A chrysomya megacephala, commonly known as a blow fly.
A chrysomya megacephala, commonly known as a blow fly. Muhammad Mahdi Karim

Climate change has spurred the spread of invasive insects that devour crops, destroy homes, and spread disease. Now, rising temperatures are driving cadaver-eating blow flies to migrate north in search of cooler weather, with consequences for forensic scientists who rely on them to solve crimes. Blow flies are drawn to dead bodies, both human and animal. They land on a fresh corpse within minutes of death. The females take a quick taste to make sure it’s good food for their larvae, then lay hundreds of eggs. Once hatched, the maggots begin feasting on soft tissues, hastening decomposition. In doing so, they become key players in murder investigations by helping forensic scientists determine when a person died. Blow flies have proved critical in countless homicides by supporting the innocence or guilt of suspected killers. Forensic entomologists can do this because they know the lifecycle of blow flies found in their region. But now, as blow flies move northward, driven by rising temperatures to find more comfortable homes, these death inquiries could become flawed as newcomers who resemble resident blow flies produce offspring that grow on a different schedule.

Lucilia cuprina, more commonly known as an Australian sheep blow fly.
Lucilia cuprina, more commonly known as an Australian sheep blow fly. fir0002

In the Midwest, for example, a new type of blow fly species — one that typically lives in southern states — has taken up residence. Lucilia cuprina has been spotted in parks and other public places more than two dozen times in Indiana between 2015 and 2017. It looks a lot like its sister species, Lucilia sericata, which is widely present in the state, but its maggots don’t act the same. Mistaking one for another could cause big headaches for crime solvers.

“The larvae of both species are difficult to tell apart, thus it’s possible a forensic investigator might end up with an inaccurate time of death,” said Christine Picard, associate professor of biology at Indiana University-Purdue University Indianapolis School of Science. “If, for example, a maggot is identified as L. sericata but actually is L. cuprina, the time of death could be an underestimate. Different species will have different development times. This could have implications involving decomposing remains. Forensic scientists need to be aware that this new species is present so they don’t make this mistake.” Picard, who studies the behavior of blow flies around the world, said a member of her lab first identified the migrant blow fly several years ago.

Blow flies
Blow flies Journal of Insect Science

“It was a blow fly in Indiana that we hadn’t seen before, and so we have been paying attention,” she said. “We do a ton of fly collection — catching blow flies is easy, all you need is a container of rotten meat and a fly net — and it was like ‘holy cow, what are you doing here?’ This species is pretty well known in Florida and Texas, where I have colleagues I speak with regularly, but here in Indiana, it was unexpected. In this particular case, a PhD student was out doing her normal collections — she would go out every two weeks during the summers, collecting from around Indianapolis for her dissertation work — and as she was identifying them, she noted [the interloper species].”

Picard and her colleagues recently published a paper describing the appearance of this blow fly in Indiana, which appeared in the Journal of Insect Science. “As temperatures change and increase, the distribution of these insects will continue to change as well,” Picard said. She predicts blow flies will continue to migrate northward in search of cooler climates and new sources of food.

Rest assured that, while blow flies may be unpleasant, they don’t bite and, “other than the accidental deposit of bacteria from their feet possibly getting onto some food you might be eating,” they don’t transmit diseases, Picard said. “They act primarily as nature’s recyclers, not causing any real damage.”

Lucilia sericata, commonly known as a greenbottle fly.
Lucilia sericata, commonly known as a greenbottle fly. Calibas

“Blow flies live in a really competitive environment. They need a very valuable resource — a dead animal — to survive, so they compete with other blow flies and with scavengers,” she said. “Their ability to find a dead animal and grow very quickly makes them competitive. But there is a finite number of dead animals out there. So, they will continue to expand their distributions as long as they can survive.” Rising temperatures allow blow flies to extend their reach into areas that were previously too cold to inhabit.

“Forensic scientists should be aware, but everyone should care,” Picard said, noting that while blow flies are largely harmless, other species are not. “Even though these particular flies don’t carry disease, many others do. If they are expanding their ranges, there is no doubt other insects are as well.”
