Walter Renwick is probably more thoughtful about urban ecology than most other residents of Los Angeles in the US. The house he lives in with his wife and two young daughters is outfitted with solar panels and he collects rainwater in barrels, but he admits that his true passion was for bugs. He calls them his "guilty pleasure."

When he entertains friends at his home in the Silverlake neighborhood, many admire his efforts at sustainability, and most even appreciate the bins of worms he has chewing away, slowly converting trash into compost. But his colony of black soldier flies, harmless insects that are remarkably efficient at converting food waste into chicken feed, tend to creep people out. "As such, I was always somewhat embarrassed by my fascination with these critters," he admits.

Then he got to know Lisa Gonzalez. She's assistant collections manager in entomology at the Natural History Museum of Los Angeles (NHM) and was the first proper entomologist that Renwick had the pleasure of meeting. She got in touch with Renwick at first because he'd established himself as a local expert on black soldier fly colonies. She was helping to design an exhibit featuring the species at NHM. From there it didn't take much convincing before Renwick agreed to put a malaise trap in his backyard, a tent-like contraption designed to collect insects for analysis by NHM researchers.

I bragged that I could find a new species anywhere

Why were the scientists at the Natural History Museum interested in seeing what insects were flying around Renwick's backyard?

Los Angeles is a city known for many things: movies, traffic, smog, earthquakes, yoga, kale. Urban spaces such as this are not usually known for their biodiversity, and LA is especially urbanised, a sea of concrete with a mosaic of green islands – city, county, state, and national parks – scattered across the landscape, plus lots of backyards, many of which with those stereotypical LA swimming pools.

But it turns out there's plenty of nature still waiting to be found, even in the concrete jungle. At least, that's according to NHM entomologists Emily Hartop and Brian Brown, who have discovered thirty insect species never before known to science, all in LA backyards such as Renwick's. Their findings will be described in April 2015 in the journal Zootaxa.

Hartop and Brown didn't set out to discover new bugs in Los Angeles, at least not at first. "I was talking to a [museum] trustee and I bragged that I could find a new species anywhere," said Brown, the museum's curator of entomology. She took the bait. Her home was in Brentwood, a wealthy neighborhood on the western side of Los Angeles, just a few miles from the beach. "And then, I thought: oh crap, now I've actually got to do it," Brown recalls.

From those three specimens, we had a new species, a new North American record from Europe, and a new North American record from Africa

He deployed a malaise trap in her backyard. Bugs fly into the tent and get funneled towards an opening at the top. There, they fall into a jar filled with alcohol where they remain preserved until a researcher retrieves them.

The first thing Brown did was separate the bugs on which he's an expert, tiny insects called phorid flies, discarding all the wasps, bees, beetles, leafhoppers, moths, butterflies – everything caught by the trap that wasn't a phorid.

The first one he plucked out was a 2.5 millimetre yellowish fly. He tried comparing it to known species of phorids, but he couldn't match it to anything. It turned out to be an undescribed species, hitherto unknown to science.

One she called "bunny" because its genitalia reminded her of bunny ears

Then he pulled out a brown fly and saw something on the middle leg that looked familiar. He realised it was a species known only from Europe. Another phorid he pulled out was known only from the Canary Islands and the Seychelles, both off the coasts of Africa.

"So from those three specimens, we had a new species, a new North American record from Europe, and a new North American record from Africa. After that I didn't bother looking through any more of the sample because I had enough for an evening program," he says.

But that discovery laid the foundation for what would become known as BioSCAN, a research program at NHM that relies on citizen scientists who have allowed the researchers to install malaise traps in their backyards.

Brown's idea, inspired by his unexpected discovery, was to set out an array of traps throughout Los Angeles on a gradient of urbanisation. Some traps would be placed at homes close to the natural landscapes of LA's mountain ranges. Others would be in the middle of downtown, as far as you could get from what we typically think of as nature. The idea was to see if there was a statistical relationship between urbanisation and biodiversity.

The truth is that scientists haven't been paying all that much attention to their own backyards

Three years after Brown's "bug bet," Hartop was busy examining the specimens collected in the volunteers' backyards. She started to realise that some of the flies had unique characteristics. One she called "bunny" because its genitalia reminded her of bunny ears. Gonzalez christened one "sharkfin" because of the shape of one of its legs.

Hartop ran into the same problem that Brown had several years earlier: she was unable to match many of her nicknamed specimens to known species. She even took them to the University of Cambridge, in the UK, to meet with Henry L. Disney, another phorid expert. Though he's retired, he's still active in the research community and is an expert on a single genus from within the phorid family, called Megaselia.

Together, the three researchers realised that they had discovered thirty new species of phorid flies, all from the genus Megaselia. Those new species come from among 10,000 specimens collected over just three months.

There are around 4,000 known species of phorid flies, but Brown says that there may be as many as another 35,000 species waiting to be discovered or formally described, most of them occurring in the tropics.

The researchers initially thought that being close to the mountains would be a predictor of high diversity, but some collection sites near the mountains had low diversity, and some downtown sites were surprisingly high in insect diversity. It's not clear what might explain species diversity from site to site, but Brown suspects that moisture might play a role, which could come down to something as simple as artificial irrigation.

We've got new, undescribed species all around us, and what's really frightening is the habitat destruction that's happening

As far as cities go, Los Angeles may be particularly suitable for a high level of biodiversity.

In part, that’s because it rests in the middle of the California Floristic Province, one of a number of global biodiversity hotspots, which are so designated due to their high proportion of endemic species, found there and nowhere else on the planet. The relatively warm climate year-round allows a lot of critters from a lot of parts of the world to thrive. Most insects get introduced through the nursery trade, their eggs hiding on plant leaves and in soil. Others stow away in cargo; Los Angeles hosts two major international shipping ports.

That's why it isn't clear whether these new species have been evading the prying eyes of scientists in Los Angeles all along, or whether they're new arrivals to the area. The truth is that scientists haven't been paying all that much attention to their own backyards in the first place, at least not in a systematic, rigorous, empirical way. Biologists tend to turn their eyes towards more exotic, relatively undisturbed locales. But, as this study clearly demonstrates, a tremendous amount of biodiversity is waiting to be discovered, even in the most heavily urbanised parts of our planet.

Brown hopes that studies like his will help connect people to nature. "People need to be associated with biodiversity," he argues. "We need interaction with it to be healthy, both mentally and physically."

He tells me about the Japanese practice of "forest bathing" or Shinrinyoku. By some accounts, the practice, which involves what is essentially a nature walk, can actually promote human health and wellbeing. Biologist E.O. Wilson similarly argued that humans have a strong "urge to affiliate with other forms of life."

I have developed a much greater appreciation for all that goes on when we are not looking

Dean Pentcheff, BioSCAN project coordinator, agrees. "Unless our biodiversity is healthy, we can't be healthy," he says, but his argument is more practical. "Biodiversity is the engine of ecosystem services." Without a diversity of species, the things we need to survive such as oxygen, clean water, and food could be in short supply.

Julian Donahue, another Los Angeles resident whose backyard hosts a malaise trap, is in it strictly for the science. It's important for him that LA's biodiversity be catalogued, and as quickly as possible.

His motivation is perhaps unique: he's a lepidopterist and before retiring, worked as curator of entomology at NHM for many years. "We've got new, undescribed species all around us, and what's really frightening is the habitat destruction that's happening. And those species are going extinct before they're even known to science," he says. "It's very frustrating not to be able to know what our fauna is before it disappears."

It’s just wild to imagine all that going on in your backyard every day

Each of the new species was named for the resident in whose yard the flies were discovered. The species discovered in Donahue's backyard is now known as Megaselia donahuei. The one named for Renwick and his family is called Megaselia renwickorum.

As for Renwick, he's thrilled to have found a like-minded community of amateur and professional entomologists to share his love of insects. "It was as if I had found a home for this weird little part of me," he says. "I have developed a much greater appreciation for all that goes on when we are not looking… It’s just wild to imagine all that going on in your backyard every day," he adds.