Forensic entomology is a scientific discipline that interprets insect evidence for forensic and legal purposes [1,2]. Insects have been shown to be useful for estimating the post-mortem interval (PMI) or time since death. There are two ways to use insects to determine the PMI. The first is based on the developmental rates of insects that arrive first on the remains considering the environmental conditions at the scene. The second is based on changes in the characteristics of the insect community associated with the corpse over time [2]. If insect data are unavailable, using succession data to determine the sequence of colonization by different arthropod species can be a key method for estimating the PMI [3].

Flies of the families Calliphoridae, Sarcophagidae, and Muscidae are prominent invaders in the earlier decomposition stages [4]. In contrast, in the later stages of decomposition wherein the corpse becomes dry and skeletonized, Coleoptera is considered the main entomological evidence for forensic investigators to determine PMI [4]. In particular, larder beetles (Dermestidae) are often found as common pests in stores or homes [5], and these beetles have been reported to feed on dry human corpses [4,6,7]. Under dry and warm conditions, they frequently appear in numerous numbers on the dried bodies of dead animals [8]. Female Dermestid beetles oviposit eggs, and the larvae hatch within a few days. Both adult beetles and larvae are capable of feeding on animal bodies. Dermestid beetles and their larvae possess strong mouthparts, allowing them to masticate both tissue and hard materials, such as bone. As a result, they can feed on animal carcasses, destroying soft tissues, including the skin, organs, cartilage, and consuming the corpse down to the bones. In France, Dermestidae was observed in 81 real cases of human corpses in the decay, dry, and skeletonization stages [9].

While most forensic entomological applications have been human death investigations, recent years have seen an increase in forensic entomology cases involving animals [10-12]. In Korea, the companion animal market expanded from 500 billion won in 1996 to 1.8 trillion won in 2010 and steadily grew to 6 trillion won in 2020 [13]. In addition, the number of people prosecuted for violating the Animal Protection Act has gradually increased [14]. Therefore, there is a need to evaluate the employment of forensic techniques in animal cruelty investigations, especially where abuse and negligence leading to death is being considered.

We recently encountered a case in which the ecological features of Dermestidae were used to estimate the minimum post-mortem interval (PMImin) of animal carcasses. This manuscript reports the first case study to estimate PMImin using insect growth rates for cat carcasses in South Korea.

On July 13, 2022, 17 neglected cats were found dead at a villa in Daegu Metropolitan City and police were notified. The carcasses were discovered in four different rooms: three carcasses at the entrance, four carcasses in the living room, six carcasses in room 1, and four carcasses in room 2. As almost all carcasses were dried or skeletonized (Fig. 1). When we arrived at the site, the police were already cleaning up the scene. Therefore, entomological evidence was collected as the first step to estimate the PMImin using insects. The carcasses were covered in several dead adult flies (Lucilia sericata, Calliphora vicina), pupae (Sarcophaga crassipalpis, S. dux), larvae (S. crassipalpis, S. dux), and empty pupae (L. sericata determined after [15]), and many adult larder beetles, larvae, and larval skins (Dermestes haemorrhoidalis determined after [16]) were scattered in the area (Figs. 2, 3).

Temperature affects the decomposition rate of carcasses and the developmental rate of insects, which can help investigators identify how long the animal has been deceased. However, estimating the room temperature was restricted to preserve the scene for the police investigation; as a result, the room temperature was not measured upon arrival at the scene. Instead, we temperature and relative humidity data from the Sinam District Meteorological Observatory (attitude 59 m) in Daegu Metropolitan City, which is the closest observatory to the scene (about 6.1 km), were used to obtain temperature information. We collected temperature data for the 2 weeks before July 13. Animal cadaver experiments were conducted at Kosin University during June to July (unpublished data) (Fig. 4), which demonstrated that an animal carcass similar in size to the last presumed dead cat carcass required at least 2 weeks to transition from the fresh to the dry stage. The support vector machine model was used to correct for the difference between the temperature data provided by the Korea Meteorological Administration and the actual temperature recorded at the site [17]. Based on surrounding temperature (average 30.4±0.5°C) and relative humidity (average 67.6±12.8%) data during the preceding 2 weeks prior to the investigation, the temperature and relative humidity of the scene were estimated to be 30°C and 65%.

At 30°C, we confirmed that similarly sized animal cadavers weighing approximately 5 kg took at least 3 days to transition from the fresh stage to the decay stage and that it took at least 4 days for Dermestidae to approach the cadavers (unpublished data). In addition, based on the data published by Coombs (1979), we considered that it takes approximately 40.5 days for D. haemorrhoidalis to mature at a temperature of 30°C and 65% relative humidity (from egg to larva: 4 days, from larva to pupae: 31.5 days, form pupae to adult: 5 days), according to previous research [18]. Therefore, the PMI was assumed to be at least 44.5 days, indicating that the animals perished on May 31 at the earliest. This estimate fit well with the suspects’ statements.

To accurately estimate PMImin, various factors that can affect decomposition, including temperature, humidity, insect access and activity, rainfall, sunlight exposure, soil pH, depositional environment, and trauma [19-22], must be considered. However, estimating these variable factors in a criminal scene can be difficult due to investigator's lack of awareness of their importance. In this case, it was also difficult to measure various factors because of the limited and control of field workers. Therefore, we determined the most reasonable PMImin for the given condition using the Dermestid beetle.

Dermestid beetles thrive on protein-rich media [23]. Generally, male beetles excrete pheromones to attract females, and both female and male beetles react similarly to these pheromones. Therefore, numerous adult beetles will be attracted to a corpse within a very short time [24].

The occurrence of few necrophagous flies at the scene indicated that the conditions for their activities were limited. There could have been various factor like temperature, fur, competition between species.

This case study evaluated entomological data to determine the PMImin for the carcasses of domestic cats. Using the growth information of black ladder beetles, the unpublished decomposition rates of animal cadaver experiments, and regional temperatures estimated from meteorological observatory data, we estimated a PMImin of 44.5 days. Necrophagous insects have been reported to have some difference in growth rate depending on the type of food, so it is judged that these aspects need to be considered to accurately estimate the PMImin [25]. Nevertheless, this estimate was later confirmed through police investigation and found to effectively match the statement of the suspect.