Fly Colonies Help Calculate Time of Death of Car Trunk Cadavers
Fly Colonies Help Calculate Time of Death of Car Trunk Cadavers

Fly Colonies Help Calculate Time of Death of Car Trunk Cadavers

Using pigs as human proxies, forensic entomologists reveal how bodies in vehicles decompose differently from those dumped outside.

Ashley Yeager
Ashley Yeager
Jul 13, 2020

ABOVE: Researchers who loaded car trunks with pig carcasses and set the vehicles on fire still found abundant entomological evidence that could help estimate when the body was placed there.
© GAIL ANDERSON

The six cadavers all wore the same clothes: red t-shirts, plaid boxers, and cargo shorts. They’d been shot in the head and then stuffed into the trunks of old, beat-up cars or deposited in densely shaded spots of forest in Maple Ridge, British Columbia. All died on July 24, 2007. Graduate student Stacey Malainey of Simon Fraser University checked on the six pigs, which served as proxies for human homicide victims, twice a week from the day they were killed, for nearly a month.

When it comes to murder, cadavers are most commonly found dumped in the bushes or the forest. “But there are a remarkable number that are concealed, and particularly concealed in vehicles—in old, junker vehicles,” says forensic entomologist Gail Anderson, Malainey’s supervisor at Simon Fraser. Having worked with the police for more than 25 years, Anderson has seen her share of bodies left to decompose in the trunks of cars. Using her understanding of insect development on cadavers, she’s able to roughly estimate the minimum amount of time each victim has been dead—a crucial piece of information for corroborating alibis and other details in a criminal investigation.  

Researchers typically use insects to calculate “what we call a ToC, or time of colonization,” says Lauren Weidner, a forensic entomologist at Arizona State University who was not involved in Anderson’s work. “We’re trying to figure out how long [the insects have] been there, so we can help determine how long a body has been there.” While there’s been a lot of research on how insects colonize bodies left outside, forensic entomologists have often wondered what happens when a body is confined or concealed. 

People think they can get rid of a lot of evidence setting fire to things, and of course, they don’t.

 —Gail Anderson, Simon Fraser University

In a series of studies, Anderson and others have shown that pig cadavers decompose differently when wrapped in sheets or stashed in buildings versus just being dumped outside; even zipped suitcases change the way insects inhabit the carcasses stuffed inside. In the new study, Anderson and Malainey wanted to determine how long it would take for insects to get inside cars and colonize dead bodies.

Every few days that summer, Malainey went to photograph the pigs, record their decomposition rate, and collect insect samples. However, she couldn’t just pop the trunk; that would risk giving insects easy access to the cadavers. So on each visit, she draped a huge piece of plastic around the rear end of the car, taping it to the rear windows and spreading it out like a train of a wedding dress. Then, she’d duck under the plastic and pull it down to ensure it touched the ground the entire time she had the trunk open. After she’d braved the stench of putrefaction to collect her data, she’d slip out from under the plastic train, then remove it, and roll it up. “We were very careful,” Anderson says.

On the third day, the pigs left outside were bloated and covered in fly eggs and early stage larvae, mainly of blue bottle flies (Calliphora vomitoria and C. latifrons) and blow flies (Lucilia illustris). The pigs in the car trunks were bloated, but only one of the three had eggs, specifically from the blow fly Phormia regina. By day six, the pigs outside had larvae that were further along in development and moved as masses of maggots, while the pigs in the trunks were now all colonized by insect larvae in earlier stages of development.

About a week later, the temperature inside the exposed dead pigs started to rise, as the maggots decomposed the dead flesh. Inside two of the cars, however, the pigs had already been reduced to mere skeletons, with pupae—the developmental stage between larva and adult insect—scattered throughout the trunk and car, with some even nestled into the front driver’s side floor carpet. The temperature of the car plus the pig was warmer than a pig sitting outside, allowing any hatching insects to grow faster and consume dead tissues more quickly than those in pigs dumped outside. On any given day, the temperature inside the cars was 10–25 °C higher than the ambient outdoor temperature, which peaked at about 25 °C. 

On the last day Malainey visited the pigs, day 28 of the experiment, the two skeletonized, car-stored pigs were infested with thousands of live and dead blow flies, while only one exposed pig showed evidence that adult flies had emerged. However, in the third car, which had the highest recorded temperatures, the pig was still intact and bloated, with far fewer larvae and no maggot masses—an oddity in the observations at the time. 

Overall, the work supports existing hypotheses on how stashing a cadaver inside a car affects insect colonization, says Daniel Martín-Vega of the University of Alcalá in Madrid. Car trunks lengthen the time to colonization, because they cause a delay in female blow flies accessing a body to lay eggs. But cadavers in cars can become warmer than those in open air, quickening the rate at which female blow flies’ larvae consume the cadaver, he writes in an email to The Scientist. The study provides “reference data for eventual cases taking place in similar scenarios.” And, it suggests that certain species of flies—P. regina and Protophormia terraenovae in the study—might stick around until they find a way to colonize cars while others look for an easier place to lay eggs, Weidner notes.

After the month of observations, Anderson and Malainey still weren’t finished with the cars: next, they lit them on fire. “That’s another common [scenario] you hear all the time: somebody dumping the car, then setting fire to it,” Anderson says. “People think they can get rid of a lot of evidence setting fire to things, and of course, they don’t.” 

From the burnt-out cars, the team recovered flies in advanced stages of development, all of which could help estimate time of colonization. They also discovered that the car in which the pig had been mysteriously preserved didn’t burn as well as the others. It turns out that the vehicle had a steel firewall between the passenger compartment and the trunk, which delayed decomposition of the pig in the first experiment and allowed the team to retrieve even more insect remains than from the other cars after the fire, Anderson says. But firewall or not, she adds, “there’s an awful lot of evidence left behind.”