Succession of Diptera and Coleoptera taxa of plastic wrapped and unwrapped carcasses in coastal New South Wales.

Succession of Diptera and Coleoptera taxa of plastic wrapped and unwrapped carcasses in coastal New South Wales.

Meirah Patterson – Ulladulla High School

This study investigated if plastic wrapping of carcasses delay the insect succession in coastal New South Wales. Piglet carcasses were used and focus was placed on Coleoptera and Diptera taxa. The succession of the taxa differed between the wrapped and unwrapped carcasses. The predominant species observed were C. Varipes and P. Perlata which experienced an insect succession delay of three and six days respectively. Delays of wrapped carcasses were also observed on the other species recorded with a few notable exceptions. C. rufifacies and P. lacrymosa colonised the wrapped carcasses three and eleven days before the unwrapped. Statistical analysis concluded that there was no significantly significant delay of insect succession on wrapped carcasses (p=0.53).

Literature review

Decomposition is a complex process that can be divided into four stages: autolysis, putrefaction, decay and diagenesis or fresh, bloat, wet and dry. Autolysis is the process by which cellular enzymes found in the body, begin to break down the cells of the organism from the inside out. The process of putrefaction is the second stage and involves the breakdown of soft tissues by micro-organisms (fungi, protozoa and bacteria) which produces gases that cause the corpse to bloat (Vass, 2001). The decay stage involves the purging of decomposition fluids and gases into the surrounding environment, as well as the saponification of the flesh of the corpse. (Australian Museum, 2018) The corpse is mummified and then the last stage, diagenesis, the altering of bone components occurs. These four stages result in a fresh corpse turning ultimately to a pile of bones (Vass, 2001).

Schoenly (1987) and Davies (1990) found that forensic entomology is of vital importance in assisting criminal investigations. Studies into forensic entomology have provided knowledge and tools which can assist in determining time of death (Post Mortem Interval or PMI) as well as determine whether or not a body has been moved from the original location of death, based on the examination of species variation found on the body (Joseph, et al., 2011). Recent studies have found that Dipteran species are the most commonly found in forensic investigations. Within this order the main species are Calliphoridae (blow flies), Muscidae (house flies) and Sacrophagidae (flesh flies). Insects from the Coleoptera order are also common colonisers of carcasses. (Joseph, et al., 2011) Coleopteran and Dipteran taxa are consequently the most significant to forensic entomology.

Voss et al. (2011) investigated the succession patterns of nineteen different species from the Diptera and Coleoptera orders onto clothed and unclothed pig carcasses during decomposition. They found that Diptera Calliphoridae Calliphora dubia colonised all of both clothed and unclothed carcasses consistently within the first twenty-four hours. A number of other species also colonised the carcasses in the early stages of decomposition: Calliphora Albifrontalis, Chrysomya Magacephala, Chrysomya Rufifacies and Lucilia Sericata (Diptera: Calliphoridae). Voss et al. (2011) noted that Musca Domestica and Musca Vetustissima (Diptera: Muscidae) were regularly observed throughout the decomposition process on all carcasses. It was also observed that Australophyra rostrata, Chrysomya varipes, Piophila casei and different species from the Sarcophagidae family colonised later. After carcasses had progressed onto the dry remains stage, Trogidae Omorgus tatei, Tenebionidae Helea castor, Silphidae Ptomaphilia lacrymosa and Scarabaeidae Onthophagus binodis (Coleoptera) were observed. Voss et al. (2011) conducted an investigation into the decomposition and insect succession of clothed and unclothed carcasses in Western Australia. Carcasses of 10 pigs were used to model human decomposition. The domestic pig is universally considered to be the preferred animal model for human decomposition due to similar internal anatomy, fat distribution, chest cavity, omnivorous diet and lack of heavy fur. (Schoenly, et al. 2007)

Study site included a wildlife reserve, 23km south of Perth in Western Australia. Trials lasted 98 days and two were run separately in autumn of 2001 and 2003. (Voss, et al., 2011) The investigation provided results which showed that the ‘progression of decomposition differed between clothed and unclothed carcasses. However, this article did not give any clear indication that there was a significant difference between the insect succession of clothed and unclothed carcasses. (Voss, et al., 2011).

Another recently published article, Dautartas (2009) detailed the effect of various coverings on the rate of human decomposition. Investigations were conducted at the University of Tennessee, Knoxville, USA and human cadavers were used. They found that there were not any statistically significant differences in rates of decomposition between the bodies that were subject to different treatments (plastic tarp covering, cotton thermal blanket covering and uncovered). However, there were differences in the visual analysis of the corpses (Dautartas, 2009). This article considered the insect activity surrounding decomposing corpses and the affects that it may have. However, the main focus was rate of decomposition rather than insect succession and how it is affected by certain coverings.

Both of these articles are very informative; however, they were conducted in environments that are very different to that of coastal NSW. Perth experiences a significantly different climate compared to Ulladulla (town close to study site). Perth’s mean maximum temperature (℃) in Autumn is 25.9, mean minimum temperature is 13.7 and the mean rainfall (mm) is 48.3 (Bureau of Meteorology A, 2019). Compared to 21.3℃, 14.1 ℃ and 102.9 mm in Ulladulla on the south coast of NSW (Bureau of Meterology, 2019). These major differences in climate can significantly affect insect species populations and reproduction (Khaliq, et al., 2014). Thus, there is a great need for investigations similar to those conducted by Voss et al. (2011) and Dautartas (2009) to be conducted in the coastal area of NSW, in order to have information and findings that are relevant to professionals and forensic investigations in that area.

Scientific research question

Does the wrapping of a body delay the insect succession of Coleoptera and Diptera taxa?

Hypothesis

Hypothesis: Plastic wrapping of carcasses will delay Diptera and Coleoptera taxa succession.

Null Hypothesis: Plastic wrapping of carcasses will not have an effect on Coleoptera and Diptera taxa succession.

Methodology

Study site

A study of the succession of Diptera and Coleoptera taxa on decomposing piglet carcasses was conducted on land situated 16km north of Ulladulla, New South Wales (35°14'07.5"S 150°25'53.6"E) The study site included coastal bushland with carcasses positioned in a small clearing (~12m²) and had an exclusion zone of 50m. Site was privately owned, manage by the owner and secured with fencing.

Figure 1. Satellite image indicating study site.

Animal model

Six piglet carcasses were used in the study as models for human decomposition. Due to availability and institutional restrictions, piglets which had died from natural causes such as stillbirth were used. Following death, piglets were frozen and remained so until the study began. Three piglets were wrapped separately in 1m x 0.3m of Low Density Polyethylene (LDPE) (piglets 1, 2, 3) and the other three were left unwrapped (piglets 4, 5, 6) acting as controls. Piglets were each placed in a PVC rainwater pit bucket and arranged within study site. Buckets contained a small amount of dirt and leaf litter. Six 10mm holes were drilled into sides of buckets to allow insects access to carcasses. Buckets were sealed with a concrete paver. Piglets were protected from scavenging animals by the thick buckets and pavers which were secured to the ground using strapping. Appropriate PPE was used when handling carcasses as protection from biohazards

Figure 2. Wrapped piglet 1, Day 4

Figure 3. Unwrapped piglet 6, Day 4

Sampling procedure

The study was conducted during autumn, it commenced on the 11th March 2019 and ran for 30 days. During the sampling procedure, replicates were observed and photographed. Sampling of adult insect fauna was conducted between 1300 and 1800 h on alternating days. Diptera were collected using an insect net and Coleoptera were collected with tweezers. All specimens were kept in sampling jars labelled with the day and replicate number. Specimens were killed at -17℃ and kept at this temperature for preservation. Gloves, goggles and organic vapour mask were used during sampling. Photographs of samples were sent to Blake Dawson, PhD Candidate University of Wollongong, for identification

Results

Table 1. DIptera and Coleoptera species succession (days) of wrapped and unwrapped carcasses

Figure 4. Coleoptera and Diptera succession of unwrapped carcasses

Figure 5. Coleoptera and Diptera succession of wrapped carcasses

Succession

The succession patterns of Coleoptera and Diptera taxa of wrapped and unwrapped carcasses are presented above in Figures 1-2 and Table 1. Throughout this study, a total of twelve species were collected including seven of the Diptera taxa and five of the Coleoptera taxa. Specimens collected were adults; eggs, larvae and pupae were not included in this study.

Diptera Calliphoridae Chrysomya Incisuralis was the first insect to colonise both the unwrapped and wrapped carcasses. Its arrival was recorded on days 5 and 6 respectively. However, C. Incisuralis did not consistently colonise all piglets, only replicates 3 and 4. Other calliphorid colonisers included Chrysomya Varipes and Chrysomya Rufifacies, which arrived before day 10 for both unwrapped and wrapped carcasses. Diptera Muscidae Australophyra Rostrata first colonised unwrapped replicates on day 8 and wrapped replicates on day 13. Of the Diptera taxa, Muscidae Hydrotaea Spinigera, Calliphoridae Chrysomya Nigripes (only on piglet 2) and Lauxaniidae Sapromyza Spp. (only on piget 3) were first recorded between days 9 and 16. H. Spinigera was latest Dipteran species and was last recorded on day 21 (wrapped piglet 2).

Coleoptera taxa began colonising the replicates in the later stages of the study, after the Diptera taxa, from day 9 onwards. Coleopteran colonisers included Silphidae Ptomaphila Perlata, Silphidae Ptomaphila Lacrymosa, Histeridae Saprinus Cyaneus Cyaneus, Staphylinidae Creophilus Spp. and Dermistidae Dermestes Spp. These species colonised the carcasses between days 10 and 27.

Of all the species recorded, only two consistently colonised all carcasses, C. Varipes and P. Perlata. Remaining species colonised at least one wrapped and one unwrapped replicate with the exception of C. Nigripes, Sapromyza Spp. and Dermestes Spp. which only colonised piglets 2, 3 and 4 respectively.

Discussion

This study observed the impact of plastic wrapping on the succession of Diptera and Coleoptera taxa of piglet carcasses. It was evident that there were differences in the insect succession of the wrapped and unwrapped carcasses. Of the species collected, ten were observed to colonise the unwrapped carcasses before the wrapped carcasses. The length of this delay ranged from one day to nine days with an average of four days. S. cyaneus cyaneus experienced the longest delay of nine days and C. incisuralis and Creophilus Spp. experienced the shortest of one day. H. spinigera and A. rostrata experienced delays of three and five days respectively. The two predominant colonisers, C. varipes and P. perlata experienced delays of three and six days respectively. To test the hypothesis and to determine whether these differences were statistically significant, an f-test and then t-test was conducted for a number of the species. It was concluded that the delay of the above species was not significant (C. varipes: df=2, α=0.05, P=0.53). It was also observed that two of the species recorded, C. rufifacies and P. lacrymosa, colonised the wrapped carcasses before the unwrapped. They had a delay of -3 and -11 days respectively. Additionally, species C. nigripes, Dermestes Spp. and Sapromyza Spp. did not colonise both wrapped and unwrapped carcasses and thus did not have a period of delay.

While piglets are easily available, due to their smaller size and weight, they are not the optimal analogue for investigating human forensic entomology. Therefore, in order to better understand the effect of plastic wrapping on insect succession in relation to forensic investigations of human remains, a study must be replicated with adult pigs which are more similar to the human body, or with cadavers. It would also be beneficial to use carcasses which are the same size and weight and which have a known time and cause of death. This more controlled use of carcass is necessary for obtaining results with a higher certainty.

In this study, collection and identification of Coleoptera and Diptera taxa was limited to adults. If the collection and identification of species eggs, larvae and pupae could be performed, a more detailed view of the entomology present on wrapped and unwrapped carcasses during the stages of decomposition would be produced. Additionally, to better understand the succession of insects, specimens need to be collected daily and over a longer period of time, depending on the size of the carcasses; for example, 98 days for adult pig carcasses. The identification of specimens collected by an entomologist in person rather than through photographs would ensure the correct identification.

While it was concluded that the plastic wrapping did not significantly delay insect succession, differences were observed between wrapped and unwrapped replicates. This may have been the result of the wrapping effecting the stages of the carcass decomposition which then had an effect on insect succession or vice versa. Other coverings such as blankets, clothes and tarps may also affect decomposition or insect succession directly. Comparing the impact of different coverings would also be beneficial to forensic entomology investigations.

The containers used in this study did not closely mimic a true crime scene and may have delayed the time that the Diptera taxa first colonised the carcasses. In the study conducted by Voss et al. (2011), it was found that Calliphora dubia Macquart (Diptera: Calliphoridae) colonised pig carcasses within the first 24 hours. In this NSW study, C. Incisuralis was the first coloniser and arrived on day 5. In order to successfully mimic a crime scene and to reduce the prevention of insect species, metal cages should be used to enclose and protect carcasses. If adult pigs or human are to be used, the bio exclusion zone should also be modified to at least 500m to meet these changes in carcass.

As the climate significantly varies throughout Australia, the results obtained from this study conducted in coastal NSW may not be entirely relevant to other parts of the country. In the investigation conducted by Voss et al. (2011), the arrival of C. dubia was significant to the study. However, in this NSW study, C. dubia was not recorded to colonise any of the carcasses at any time.

During this study, it was observed that a number of species including C. incisuralis, C. varipes, A. rostrata and Creophilus Spp. were present for longer periods on the wrapped carcasses compared to the unwrapped carcasses. This may be the result of the plastic limiting the evaporation of decomposition fluids which then attract the insects for a longer amount of time. It was also noted that there was an overlap of the succession of the Coleoptera taxa and Diptera taxa on the wrapped carcasses, but not on the unwrapped carcasses. This difference may be due to the wrapping prolonging the decay stage of decomposition and thus effecting the length of species succession.

Conclusion

This study aimed to answer the research question, “does the wrapping of a body delay the insect succession of Coleoptera and Diptera taxa?” Piglet carcasses were used as an analogue for human cadavers and were wrapped in plastic. C. Varipes and P. Perlata were the predominant species observed to colonise all of the carcasses. A delay of three and six days was observed for both species respectively. C. rufifacies and P. lacrymosa suggested that the wrapping did not result in a delay of the wrapped carcasses, as these species experienced delays of 3 and -11 days respectively. While the majority of the results demonstrate that there was a delay in the succession of the wrapped carcasses, it was concluded that this difference is not statistically significant (p=0.53). Thus, the null hypothesis is accepted, “Plastic wrapping of carcasses will not have an effect on Coleoptera and Diptera taxa succession.”

Acknowledgements

I thank Dr Maiken Ueland, Blake Dawson and Professor James Wallman for their advice and expertise during this study. I also thank Dr Samantha Kneeshaw and Australian Pork Ltd. for their invaluable assistance.

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