Insecticides and Forensic Entomology: Understanding effects on blow fly (Diptera: Calliphoridae) oviposition, development, and carcass decomposition

Blow flies (Diptera: Calliphoridae) are forensically significant insects known for their rapid detection and colonization of decomposing remains shortly after death. By ovipositing on carcasses (human or animal), blow flies start a biological timeline that enables forensic entomologists to estimate the minimum postmortem interval (minPMI) based on their developmental progression. However, numerous biotic and abiotic factors influence blow fly behavior and the decomposition process. These include inter- and intra-specific competition, parasitism, and scavenging, as well as environmental variables such as precipitation, wind, sun exposure, and temperature, all of which have been reported to affect the decomposition rate and insect colonization patterns. Additionally, anthropogenic factors, such as the concealment of remains—whether through physical barriers or chemical means to mask decomposition odors—have been shown to alter insect succession and delay colonization.

The research presented in this dissertation investigates the effects of two commercially available products: Talstar® P Professional, which contains 7.9% bifenthrin (a pyrethroid) as the active ingredient, and Arena 50 WDG, which contains 50% clothianidin (a neonicotinoid) as the active ingredient, on key forensic entomology parameters. Specifically, this study examines 1) blow fly development under controlled laboratory conditions, 2) oviposition behavior in both laboratory and field, 3) species composition of blow flies colonizing remains, and 4) decomposition progression in pig carcasses within the state of Indiana.

The first chapter of this dissertation introduces the field of forensic entomology and its importance in criminal investigations. This chapter also describes the environmental, biotic, and anthropogenic factors that influence blow fly activity and carcass decomposition. In the second chapter, the effects of bifenthrin and clothianidin on the oviposition behavior of Lucilia sericata and Phormia regina were evaluated under controlled laboratory conditions using multiple insecticide concentrations. In the field, stillborn pig carcasses were treated with the same concentrations to assess oviposition timing and site preference. Although statistical analyses revealed no significant effect of bifenthrin or clothianidin on oviposition timing in laboratory trials, bifenthrin showed a positive trend between concentration and time to oviposition for both species. Species-specific and insecticide-specific responses were observed, including high mortality rates associated with clothianidin exposure. In field experiments, bifenthrin significantly delayed oviposition, while clothianidin, at high concentrations, completely inhibited oviposition. There was, however, no effect of insecticide on oviposition site preference.

The third chapter determined the influence of bifenthrin and clothianidin on the diversity and relative abundance of blow fly species colonizing small pig carcasses. While bifenthrin exposure negatively affected the number of flies reared, this effect was not statistically significant. In contrast, clothianidin significantly reduced blow fly abundance (p = 0.05), with no colonization occurring at the highest concentration. Species diversity varied between insecticide treatments. In the bifenthrin experiment, four species were identified: P. regina, L. illustris, L. coeruleiviridis, and L. sericata. However, only L. sericata and L. coeruleiviridis were recovered in the clothianidin-treated carcasses.

In the fourth chapter, the effects of bifenthrin and clothianidin on pig carcass decomposition were evaluated over a 20-day observation period using qualitative and quantitative assessments, including Total Body Score (TBS) measurements. Carcasses progressed through the expected decomposition stages—fresh, bloat, active decay, advanced decay, and skeletal remains—in both insecticide trials. Although statistical analyses suggested no significant difference in decomposition between treatments of both insecticides, the trends observed and TBS values suggest a delay in decomposition at the high concentrations.

In chapter five, the impact of bifenthrin and clothianidin on the development of L. sericata and P. regina was examined by rearing eggs on insecticide-treated liver at low, medium, and high concentrations. Key developmental parameters assessed included larval growth (measured by length), pupation and adult emergence rates, adult size, sex ratios, and overall development duration. Both insecticides had significant effects on larval growth, with bifenthrin significantly reducing P. regina adult size, while clothianidin significantly delayed P. regina development at higher concentrations.

Altogether, these findings highlight the potential forensic implications of insecticide exposure on blow fly colonization and carcass decomposition, emphasizing the need for forensic entomologists to consider chemical contamination as a confounding variable in minPMI estimations.