated in the presence of conspecifics and heterospecifics in the laboratory, and the fitness of each species was measured. In heterospecific treatments, the survival and body size of P. regina increased even at high ambient temperatures. In contrast, the survival of L. sericata remained unaffected by density or presence of heterospecifics, whereas body size increased in L. sericata-dominated heterospecific treatments depending on temperature and density. The negative effects of density were observed at high ambient temperatures, suggesting that density impacts are a function of ambient temperature. Overall, species coexistence was dependent on temperature, which mediated the outcome of species interactions.

Fall Decay Deceleration in Northern Latitudes: Merely a Matter of Cold?

The decomposition of cadavers and large vertebrate carcasses is the result of complex processes primarily influenced by ambient temperatures. Thus, low temperatures can alter decomposition by curtailing tissue autolysis and bacterial decomposition, and by limiting insect activity contributing to necromass removal. In this study, we tested whether carcass decomposition rate is modulated not only directly by temperature and insect occurrence, but also indirectly by the mediation of interactions among insects by ambient temperature. To test this, a comparative analysis of the decomposition of domestic pig carcasses in summer and fall was conducted in Atlantic Canada. The results indicated that carcass decomposition standardized to account for seasonal differences was significantly decelerated in the fall as opposed to the summer during the later decomposition stages and was sometimes incomplete. Moreover, the arrival, presence, and departure of insects from carcasses during ecological succession differed between summer and fall. Necrodes surinamensis (Fabricius) (Coleoptera: Silphidae) and Creophilus maxillosus (Linnaeus) (Coleoptera: Staphylinidae) maintained higher abundances late during succession in the fall than in the summer and their abundance was related to a decline in decomposition rates, probably because these species feed on dipteran larvae promoting necromass removal. These results demonstrate the variability in response to environmental parameters of insects of forensic importance and support the idea that slowed decomposition in the fall may be exacerbated by changes in interspecific interactions among insects. Furthermore, these results suggest that successional studies of insects carried out in the summer have little forensic utility for cadavers found in cold weather conditions.

Blow Flies (Diptera: Calliphoridae) and the American Diet: Effects of Fat Content on Blow Fly Development

Forensic entomology is an important field of forensic science that utilizes insect evidence in criminal investigations. Blow flies (Diptera: Calliphoridae) are among the first colonizers of remains and are frequently used in determining the minimum postmortem interval (mPMI). Blow fly development is influenced by a variety of factors including temperature and feeding substrate. Unfortunately, dietary fat content remains an understudied factor in the development process, which is problematic given the high rates of obesity in the United States. To study the effects of fat content on blow flies we investigated survivorship, adult weight, and development time (overall and by sex) of Lucilia sericata (Meigen) and Phormia regina (Meigen) on ground beef with 10%, 20%, or 27% fat. As fat content increased, survivorship decreased across both species with significant impact to P. regina. While P. regina adults were generally larger than L. sericata across all fat levels, only L. sericata demonstrated a significant (P < 0.05) difference in weight by sex. Overall development times did not vary by fat content, excluding 27% for P. regina. Additionally, development times did not vary by sex for P. regina but did differ (P < 0.05) for L. sericata with females taking longer to develop. These findings provide insight into the effect of fat content on blow fly development, a factor that should be considered when estimating an mPMI. By understanding how fat levels affect the survivorship and development of the species studied here, we can begin improving the practice of insect evidence analysis in casework.

Coexistence and intraguild competition of Chrysomya albiceps and Lucilia sericata larvae: case reports and experimental studies applied to forensic entomology

The larval development of Chrysomya albiceps and Lucilia sericata is a well-known and valuable tool for estimating the postmortem interval (PMI). The third instar larvae of Ch. albiceps are facultative predators of the larvae of other necrophagous dipterans, and this behaviour is undoubtedly an important factor to consider. Both species are found together during human autopsies in Spain and other parts of Europe, where they cohabit, so a precise knowledge of their larval coexistence data is essential. The aim of this study is to investigate their coexistence and the intraguild predation of Ch. albiceps on L. sericata under experimental conditions and in real case reports. To analyse intra- and interspecific competition, four densities were used under controlled abiotic conditions [25ºC, 60-70% RH and 12:12 (D:N)]. The experimental data were compared with data corresponding to their coexistence in natural conditions, so annual activity and forensic case reports for both species were studied. The results indicate that the mortality of immature specimens in both species, the preimaginal developmental time, and adult size were affected by competition. In natural conditions, adult coexistence during the spring-summer period was confirmed. The simultaneity of the two species colonising human corpses is frequent in the studied area, with Ch. albiceps being the dominant species, and L. sericata the relevant species for estimating the minimum postmortem interval.

Domestic Filth Flies in New Haven, Connecticut: A Case Study on the Effects of Urbanization and Climate Change by Comparing Fly Populations after 78 Years

Simple Summary

Domestic filth fly population data were collected in the summers of 1942–1944 in the urban city of New Haven, Connecticut, during a polio epidemic. The current survey was completed 78 years later by setting out a weekly trap in the same region during June–September over a two-year period. Results indicate that the fly population has changed in the city, with 16 fewer species trapped overall, and there have been changes in the fly species trapped. Some species have increased in abundance, notably Lucilia coeruleiviridis, while numbers of the common Lucilia sericata have decreased, and Lucilia illustris was absent. Changes in land cover and climate were also assessed to show that the trap site has experienced significant habitat change, together with an increase in the average temperature and rainfall. Fly numbers were significantly affected by temperature and rainfall in both the 1940s and the current survey. The results of this study suggest the prolonged period of urbanization of the region is influencing the domestic filth fly population.

Abstract

Changes in common and widespread insect populations such as the domestic filth fly in urban cities are useful and relevant bioindicators for overall changes in the insect biomass. The current study surveyed necrophagous flies by placing a weekly trap from June–September over a two-year period in the city of New Haven, Connecticut, to compare data on fly abundance and diversity with data collected 78 years earlier. Climate and land cover changes were also assessed in combination with the fly population for each period. The survey results suggest the domestic filth fly population is now less diverse with decreased species richness and changes in the relative abundance of species. In both surveys, 95–96% of the population was composed of only three species. The current survey data indicate the numerical dominance of Lucilia sericata has decreased, the abundance of several species, notably Lucilia coeruleiviridis, has increased, and Lucilia illustris is absent. Species that showed a significant interaction with temperature in the 1940s survey have now increased in abundance, with several of the trapped species continuing to show an interaction with temperature and rainfall. Analysis of the land cover and climate data characterizes the trap site as a region exposed to a prolonged period of industrialization and urbanization, with only 7% of the land cover remaining undeveloped and over 50% impervious, coupled with an increase in temperature and rainfall. This study serves as a model for changes in domestic filth fly populations and other insects in similarly highly urbanized established cities.

Balance between larval and pupal development time in carrion blowflies

Several studies have highlighted the impact of environmental factors such as food type or larval density on the development of blowfly larvae. We investigated how changes in development speed (due to larval density and group composition) are divided among feeding and post-feeding stages. Even if these parameters impinge only on feeding larvae, they may ultimately also affect their subsequent development, and especially metamorphosis duration. Therefore, this study analysed the effect of larval density and group composition on the rhythm of necrophagous blowfly development. Based on laboratory studies, we highlighted that Calliphora vicina individuals with a fast development during their feeding phase developed slower in the later post-feeding phase (i.e., they had a compensatory effect). Lucilia sericata, a calliphorid species also frequently found on carrion at the same time as C. vicina, showed a different developmental strategy by not making its post-larval development speed dependent on the larval development speed. Finally, while a compensatory effect may exist, variations in the development rate more often accumulate through life-stages and resulted in a larger variability for later development instars. In this respect, the inclusion of detailed development duration covering all life stages, including post-feeding, is recommended in future studies, especially those dedicated to forensic entomology.

Looking Back to Move Forward: How Review Articles Could Boost Forensic Entomology

The Locard's exchange principle (1930) holds that the perpetrator of a crime leaves traces behind that can later be sampled and used as forensic evidence [...].