TEAM BUFO'S NEW IDEAS ON CANE TOAD CONTROL

We urgently need new ideas on toad control. In other pages on this website, we've explained why toads are so difficult to control, and why previous ideas on cane toad control really haven't worked. Although some scientists hope to create genetically modified viruses to kill toads, we are skeptical about the collateral damage that would arise from that approach, even if the technical obstacles could be overcome (which seems unlikely).

When Rick began his work on toads, he doubted that we would find ways to control them; cane toads are incredibly effective invasion machines! However, our research has revealed more and more chinks in the toad's armour, to the point that we are optimistic about the possibilities for toad control (though not eradication).

Team Bufo's research hasn't been designed for this purpose - our funding from the Australian Research Council is to understand toads and their effects, not to control them - but clearly it would be a terrific spin-off if that knowledge could be used to reduce the impact of cane toads on Australian biodiversity.

So, we've put special effort into parts of the research that might help in this way. That effort has been helped by extra funding from the Australian government (Department of the Environment, Water, Heritage and the Arts) to look at pheromones and spawning-site choice in toads; and from the Invasive Animals Co-Operative Research Centre to explore effects of parasites on toads in Queensland. The results from that work, in combination with our other recent results from the studies at Fogg Dam, are really encouraging.

In terms of ways to reduce the numbers and body sizes of cane toads, we have made three exciting discoveries:

Alarm chemicals (“pheromones”)

First, Mattias Hagman discovered that cane toad tadpoles have a special "alarm pheromone" - a chemical substance they produce when they are frightened or injured. It travels through the water and alerts nearby tadpoles that something is wrong - most likely, a predator is attacking one of your brothers or sisters - and the toad tadpoles flee as fast as they can from the chemical. Tadpoles of Australian native frogs don't react to the toad chemical - they probably don't even detect it.

When we added water containing the alarm pheromone to this tank (near left corner), the toad tadpoles fled from it (so are clustered in the top right corner).

What use is it to frighten toad tadpoles? Well, it stresses them. In trials in outdoor ponds, adding the alarm chemical every day for a couple of weeks killed about half the toad tadpoles in our ponds. And when the survivors turned into little toads, they were tiny - less than two-thirds the weight of their brothers and sisters from the ponds next door where we hadn't added the alarm chemical. So, adding the alarm chemical to ponds can kill many toad tadpoles without affecting frog tadpoles, and can "miniaturise" the emerging toadlets.

Exposing tadpoles to the alarm pheromone makes them transform into toadlets at a much smaller size (on the right) than normal (on the left).

Effects of parasites on baby toads

Second, Crystal Kelehear studied a lungworm parasite that is common in Queensland cane toads, but has only recently arrived in Northern Territory cane toads (the parasites slow toads down, so have lagged behind the fast-moving toad invasion front).

Crystal infected baby toads in the laboratory, and found that the lungworm was devastating for the tiny animals - about 30% died, and the remainder grew more slowly, moved more slowly, and so on. The parasite was especially lethal to very small toads – so transforming from the tadpole stage at a small size (as happens if the tadpoles are exposed to the alarm pheromone) makes the baby toads really vulnerable to the lungworm.

Parasitic nematode worm in a cane toad’s lung

This is really encouraging - we might be able to spread the lungworm parasites around, especially to toads at the invasion front, and kill many of the baby toads as they emerge from the water. And we can increase the effect of the lungworm by spreading the alarm chemical around at the same time, to “miniaturise” the baby toads. But there's a problem - would we also kill lots of native frogs? If so, the parasite can't be used to control toads. But it turns out this isn’t a problem – read on!

What is the parasite that lives in toad lungs?

Third, Dr Sylvain Dubey has been examining just what kinds of lungworm parasites occur in toads and frogs. The parasites all look about the same, and there has always been a lot of uncertainty as to which species were which. The only reliable way to identify them is to look at their DNA sequences - a lot of work! But Sylvain has now compared DNA from lungworms found in toads (both in Australia and South America) to that from Australian frogs (mostly from Queensland, where the toads and frogs have lived side-by-side for more than 50 years).

Dr. Sylvain Dubey's skills in molecular biology gave TEAM BUFO an exciting breakthrough.

And the result amazed us.

Scientists have always thought that the cane toads left their native parasites behind in the process of being moved to Australia - and so, the lungworms in Australian cane toads were believed to be a species (Rhabdias hylae) that is common in Australian frogs. This would make it difficult to use the lungworms to control toads - because if we made the worms more common or more infective, we'd cause problems for native frogs as well.

But Sylvain's results show that we don't have to worry about this kind of collateral damage. All of the lungworms that we have found in Australian cane toads so far are Rhabdias pseudosphaerocephala - a South American species! So the toads have brought the parasite with them, not picked it up from the Queensland locals!

And even better, all of the lungworms we have identified from the Queensland frogs are genuine Aussie species - none have been carrying the cane toad's parasite! Dr. Ligia Pizzatto has been checking this important issue by trying to infect native frogs with the toads' parasite - and so far, none of the frogs have become infected.

Both frogs (left) and toads (below) have parasitic worms in their lungs, but the worms are different species.

So, we have discovered a powerful new biological control for cane toads in Australia!

In summary, we have found a parasite that kills toads, but does not infect Australian frogs; and an alarm chemical that kills tadpoles of toads, but not Australian frogs. And a combination of the two weapons will work even better, because the alarm chemical makes toads transform from tadpoles into toadlets at a smaller size, and so makes them even more vulnerable to the parasite.

There's a lot more research still needed before we fully understand exactly what goes on when the parasite meets the toad. For example, how common are the parasites in areas of Queensland where the toads are common, compared to places where toads are rare - and especially, in places where the toads used to be common but have become rare? Are the parasites responsible for those variations in abundance? We still have some questions like these to answer, but we think that TEAM BUFO’s new control strategy has terrific potential, and could be implemented as soon as we have answered those questions. We are hoping to work with the Western Australian state government to start conducting exactly that research.

NEWS ARTICLES

You can read the transcript or listen to the podcast of Rick's interview with Tony Eastley on ABC radio's AM program discussing Rick's cane toad control proposal.

ABC News article about Rick's cane toad control proposal.

The University of Sydney NEWS article about Rick's new control ideas.

footer for cane page