The effect of rainfall on blowfly (Calliphoridae) activity and decomposition on recently deposited animal remains
McLeod, Elizabeth Van Hoven
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The accurate estimation of the postmortem interval (PMI) is one of the most important determinations in a forensic investigation of decomposing human remains. Forensic entomology has gained popularity in death investigation due to its reliability and precision in the estimation of the minimum postmortem interval (mPMI). Forensically significant insects are mainly necrophagous species, which feed only on decomposing animal matter, and the most common necrophagous insects are the blowflies (Diptera: Calliphoridae). Estimations of the mPMI by entomological methods are made using the known developmental rates of various species of blowfly and via the successional patterns of the carrion insect community in a given region. It is generally assumed that blowflies oviposit quickly after death, so in many cases this time may equate to the time since death. The precision of mPMI estimations based on the developmental rates of blowflies often relies on this assumption. Rainfall may effect decomposition by inhibiting access of insects to the cadaver or carcass for oviposition. The current study investigated the effects of rainfall on blowfly activity, behavior, and overall decomposition of decaying animal material in an outdoor environment in the northeastern United States, conducted at the Boston University Outdoor Research Facility (ORF). It was hypothesized that natural rainfall, typically light to moderate in the geographic area of study, will disturb initial blowfly activity by acting as a physical barrier, diminishing access to the remains, and creating a delay in colonization and subsequent larval development. This hypothesized delay would result in an underestimation of the mPMI by entomological methods when rainfall has occurred. Also examined were several questions about the nocturnal behavior of blowflies and their activity in heavy rain. In the experimental trial 12 pig (Sus scrofa) heads were exposed under normal conditions (N; no rain controls), and 15 pig heads were exposed under rainy conditions (R; rain treatment), split into uncovered (N, n=6; R, n=5), covered (NC, n=5; RC, n=5), and covered partially (RCP, n=5) treatments. Additionally, there were three pig heads used in a preliminary trial and three pig heads exposed in an active rain trial. Generally, the results show that while a negative correlation exists between the amount of rainfall experienced and the coded number of flies observed, the light to moderate rainfall typical of many rainy days in the northeastern Unites States will not totally inhibit blowfly activity or disturb established maggot masses. A one-way analysis of variance (ANOVA) determined that there was no statistically significant difference (p > 0.05) between the N, NC, R, RC, and RCP treatments in the number of days it took to reach the advanced decomposition stage. While constant, heavy rainfall may inhibit blowfly activity; the results suggest that the irregularity of natural rainfall would rarely produce the conditions necessary for this to make a significant impact estimation of the PMI by entomological methods, although further studies are needed to confirm this conclusion. The results show a positive correlation between solar radiation and the coded number of flies observed. Time of day as a function of the coded number of flies observed during the first 48 hours of exposure forms a bimodal bell curve, confirming that blowflies are diurnal in their natural environment. Additionally, evidence of scavenging by turkey vulture (Cathartes aura) and some unknown animal(s) was observed. The results of this study illustrate the complicated, multivariable nature of the process of decomposition. This study provides preliminary data on the effect of rainfall on blowfly activity and overall decomposition, while future studies will be required to determine the effects of the duration and the intensity of rainfall.