Can DNA from the bellies of parasites lead us to rare frogs? We’ve been in the field to find out.

A Mountain Stream Tree Frog being parasitized by two Sycorax midges – note their abdomens engorged with blood
A Mountain Stream Tree Frog being parasitized by two Sycorax midges – note their abdomens engorged with blood Image: Tim Cutajar
© Australian Museum

We’ve been in the forest studying the interactions between frogs and flies, but not the interactions you’re thinking of. We all know that frogs rely mostly on invertebrates for food, but what we’re interested in are the flies that feed on frogs! We’re researching whether specialist midge and mosquito species that feed only on frog blood can help us locate rare or elusive frogs in order to inform their conservation.

Being more threatened with extinction than any other vertebrate group, frogs are an obvious conservation priority. But to conserve species, we need to know about them, including where they live. In many places, and for many species, we simply don’t have that kind of information. There are a number of causes for this lack of data; some frogs are excellent at hiding their presence through camouflage or secretive behaviour, some live in habitats that are too complex for scientists to search in, while others are just plain rare.

Many survey techniques have been developed over the years for frogs, from the good old ‘get out there and look’ to automatic frog call recording devices and even collecting water samples and analysing DNA that may have been left by frogs in their aquatic environment (called eDNA). All have their merits, none are completely effective. There is another option though: frog-biting midges.

A resting Sycorax midge
A resting Sycorax midge Image: Tim Cutajar
© Australian Museum

On every continent on Earth where there are frogs, there are also midges and mosquitos that feed on frogs’ blood. At least some of these find food in a perhaps unexpected way – by listening out. Male frogs call to attract a mate, but many also inadvertently attract bloodthirsty flies. With these flies, the search party for rare frogs during surveys might be increased from one or two scientists to thousands.

Sycorax midges feeding on a threatened Stuttering Frog
Sycorax midges feeding on a threatened Stuttering Frog Image: Tim Cutajar
© Australian Museum

The most common question I get is whether we follow flies to the frogs. They can fly, and unfortunately we cannot, but luckily there’s an easier way. Over the last week in the field, we’ve been having the flies come to us. With a couple of small traps and speakers playing pre-recorded frog calls as bait, we’ve attracted over a thousand frog-biting midges. So we have thousands of midges, but how does that help us find the frogs?

Traps set up for collecting Frog-biting Midges
Traps set up for collecting Frog-biting Midges Image: Tim Cutajar
© Australian Museum

Here’s the essence of the project. We’re now going to select the fattest, fullest of those midges and extract DNA from the meals in their bellies. Assuming they’ve been feeding on frogs, it should be frog DNA. We then analyse that DNA and identify the species of frog it belongs to. Hopefully then, we'll have detected a frog species in the forest that we may not have seen or even heard when we were there.

This method is called invertebrate-derived DNA (iDNA). It has already been used to survey for mammals and birds, and with good success. A few studies have used iDNA to confirm the identity of the hosts of some frog-feeding flies, but this will be the first time it’s tested as a survey technique for frogs. Fieldwork is over. The next step is getting into the lab to see what we have, so, check back to find out!

Tim Cutajar, Research Assistant & Honours student, Herpetology, AMRI & UNSW


Thanks to the Frog and Tadpole Study Group of NSW for providing a research grant to assist this work, Peter Beard, National Parks and Wildlife Service and the Forestry Corporation of NSW for approving fieldwork, Brittany Mitchell for help in the field, Dr Cameron Webb (University of Sydney) for providing sampling equipment, and Dr Jodi Rowley for continued support throughout the project.