Effective conservation depends on understanding how biodiversity is distributed across space, but this is quite a challenge when some species, including many frogs, are much more difficult to find than others. At the Australian Museum and UNSW Sydney, we’ve developed a new method for detecting secretive frog species in the field. With the help of parasites, we can get hold of frogs’ blood (and therefore DNA) without having to find them. This unconventional new technique has the potential to contribute new information and improve the conservation of endangered frogs.

Bleating Tree Frog with Sycorax fly

Frog-biting midges are parasites that find frogs by following the sound of their calls.

Image: Timothy Cutajar
© Australian Museum

You may think that in the Anthropocene, the age in which humans have well and truly taken over the planet, we might at least know what other species we share it with and where they are. However, there are still huge amounts we don’t know about biodiversity. In fact, some of most threatened species are also some of the most poorly known and difficult to find. This is a big problem when you consider that effective conservation depends on knowing where species live – you can’t protect something you don’t know is there.

Many frog species are highly threatened and hard to find, which is why frogs are among the most common animals to be ‘rediscovered’ after being missing for years. For every rediscovered frog species, there are likely many more that elude us. For four years, we’ve been trying to find a frog that’s been feared extinct since the ‘70s. We haven’t found it yet, but our tireless search hasn’t been for nothing. What we did find were lots of frog-biting midges sitting on other frogs and sucking their blood, and that gave us a crazy idea...

A Mountain Stream Tree Frog being parasitized by two Sycorax midges

A Mountain Stream Tree Frog (Litoria barringtonensis) being parasitized by two Sycorax midges – note their abdomens engorged with blood.

Image: Timothy Cutajar
© Australian Museum

Frog-biting midges are parasitic flies that feed on frogs’ blood. Some of them have evolved an amazing mechanism for finding frogs. We don’t know exactly how yet, but frog-biting midges follow frogs’ mating calls to find their bloodmeals. Seeing so many midges getting fat on frogs’ blood got us thinking: why search for a needle in a haystack alone when thousands of flies are probably already doing it? Could these midges hold the key to detecting elusive frog species?

As it turns out, scientists have used parasites to detect secretive animals before, using invertebrate-derived DNA (iDNA) sequencing. iDNA is the DNA contained in the meal of a parasite, and that parasite’s unfortunate host could be an elusive threatened species. iDNA can be extracted, sequenced and read, which can be an effective way of detecting mammal species. But until now, no one has ever tested whether iDNA might be useful for frog surveys. We had a big question; can iDNA from frog-biting midges be used to detect frogs?

Traps set up for collecting Frog-biting Midges

Traps set up for collecting Frog-biting Midges.

Image: Tim Cutajar
© Australian Museum

We headed to the forest for a week to trap frog-biting midges and find out. We brought speakers with us and played pre-recorded frog calls to attract midges from deep in the forest. While our traps were running, we also walked up and down streams like we usually do, looking for frogs.

At the end of the week, we brought our midges into the lab and picked out the ones with the fattest, juiciest, most blood-engorged abdomens. These were the flies we hoped would answer our big question. We extracted blood from the midges’ abdomens (hoping it was from frogs), and then sequenced the DNA.

With bated breath we awaited results, and it was worth it. Thirty midges had provided host DNA sequences of four frog species! Three species were detected through iDNA in places where we failed to detect them during our traditional searches. In fact, despite neither sight nor sound of the particularly attractive Davies’ Treefrog (Litoria daviesae) in the field, we detected it five times at two streams through iDNA!

Davies Tree Frog

Davies Tree Frog (Litoria daviesae), one of the species detected in this study through iDNA.

Image: Jodi Rowley
© Australian Museum

The icing on this decidedly blood-red velvet cake is that Davies’ Treefrog and another of the species our midges helped find are threatened species. We’ve shown that iDNA from frog-biting midges can increase the detectability of threatened, elusive frog species. This totally new frog survey technique may lead to more frog rediscoveries, or even the discovery of new species, which would mean better-informed conservation practices. We’re excited to get back into the forest, so watch this space to see what frog species iDNA turns up!

Timothy Cutajar

Research Assistant, Herpetology, Australian Museum Research Institute

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We are grateful to Brittany Mitchell for help with fieldwork, John Martin for photographing specimens, Dr Cameron Webb for the loan of traps, Dr Dan Bickel for early advice and the use of microscopy equipment, and Dr Art Borkent and Dr Greg Curler for their expertise and help identifying dipteran specimens. This work was supported by the Frog and Tadpole Study Group of New South Wales.