Naturally occurring tadpole-eating predators have been too efficient in reducing cane toads in a recent study to test pheromones in water.
In April and May this year, PhD student Sam McCann from the University of Sydney tested several tadpole-control mechanisms in 30 artificial ponds constructed in a gravel pit just outside of Kununurra.
Program Leader Cane Toad Strategy for WA at the Department of Parks and Wildlife, Corrin Everitt said the aim was to determine which of several tadpole control mechanisms was most effective at reducing toad tadpole survival.
These methods included luring them into traps, supressing their growth, and mimicking high predation risk. These specifically used the manipulation of species-specific tadpole pheromones (chemicals released only by toad tadpoles) to trick tadpoles into responding a certain way.
However, it was the presence of diving beetles, dragonfly larvae, fishing spiders and water bugs all naturally occurring tadpole eating predators that resulted in all toad embryos and tadpoles being consumed before the end of the experiment.
This made it impossible to test the pheromones because the insects ate the eggs.
A strong negative correlation between the number of predators and the time tadpoles survived was recorded, Ms Everitt said.
According to Ms Everitt, the unexpected results were probably due a poor wet season. They have, however led to many new questions regarding natural predator densities.
Follow up experiments will be run in the ponds over the next year to find out what determines the presence and density of particular predators in water bodies across the toad range, and how this might influence the effectiveness of control efforts, she said.
This research is supported by Rangelands NRM through funding from the Australian Government’s National Landcare Programme.
Images (courtesy of Department of Parks & Wildlife):
[Top Left]: Dragon fly nymph
[Right]: Experimental ponds near Kununurra