Flatland forensics: Analyzing insect succession on decomposing remains in a High Plains agricultural environment
Forensic Entomology is the analysis of arthropods and their respective life cycles in medico-legal investigations, typically to determine a time period in which cessation of life had occurred. The process of human decomposition causes multiple chemical changes in a corpse, which attract a diversity of insects in a usually predictable pattern (i.e. developmental succession of the decomposer insect community). This pattern of insect succession can assist death investigators in determining a deposition period known as a post- mortem interval (PMI). Insect succession is largely affected by the environment and ecology of the location where decomposition occurs. Because of this, it is necessary to understand how various environments influence insect succession, and how this information can support death investigations.
Agricultural croplands have a unique environment that is not found in other areas due to the lack of plant diversity, intensive management with tillage and chemical applications, as well as the presence of irrigation. Despite the uniqueness of croplands ecology, and its ubiquity throughout the United States, no forensic entomology study has been conducted in a cropland environment in the United States at the time of writing. This study will impact the field of forensic science by filling a gap of knowledge regarding deposition sites in agricultural environments, one of the largest land use categories in the southern United States, thus supporting determinations by death investigators in estimating Post-Mortem Intervals involving insect succession. Furthermore, this project will be a first to document decomposer insect biodiversity in these systems.
Previous research conducted in Eastern Europe and Saudi Arabia has focused on agricultural environments, targeting the analysis of insect succession patterns on human analogs. These studies found differences in both insect population size and diversity. The focus of this study is to analyze insect succession patterns and decomposition rates in cropland environments in the major agricultural region in the Southern High Plains of western Texas. Human analogs (i.e. pigs) weighing between 120-135 pounds were placed in scavenger proof cages in four different locations representing two habitats: 1) wild grassland areas (i.e. controls) in ruderal habitats and 2) irrigated cotton fields. Insects were collected using pitfall traps, pan traps and by hand using a sweep net at the perimeter of cages. Decomposition of pig carcasses was scored and photographed.
Each type of collecting device produced one sample of the insect community on one specific date, and data was also composited across methods to yield a total community sample for a specific date. After field sampling, insects were stored in isopropyl alcohol and identified using keys of regional fauna. Indicator species analysis was used to determine affinities of insects for one of the two habitat types (wild grassland patches or irrigated cotton fields). Following tests of data normality, ANOVA was used to compare 1) taxonomic richness, 2) total number of individuals, and 3) relative abundances of focal species across space and time.
This study found a significant difference in taxonomic richness between the two studied sites, as well as a difference in relative abundance for five dominant families in the orders Coleoptera and Diptera. Six indicator species were identified for each of the habitats studied. The decay rate for the cropland environment was faster than the grassland habitat, and differences in onset and duration of decomposition stages was observed. Through a better understanding of insect succession patterns in a cropland environment on decomposing mammalian carcasses, this study helps death investigators in determining Post-Mortem Interval in row-crop region where information is lacking. Furthermore, there is a need for regional insect decomposition and succession studies to be conducted to support forensic investigations associated with different land management practices in agriculture.