Int J Legal Med DOI 10.1007/s00414-014-1094-x ORIGINAL ARTICLE Development and validation of a new technique for estimating a minimum postmortem interval using adult blow fly (Diptera: Calliphoridae) carcass attendance Rachel M. Mohr & Jeffery K. Tomberlin Received: 6 May 2014 /Accepted: 6 October 2014 # Springer-Verlag Berlin Heidelberg 2014 Abstract Understanding the onset and duration of adult Introduction blow fly activity is critical to accurately estimating the period of insect activity or minimum postmortem interval (minPMI). When an animal dies, it attracts a variety of scavengers that Few, if any, reliable techniques have been developed and break down the body and return its nutrients to the soil [1]. consequently validated for using adult fly activity to deter- The preeminent mechanism of that mechanical breakdown is mine a minPMI. In this study, adult blow flies (Diptera: the feeding of immature arthropods such as blow flies Calliphoridae) of Cochliomyia macellaria and Chrysomya (Diptera: Calliphoridae). Although the mobile adult insects rufifacies were collected from swine carcasses in rural central arrive before their progeny, blow fly larvae have predictable, Texas, USA, during summer 2008 and Phormia regina and thermally dependent development, which can be used to esti- Calliphora vicina in the winter during 2009 and 2010. Carcass mate a “postmortem interval” (PMI) [2]. Therefore, most attendance patterns of blow flies were related to species, sex, existing research on insect/carcass interaction has focused on and oocyte development. Summer-active flies were found to the immature stages in studies with human decedents [3, 4], arrive 4–12 h after initial carcass exposure, with both dogs [5], and swine [6]. In contrast to a strictly developmental C. macellaria and C. rufifacies arrivingwithin2hofone perspective, however, Tomberlin et al. [7] described the insect another. Winter-active flies arrived within 48 h of one another. activity on a carcass as a series of physiological and behav- There was significant difference in degree of oocyte develop- ioral responses of insects to specific cues provided by the ment on each of the first 3 days postmortem. These frequency carcass and its associated microbial flora. In this interaction- differences allowed a minPMI to be calculated using a bino- based framework, carrion use by insects is dictated by the mial analysis. When validated with seven tests using domestic process of neurosensory detection and behavioral activation, and feral swine and human remains, the technique correctly followed by searching and location of the carcass prior to estimated time of placement in six trials. colonization, defined as the period of insect activity (PIA) [7]. The portion of the PIA encompassed by the initial detec- tion of carrion by an adult insect until colonization is defined Keywords Forensic entomology . Decomposition ecology . as the pre-colonization interval (pre-CI). The pre-CI is little Cochliomyia macellaria . Chrysomya rufifacies . understood although recent work by several authors has Pre-colonization interval . Postmortem interval shown that factors such as temperature, humidity, and wind speed can have a significant impact on the duration of the pre- CI in species such as blow flies [8–11] and beetles [12, 13]. Adult insects represent the most mobile phase of the arthropod carrion breakdown community, controlling overall geographic R. M. Mohr (*) Department of Forensic and Investigative Sciences, West Virginia distribution, patch-based species interactions, and aggrega- University, Morgantown, WV, USA tion. Mobility also influences the speed at which individual e-mail: [email protected] insects can reach any given carcass and colonize it. Adult insect behavior can be highly variable, based on R. M. Mohr : J. K. Tomberlin Department of Entomology, Texas A&M University, College Station, species ecology and biology. From a decomposition ecology TX, USA perspective, central Texas makes an excellent study system. Int J Legal Med The extreme high heat of the summer promotes rapid decom- this study was to test and validate a novel method of PIA position, while the mild winters prevent frozen preservation. estimation based on the ovarian physiology of adult blow flies. Approximately ten species of Calliphoridae are known to inhabit carrion in Texas with variable seasonal occurrence [14]. Two particularly important summer-active species are Methods Cochliomyia macellaria (F.) (Diptera: Calliphoridae), the na- tive secondary screwworm [15, 14], and Chrysomya rufifacies For each trial, three 60–80 kg white commercial swine (Sus Macquart (Diptera: Calliphoridae), the invasive hairy maggot scrofa domesticus L.) were obtained from a commercial abat- blow fly. This species is facultatively predacious and canni- toir. Each pig was killed by humane cranial trauma to avoid balistic as third instars, and its apparent intraguild advantage any tranquilizer effects [23] and to better mimic traumatic may be shifting the geographic distribution and population human death [24]. The Texas A&M University Institutional size of other native blow fly species [16–18]. Animal Care and Use Committee required no animal use In the winter, two commonly encountered blow flies are protocol, as the swine were deceased at the time of acquisition. Calliphora vicina Robineau-Desvoidy (Diptera: Calliphoridae) The experimental animals were killed at approximately 07:30 and Phormia regina Meigen (Diptera: Calliphoridae). C. vicina for the summer trials and 09:30 for the winter trials. Within 1 h is the most cold hardy of the local flies, active even in the depth of death, the swine were placed in rural pasture near Snook, of winter [15]. P. regina is a widely distributed blow fly, active Texas (30° 26′ 14″ N/96° 25′ 12″ W). Each pig was placed in in both spring and summer in colder climates. This species is full sun along a north/south line approximately 40 m apart. To one of the more generalist blow flies, accepting much older prevent nocturnal vertebrate scavenging, carcasses were placed carrion as a colonization site than other species [15, 19, 20]. beneath a wire cage each evening. For the summer trials, trial 1 Because baseline pre-CIs are not known for local blow fly was conducted from 7 to 9 August 2008 and trial 2 from 5 to 7 species, a primary purpose for this study was to document the September 2008. For the winter, trial 3 was conducted from 7 to duration of the location phase and the timing of initial appear- 13 January 2009 and trial 4 from 24 February to 7 March 2010. ance at a carcass. By allowing a better estimation of pre-CI for Following pig placement, blow fly collection was attempted species encountered during casework, forensic entomologists at hourly intervals between sunrise and sunset [25] as both can better calculate minimum postmortem intervals (minPMI), preliminary observations and other authors have found only encompassing both the pre-CI and post-CI portions of the negligible nocturnal fly activity before sunrise or after sunset period of insect activity or minPMI [7]. A further purpose of [26]. Standard collections consisted of ten targeted sweeps of a documenting adult blow fly behavior was to record some of the 21-cm aerial net made within 30 s over each carcass. Flies thus physiological factors that underlie carcass attendance by collected were preserved immediately in ∼80 % ethanol. blow flies. Once known, the patterns of carcass attendance Observations were continued 3–14 days postmortem until might be linked with a deeper ecological explanation such as calliphorid third instars were observed on all carcasses, as this interspecific competition or pathogen avoidance [21]. Such stage typically would be identifiable to species level by a underlying ecological, genetic, or fitness-based reasons stabiliz- forensic entomologist and readily applied to an investigation. ing a behavior or biological process allow the behavior or In the laboratory, individual flies were removed from the process to be predictable within certain bounds. Finding pre- ethanol and externally dried before identification by species dictable trends in carrion-associated insects and linking them to and sex. Those belonging to family Calliphoridae were weighed ecology is a great goal of forensic entomologists striving to to 0.0001 g with an Adventure-Pro AV64 Ohaus scale (Pine meet the recommendations of the 2009 National Academy of Brook, NJ, USA). To assess physiological development state, the Sciences/National Research Council (NAS/NRC) report on ovaries of female flies were dissected using Anderson’s method forensic science [7]. Of particular interest is the development under a Meiji Techno EMZ-8TR microscope (Santa Clara, CA, of quantitative, stochastic, or probabilistic methods for estimat- USA) [27]. One arbitrary oocyte from each fly was measured ing portions of the PMI. As adult blow flies typically represent lengthwise to 0.1 mm while wet with ethanol. A single oocyte the first insects to arrive at a fresh cadaver, their behavior could was chosen instead of the entire ovary, both to decouple the be useful in allowing the development of an analytical tech- degree of yolk deposition from the positive effect of body size on nique by which the pre-CI can be estimated. Aside from being a overall ovary dimension [28] and to allow for a rapid dissection powerful technique in its own right, estimations of PIA based process. As 1.2 mm is a conservative estimate of the length for on adult blow flies would be useful for recently exposed ca- viable eggs of all three tested species [29], and all oocytes thus davers, where the small size of larvae, or lack thereof, impedes observed were clearly in the final stage of development prior to identification and application in forensic casework [22]. As part oviposition [30], flies with oocytes >1.2 mm were therefore of the development of any technique, particularly one used in classified as “gravid,” cf. Gordh and Headrick [31]. forensic science, the useful limits, proper application, and val- Statistical analysis (SPSS 15.0 (SPSS Corp., Chicago, IL)) idation must be investigated.