Revista: Forensic Science International Editorial: Elsevier
Yensy M. Recinos-Aguilar, Eduardo R. Chamé-Vázquez, Pablo Liedo-Fernández, Guillermo Ibarra-Núñez a
a
El Colegio de la Frontera Sur, Unidad Tapachula. Carretera Antiguo Aeropuerto Km 2.5, CP 30700, Tapachula, Chiapas, México.
115
Successional patterns of insects associated to pig carrion in the south of
Mexico
ABSTRACT
This investigation is the first work concerning the entomofauna associated with pig carrion in the south of Mexico. Two pigs were used for each of our seasonal sampllings (dry and rainy seasons) and they were placed into two Schoenly-type traps. Samples were taken at different times according to the advancing decomposition of the carcass and only adult insects were collected. The various trophic groups (necrophagous, necrophilous, opportunists, adventives and omnivorous) were associated to the different stages of decomposition (fresh, bloated decay, active decomposition, advanced decomposition and skeletal remains). A total of 43,768 samples of 57 insect species were collected. Ninety- seven (97) per cent of the collected fauna belongs to Dipterae, headed by five families (Drosophilidae, Calliphoridae, Muscidae, Sarcophagidae and Ulididae). Callihporidae stands out due to its abundance and for having been the first to arrive at the carcass, followed by species of the Sarcophagidae and Muscidae families. Ants represented most of the Hymenoptera; they appeared when the carcass was in its intermediate stages of decomposition. Coleoptera appeared during the last stage at a time when many species had disappeared. The presence of Hermetia illucens (Stratiomyidae) signaled the final stage of the decay process.
116 entomofauna, carrion decomposition
1. Introduction
Carcass decomposition generates a dynamic and varied micro-ecosystem which provides a temporary habitat and the necessary nutritional resources for an ample variety of organisms, including arthropods which feed on this organic material. [1, 2]. Arthropods are reponsible for the consumption and decay of the soft tissue of a carcass. They are also the first to arrive at the scene and they remain during the whole process of decomposition in predictable sequences [3]. This ecologic succession is influenced by the carrion's state of decomposition which is the result of the interaction of different variables such as temperature, relative humidity, type of vegetation, soil pH, season of the year and cause of death [3, 4]. Therefore, an understanding of these insects and the sequence of their arrival relative to the different factors that condition the stages of decay, sheds valuable information mainly in determining a more accurate postmortem interval (PMI) [5-7]. The succession of cadaveric insects has been studied in differnt parts of the world, primarily in temperate regions, and very rarely in tropical zones [8]. Forensic entomology is new in Mexico and studies concerning cadaveric insects are therefore scarce. [10-12]. The object of this study was to determine successional patterns of the entomofauna associated with pig carrion in a tropical region and to identify those species that might be used in forensic entomology.
117
2. Materials and methods
2.1 Study area
The area studied is located in the municipality of Tapachula, Chiapas, Mexico (N 14* 51' 50" and W 92* 19' 58", 70m above sea level). It presents a median annual temperature of between 24°C and 35°C, with rainfall that varying from 2300 to 3900mm. Cocoa trees (Theobroma cacao), zapote (Manikara zapota) and other fruit trees are characteristic of the zone. There are two defined seasons in the region: the dry season (February-May) and the rainy season (June-September).
2.2 Equipment and procedures
Two pigs (Sus scrofa L.) of approximately 20-24 Kgs were used in each experiment. The pigs were put to death with a bullet through the head and transferred to the trap, which was a variant of the trap designed by Schoenly (90cm long X 60cm tall X 60cm wide) [12]. The trap was protected by a hexagonal metallic mesh to prevent the access of other carrion-feeding organisms to the carcass. The Schoenly trap is designed to take a complete census of the insects that are attracted to the carcass and develop within it, without interfering with the normal decomposition processes.
The two pigs where checked until the process of decomposition had finalised. During the dry season, from March 11 to April 14, 2013, and during the rainy season, from July 31 to September 2, 2013. The scheduling of the data collection and the retrieval of insects caught in the trap varied througout the whole process of decay, from every five (days 1-3), seven (days 4-15), 24 (days 16-29), 48 and 72 hour intervals. During each data collection session, a physical inspection of the
118 carcass took place to determine the stage of decomposition it was undergoing. Temperature and relative atmospheric humidity was recorded on a digital thermometer (Control Company Cat. No. 4040 Traceable* Thermometer/Clock/Humidity Monitor).
2.3 Data Analysis
To establish the associated between carcass entomofauna and stage of decomposition, correspondence analyses (CA) were applied using the R Develpment Core Team statistical package [15]. The species that showed a significant link (p≤ 0.05) with the stages of decomposition were selected for constructing occurrence matrices. To identify the moment when the greater abundance of these species was identified, a scale was established taking into account the number of specimens of each species per sample compared to the total number of specimens of that same species collected. If a species presented an abundance less or equal to 30% of its total abundance, it indicated low abundance. Greater than 30% but less or equal to 60% of its total abundance indicated medium abundance, and greater than 60% of its total abundance indicated high abundance.
3. Results
3.1 Process of decomposicion and the influence of environmental factors
Five stages of decomposicion were identified, as proposed by Arnaldos [16] and Flores-Pérez [9]: fresh (Fr), bloated decay (En), active decomposition (Dac), advanced decomposition (Dav) and skeletal remains (Ro) (Figure 1).
119 Decomposition took 54 days (1284 h) during dry weather and 38 days (900 h) during the rainy season (Figure 1) (Table 1). Differences in temperature were recorded between the seasons (F=4.22; p=0.045) with a mean of 32°C during dry weather and 27°C during the rainy season. The variability of relative humidity was less (F=1.852; p=0.179), with an average of 58% for dry and 77% for wet seasons.
3.2 Cadaveric entomofauna
A total of 43,768 adult specimens corresponding with 57 species, 49 genera and 36 families of three orders of insects were collected (Table 2). Seasonal differences were observed, with the rainy season presenting the greatest richness (N=51) and abundance (63% of the total).
The order Diptera grouped 97% of the insects collected. The most important were the Drosophilidae, Calliphoridae, Muscidae, Sarcophagidae and Ulidiidae families representing 86% of the Diptera. Family abundance was related to the seasons, except in the case of Calliphoridae which was unaffected. Drosophilidae were more abundant during the rains (X2=8649.92; gl=1; p<0.001), while the rest of the families evinced a greater abundance during the dry season (p<0.001). The species which stood out due to their abundance were Drosophila sp (Drosophilidae), Atherigona sp (Muscidae), Cochliomyia macellaria and Chrysomya rufifacies (Calliphoridae).
Seven hundred and thirty-two (732) specimens of the order Hymenoptera were collected. The most abundant throughout the study (74% of the order total) was the Formicidae family, presenting a greater abundance during the rainy season (Formicidae X2=12.971; gl=1; p<0.001). Apidae were also collected evincing a
120 greater abundance during the rainy season (X2=5.143; gl=1; p=0.02). Also Vespidae, with a greater abundance during the dry season (X2=14.343; gl=1; p<0.001).
Least represented was the order Coleoptera with a mere 541 specimens and six species. The most abundant family was Staphylinidae with 46% of the specimens of this order. Histeridae was less abundant but it includes important species for the purpose of this study (e.g. Euspilotus azureus). Both families were more abundant during the rainy season (Staphylinidae: X2=144.4, gl=1, p<0.001; Histeridae: X2=49.47, gl=1, p<0.001).
3.3 Trophic relation of the insects collected from the carcasses
The insects were classified in five ecological groups according to their trophic relation to the carcass: necrophagous, necrophilous, omnivorous, opportunists and adventives. Out of the total of specimens collected, 54% were necrophagous, who feed off the carcass. The Calliphoridae (Diptera) was the most abundant. The Chrysomya rufifacies and Cochliomyia macellaria species were conspicuous for their abundance. Another important necrophagous family was the Sarcophagidae, with Blaesoxipha sp standing out. The immature stages of both of these families were observed to be present on the carcasses.
The second most abundant group was that of the opportunists, representing 41% of the total of collected species. Species within this group take advantage of the resources offered by the carcass without establishing a tight relationship with the decomposition process. The most abundant species within this group is Drosophilia sp. Among the adventives, bees were the most abundant and their
121 importance is small with regards to other groups. Onmivorous species where best represented by the Formicidae (Hymenoptera) family.
The necrophilious group was made up of species that feed on or parasitize various necrophagous organisms (larvae). It was represented by the Staphylinidae and Histeridae families of the order Coleoptera and the Braconidae, Chalcidadae and Ichneumonidae (Hymenoptera) families. Although their abundance was less, they bear a strong relationship with the carcasses' decomposition process.
3.4 Trophic groups linked to the stages of decompossition
The performed analyses demonstrated that the trophic groups are linked to the different stages of decay. In the case of the necrophagous group, the Calliphoridae family is associated to the bloating phase, active decomposition and skeletal remains (dry season, X2=3209.6, gl=15, p<0.001; rainy season, X2=2400.42, gl=15, p<0.001; Figure 3). Sarcophagidae were associated with the stages of active decomposition and skeletal remains (dry season, X2=273.02, gl=18, p<0.001; rainy season X2=97.74, gl=18, p<0.001; Figure 4). The other necrophagous families were associated to bloated decay, active decomposition and skeletal remains stages (dry season, X2=1378.53, gl=18, p<0.001; rainy season, X2=1169.66, gl=27, p<0.001; Figure 5). Necrophilious species were associated with the last two phases of decomposition in the dry period (Dav and Ro) and with the active and advanced stages during the rainy season (dry season, X2=230.5, gl 24, p<0.001; rainy season, X2=334.95, gl=15, p<0.001; Figure 6). Omnivorous species were associated to the initial stages of decomposition (Fr and En) and to the end of the process (Ro). During the rainy season, they were only
122 associated to the skeletal remains phase (dry season X2=230.5, gl=24; p<0.001; rainy season X2=334.95, gl=15, p<0.001; Figure 6). Finally, the opportunistic and adventive insects were related to the intermediate stages of decomposition (dry season: X2=544.82, gl=39, p<0.001; rainy season, X2=1298.11, gl=45, p<0.001; Figure 7).
3.5 Successional patterns
It was noted that during both periods, the species evinced a similar pattern of succession, in terms of number and sequence (Figures 8 and 9).
During both seasons, the first insects to turn up at the carcasses were the diptera. Hemilucilia segmentaria, Lucilia eximia and Mesembrinella sp (Calliphoridae) were the first species to appear during the bloated stage. At this same stage the Formicidae family made their appearance (dry season). Cochliomyia macellaria, Chrysomya megacephala and Chrysomya rufifacies arrived in this order, evincing a greater abundance during the active and advanced stages of decomposition. Species of the Anthomyiidae, Fanniidae, Miscidae, Sarcophagidae and Sepsidae families were registered during the same stages of decomposition.
During active and advanced decomposition, the carcasses presented a large number of immature states between the skin and tissue remains, thus attracting necrophilious species, especially those of the Staphylinidae and Histeridae families. The greatest abundance of necrophilious insects was found in the intermediate stages and they stayed on the body until it turned into skeletal remains.
123 diminished markedly, being subsituted by beetles and other new species of diptera such as Hermetia illucens (Stratiomyidae). During both seasons, H. illucens maintained important abundances during the skeletal remains stage. Formicidae species were observed at the end of the decaying process (rainy season). Other species, such as the opportunists and the adventives were registered mainly during the active and advanced stages of decomposition, although they were present throughout the whole process of decay, without presenting a specific pattern.
4. Discussion
The decomposition of a corpse can vary according to the season of the year in which it takes place. During the dry season, low humidity caused dehydration of the carcasses which resulted in the bodies adopting a mummified state towards the end of the sampling. This has been reported in other works [1, 18, 19, 20]. However, during the rainy season an acceleration of the process of decomposition was observed, favored by the increased atmospheric humidity, as has been described by Tantawi et al. [1] and Battán Horenstein et al. [19].
The most representative orders coincide with other studies that have taken place in America, with the Diptera order being the most abundant in all samplings [5, 10]. The Calliphoridae family is the most important one in terms of its abundance in the samplings and its associated with the stages of decay, evincing the same tendency in other investigations [21, 10, 11]. Five out of the six Calliphoridae species have been reported in other studies in America [22, 23, 24, 25].
124 decomposition [10, 11, 20, 26, 27], but in our work no representatives of this family were collected. Caballero and León-Cortés [27] reported only two Dermestidae individuals in rat carrion, although members of this family may appear more slowly in pig carrion as opposed to other coleopters [28]. Within the families registered in Hymenoptera, Formicidae was the most important one in the deay process and it has been reported as such by other authors [9, 10].
The data obtained suggest that the great biological richness and ecological complexity characteristic of the Neotropical region, has an infuence on the number of individuals, the species collected and the length of the decomposition process, showing different results to those reported by Flores-Pérez [9] for central Mexico, and by Valdes-Perezgasga et al. [10] for Mexico's northern region. The trophic groups observed were the same described in other works [30, 31, 32], with the necrophagous and necrophilous groups standing out in their association to the carcasses and evincing a similar pattern to the one described by Arnaldos with regards to pig carcasses [17]. During the first stages of decay (fresh and bloated), Lucilia eximia (Calliphoridae) was one of the first species to arrive at the carrion. Nevertheless, its low abundance during the samplings impeded a clear association with a specific state of decomposition. Due to its reduced competitive ability, this species might be being disregarded for interespecial competition with Chrysomya rufifacies (Calliphoridae) [33]. It is also important to consider that ants influence the colonization of diptera fauna, as they feed upon the carcass as well as on the body, eggs and larvae of necrophagous flies as has been observed in this study. Another species observed during the initial decomposition process was Cochilmyia macellaria, characteristic as a species linked to urban and high humidity locations,
125 generally appearing as a secondary species in the decomposition process [24]. The increase of Chrysomya rufifacies, and Chrysomya megacephala, was observed when the carcass was bloated (bloat stage) and it was maintained during tissue liquefaction (active decomposition). This behaviour has already been reported by Salazar-Ortega [7]. The last diptera to arrive at the carrion were the Sarcophagidae and the Muscidae. This is a frequently observed phenomenon in investigations of sarcosaprophagous fauna [7, 34, 35, 36].
Although many opportunistic and adventive insects are of no forensic importance, they presented a great abundance in the intermediate stages of decomposition (active and advanced decomposition). One example is the Syrphidae family which will take advantage of available proteins in a decomposing body in the absence of polen or other sugar-rich foods [36]. Saprophagous flies of the genus Euxesta (Ulildiidae) which feed on decomposing vegetable matter, may also take advantage of the provisional resources a carcass has to offer [37]. During these stages a larger number of stingless bees (Hymenoptera: Apidae; Meliponini) were registered. These were attracted by the amount of minerals and humidity present during putrefaction, as well as by the fungi that develops during these phases, all of which represent an important source of nutrition for these bees [38].
In the last phase of decomposition (skeletal remains), the necrophagous diptera diminished considerably, being replace by beetles. This succession has been previously described by several authors [7, 10, 11]. The drop in humidity of the carcass, the shortage of soft tissue on the same, and the exposure of cartilages and tendons allowed the proliferation of beetles during this final stage [26, 40]. This phase signals the appearance and increase in numbers of Hermetia illucens
126 (Stratiomyidae) both in their adult form and in their larval stage. This has been reported by Flores-Pérez in central Mexico [9]. H. illucens larvae are associated to decomposing animal or vegetable matter, excrement and the undersides of fallen tree trunks [7]; however, it is a forensically important species as they are reported at the skeletal remains stage in human corpses [40]. Their appearance marks the end of a corpse's decomposition process.
This study is the first contribution on cadaveric fauna in the south of Mexico. It generates knowledge concerning the species associated to each one of the stages of decomposition. The importance of studying the various factors that influence the decay of a carcass is highlighted here, as seasonal variations and geographical location influence the composition and colonization of insects on a carcass.
Acknowledgements
We want to express our special thanks to The National Council for Science and Technology (CONACyT) for support this research.
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