Can a Forensic Entomologist Really Calculate Time of Death?
By Denise Gemmellaro
This is the second part in a series of posts on forensic entomology. Read Part One and stay tuned for future posts in the coming weeks here on Entomology Today.
Medicolegal forensic entomology is a field of entomology focused on the study and observation insects that, because of their feeding behavior and biology, can be associated with human remains at a crime scene. Some are necrophagous (eating dead things), while others are necrophilous (loving dead things). We know, based on research conducted in the field and observations, that decomposing organic matter is indeed quite appealing for some groups of insects, such as blow flies for instance. And we don’t need Bones or CSI to know this. (Although those help for sure!) A look at our garbage bin left out in the summer can confirm.
To a blow fly, a dead body is nothing more than a great place to have a protein meal and lay some eggs and, for those eggs, to hatch into maggots, feed, and find shelter until they are ready to pupate. From the fly’s perspective, it is no different that huge plate of leftovers forgotten outside after a barbecue. It’s a precious food resource that needs to be used quickly, because it is decomposing after all, and it won’t last long.
This is why, we can assume (and it is only an assumption), that any dead body exposed to insect activity will attract blow flies within a short time from its exposure and that, shortly after that, those flies will start to lay eggs on it and start their colonization. The forensic entomologist called on a crime scene will use her or his knowledge of a blow fly’s life to interpret the ecosystem of the dead body and the maggots to trace back colonization and give a time interval of when that could have occurred. This is the part around which probably most of the confusion comes.
We all know that the million-dollar question of almost any crime scene involving a dead body is “When did it happen?” One of the reasons the importance of forensic entomology has grown is that forensic entomological analyses are known to be able to provide information on time of death or when the crime could have occurred. But is this true? Is forensic entomology so powerful to be able to actually pinpoint the day and time of death like the TV shows have told us?
The answer, as in many other situations, lies somewhere in the middle.
Yes, forensic entomology can indeed offer a lot of information regarding a potential time interval within which death could have occurred. The analysis of the species colonizing the remains, their developmental stage combined with the local temperature and combined with many other variables such as light, humidity, precipitation, conditions of the body (e.g., naked, dressed, burned, buried, wrapped in plastic, locked in the trunk of a car, and so on), plus so many more can provide a time interval for insect colonization of those remains.
Now, is that interval always synonymous with time of death, or PMI (post-mortem interval)? The simplest and most honest answer should be no, it is not. Insects have been observed on carcasses or remains as close as a few minutes after the exposure of them to the insect activity (“exposed” meaning the remains were either in an outdoor setting or an indoor environment accessible to insects). But does that mean those insects started colonization immediately after they landed on the remains? More importantly, does colonization always coincide with death? What about a very graphic scenario in which a killer stores a victim’s body in a freezer as a macabre souvenir and then takes the body out at a later time to confuse the investigators? What would happen in that case? Well, we can speculate that once the body is out of the freezer and exposed, insects may start to smell it, visit it, and maybe begin colonizing. Then a forensic entomologist could be consulted to analyze the entomological evidence found on the body; what would the result of the analysis show? A time interval for the occurrence of death? Or colonization? And how spaced out would these events be in this case? We could say they are spaced out quite significantly!
So, considering the training an entomologist typically has and considering what the entomologists should be looking at in an investigation (insects!), can a forensic entomologist calculate time of death? No, she cannot. She should not. She can surely help the investigation, but her efforts will be toward the estimation of the insect colonization of those remains. Can that be at least close to time of death? Yes, it can. Could there be exception? Yes, there can!
There have been quite a few animated debates on the terminology a forensic entomologist should use, about the way to express the results of the forensic entomological analysis, and about the potential and limitation of the field itself. And, although there are some common denominators that are starting to appear, there is still not a unanimous consensus. (For one proposed path toward greater agreement, see last week’s post “Could a Null-Hypothesis Model Bring Greater Clarity to Forensic Entomology?“)
For these reasons, it is important to debunk the myth of forensic entomology exclusively as a tool to calculate time of death. It is definitely a discipline used toward the estimation of a colonization interval (interval, not exact moment in time!), which is strictly dependent on the occurrence of death; hence, unless that person is already dead, colonization will not typically occur. But colonization interval and time of death are not always synonymous, and this should be acknowledged every time a forensic entomological analysis is conducted.
It should also be clear, though, that this does not diminish the value of forensic entomology. It actually does quite the opposite. In several cases, the fact that the estimation of colonization and estimation of death do not coincide has been beneficial to the investigations and has provided means to actually shed light upon the circumstances. But, for the sake of the discipline, out of the profound respect we should have for the justice system, and in consideration of the implications a forensic entomological report may have in a criminal investigation, clarifying this point is of paramount importance.
Integrity in this field will ensure a correct and prosperous development and implementation of forensic entomology among the scientific community as well as in law enforcement and courts of law.
The confusion and the debates among experts of the field simply prove the need for more validation studies to evaluate the methods in use today by forensic entomologists to calculate their intervals (whatever these might be), in the hope of finding a common ground that everybody agrees to operate on.
Denise Gemmellaro is a graduate student in entomology at Rutgers University and director of the Forensic Entomology Workshop held at the New Jersey School of Conservation, a two-week summer program for students and professionals to gain hands-on experience in the fundamentals of forensic entomology. Email: firstname.lastname@example.org