Thursday, April 27, 2017

April Speaker Report: The Cane Toad Challenge by Rob Capon

Our speaker Professor Rob Capon, an Organic Chemist, is a Professorial Research Fellow and Group Leader at the University of Queensland’s Institute for Molecular Bioscience. In collaboration with colleagues (University of Sydney’s Professor Rick Shine and his team) they have developed and patented a pheromone based cane toad control technology. This technology allows for the rapid, cost effective species selective trapping of cane toad tadpoles in managed waterways (i.e. dams, creeks, ponds, lakes, canals etc.).

Cane toads are prolific breeders, and the team has identified the short window in time when tadpoles are swarming at the margins of water bodies in the breeding season as an ideal opportunity to make a dent in intergenerational recruitment (that is, take out the young guns before they become adults). Their approach is both environmentally sustainable and safe, as well as being intuitive and simple to implement - if they provide the pheromonal baits. The story of how they achieved their goal of identifying the pheromone was the subject of Professor Capon’s fascinating and well-received talk.

He commenced with a brief history of the spread of and previous attempts to control cane toads. It became apparent that in order to find a practical, long term solution (the Challenge) there was a need to understand the cane toad’s chemical ecology, as had been done with insects where species specific pheromones have been used successfully in insect traps. In theory, it was believed, the same principle should hold for toads.

The problem with cane toads is their toxicity, making it important to learn how it uses their chemistry. This involved dissecting and investigating the structure of the parotoid gland. The parotoid gland is the raised area on the cane toad’s shoulders from where a poisonous, milky substance is exuded when the toad is caught by a predator. The gland contains a mass of small vesicles, each with a duct leading to a pore on the surface of the skin. What the researchers discovered was that the toxin stored in the vesicles was not as potent as that exuded onto the skin. The potency was increased by the addition of an enzyme, stored in even smaller vesicles either side of the duct. This very clever system ensured that the toads were not themselves endangered by storing a lethal cardio toxin in their bodies.

How, then, can this knowledge be used as a control method? It was already known that cane toad eggs are poisonous, as are the tadpoles. Female cane toads coat their eggs with the same toxin to protect them from predators, but the pheromone in this toxin also acts as an irresistible lure for cane toad tadpoles. Cane toad tadpoles eat the eggs from later spawnings, possibly to obtain protein, to renew their own toxin, or simply because of genetic competition. The pheromone is species specific and does not attract frog tadpoles.

Professor Capon and his team extracted the pheromone from adult toad toxin and developed a system to lure tadpoles into traps. The chemistry the tadpoles hone in on is replicated, coated on to an air stone (a porous stone used in aquariums) and placed in a very simple plastic box-trap, with simple plastic funnels fitted in holes drilled in the side. The chemical scent trail leaks from the box into the surrounding water, where the tadpoles detect it and follow it back into the trap. These traps have, for example, been used successfully on Bribie Island (10,000 tadpoles trapped in a 24-hour period) and at Thornlands (44,000 over a season).

There is a web page and video explaining the Cane Toad Challenge that members who were absent from the presentation will find interesting – and even those who did attend will be interested to see the end of the video that was cut short by a power cut. See “An ugly menace” online (

Professor Capon has donated to the Cane Toad Challenge, the travel money provided to him by the Field Nats.
(Report by Deb Ford)

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