Insect Informants

Most park visitors could do without flies buzzing around their heads or landing on their sandwiches, but the pesky critters are the very reason Hannah Chu and Christine Picard go to national parks.

That’s because Chu and Picard are scientists who have separately conducted studies of blowflies, the ones that make a habit of landing on feces and rotten meat before moving on to people’s food — and puking on it. But while blowflies can spread disease and are unquestionably gross, they perform invaluable ecological roles, including pollinating and decomposing carrion. “We would just be buried under mountains of dead things if blowflies didn’t exist,” Picard said.

They also help humans in more surprising ways — from serving as tiny detectives to possibly acting as minuscule weapons inspectors.

Chu, who has a background in forensic science and entomology, is hoping that her recently published Joshua Tree National Park study will be useful to local law enforcement agencies investigating suspicious deaths. And Picard’s work in Yellowstone and Great Smoky Mountains national parks demonstrates that the insects can be used as an efficient way to assess the diversity of vertebrates in a given area and even identify species not previously known to be there. “We’re not ready to stop looking for all the different uses that are available to us by these blowflies,” said Picard, a biology professor at Indiana University, Indianapolis.

Little was known about blowflies in Joshua Tree before Chu’s survey. In fact, a researcher who had supposedly tried to trap blowflies there decades ago told her he doubted she’d catch any in this hot and dry environment. Yet from June 2022 to November 2023, Chu placed traps at locations across the park — in the Sonoran and Mojave deserts and the transition zone where the two overlap. To improve her odds, Chu needed to pick the right bait. The most effective is a full-sized pig carcass, she said, but that would not have been a palatable option for park authorities. She experienced limited success with beef liver, pork liver and horse manure, before settling on dead squid. “The squid ended up being good because it was more wet,” she said. “So in dry conditions, it lasted longer.”

ILLUSTRATION BY JACQUI OAKLEY

She would return to pick up the traps four or five hours later, drive back to the lab and place the flies in ethanol for identification. All in all, she captured an astounding 2,699 blowflies representing 12 different species, including one that had never been formally reported in Southern California. “That was really exciting,” she said. “You wouldn’t expect for all of these little flies to be there.”

Chu’s survey is important for the park, which keeps an inventory of species documented there, but it also could assist the police in the area, which has had its fair share of untimely deaths, between homicides and hikers who fall victim to the park’s harsh conditions.

Blowflies can detect a dead body — human or animal — from miles away if wind conditions are favorable, and the insects are among the first to arrive on the scene, sometimes within minutes. Once there, the female lays up to 250 eggs, and they typically hatch a day later. The larvae, or maggots, then feed on the decomposing flesh and go through three growth stages before forming a cocoon known as a pupa, from which an adult fly emerges.

Forensic entomologists can estimate the amount of time that has passed since a fly laid her eggs based on the larva’s stage of development, which helps police narrow down the time of death and possibly confirm or rule out potential suspects. But the fly’s life cycle can last anywhere from about two to four weeks based on environmental conditions, such as temperature, and the fly species, so it’s crucial to know which flies live where. “It is that baseline data that I think is very, very valuable, because we so often don’t have that,” Chu said.

This kind of information is also essential because the presence of a fly species not known to live in the area could indicate that a body has been moved, said Jeffery Tomberlin, the principal investigator of Texas A&M University’s Forensic Laboratory for Investigative Entomological Sciences (got the acronym?). Tomberlin, who was not involved in the park studies but has participated in more than 200 criminal investigations, also said a toxicological analysis performed on the insects could show whether victims consumed narcotics before their death. Even if the body is no longer there, the victim’s DNA could be obtained from a fly larva collected on the scene. “Sometimes you can get an identification of a person based on what’s in the gut of the maggot,” he said.

That is how Picard can detect the presence of vertebrates in an area without having to spot any of them. She and a student, who is now a professor at a different university, attracted blowflies with a hunk of rotting meat, caught them with a net, dissected the flies’ gut and sequenced the DNA of the animals whose bodies or feces they fed on.

Picard’s student, who captured flies in Indianapolis public parks, found a lot of dog DNA — and one big surprise. “Out of this downtown sample, she got lion DNA, and I was like, ‘Whoa, what’s happening?’” Picard said. It didn’t take long to figure out the fly had likely visited the Indianapolis Zoo a couple of miles away.

In Yellowstone, a park with an exhaustive species list that they could use to cross-reference their results, Picard and her student collected DNA from easy-to-spot ungulates such as elk and bison, but also from small rodents that would otherwise be harder to detect. In the greater Great Smoky Mountains, they identified DNA from an armadillo, a species known to exist in neighboring Georgia that had not been reported in the park, though the animal’s presence was later confirmed by a camera trap. Picard said this sampling method is particularly beneficial in arduous terrain that researchers would have to traverse to conduct species inventories. “The big advantage is the flies are doing all of the work,” she said. “The flies are out there, sampling.”

Picard is also working with public health researchers to learn how blowflies could spread pathogens, such as E. coli, from sewage overflows. And she and her collaborators received a grant from the Department of Defense to look into whether the insect could sample the compounds found in chemical weapons — and be the literal fly on the wall. “Anyone who’s using chemical weapons typically doesn’t invite people to come in,” she said.

Picard said that basic facts about the blowflies’ ecology, such as where they go when they’re not feeding and how far they travel, remain mysteries, but these questions drive her fascination and fondness for the oft-reviled insects.

“They do all these different things in the environment,” she said. “They sort of move in a disgusting world, but they’re very attractive insects to look at.”