The cane toad (Rhinella marina) is a large, terrestrial amphibian native to Central and South America. Beginning in the early 20th century, this species was deliberately introduced to various regions, including Australia and the Caribbean islands. The primary goal was to provide biological control for agricultural pests, such as the cane beetle, on sugar plantations. However, the cane toad failed to control the pests it was meant to target and instead became one of the most destructive invasive species globally, requiring extensive control strategies.
Ecological Impact and Rapid Spread
The cane toad is considered an invasive pest due to its omnivorous diet and potent defense mechanisms. Behind each eye, the toad possesses large parotoid glands that secrete bufotoxin, a milky, toxic substance. This potent cardiotoxin is lethal to many naive native predators, including northern quolls, goannas, snakes, and domestic pets, which attempt to consume the amphibian.
The lack of natural predators allows the cane toad population to explode and spread rapidly across new landscapes. Female cane toads are prolific breeders, capable of laying a clutch of between 8,000 and 25,000 eggs in a single string spawn. This high reproductive capacity ensures rapid population recovery and geographic expansion, necessitating sustained control efforts.
Direct Physical and Chemical Removal Methods
Immediate control relies on hands-on techniques, primarily involving the physical removal of adult toads and their spawn. Manual collection, often called a “toad muster,” is a common community-based method conducted at night when the toads are most active. Specialized traps and netting are also used to remove the black, string-like egg masses and tadpoles from water bodies before they can mature.
Once collected, the toads must be humanely euthanized using approved chemical methods. A high-concentration citric acid solution, often applied as a spray, is one of the most effective agents. A 16% aqueous citric acid solution causes rapid mortality through skin absorption and osmotic shock. This method is favored because it is species-specific, poses minimal risk to native fauna, and is widely available for community use.
Targeted Biological and Genetic Control
Long-term control efforts focus on developing biological solutions that suppress populations. Researchers are investigating synthetic pheromones, which cane toad tadpoles are naturally attracted to, to create olfactory lures for trapping. This approach exploits the tadpoles’ cannibalistic behavior, drawing them to toxins exuded by other toads, allowing for the removal of large numbers of juvenile toads from breeding sites.
Advanced research is exploring the potential of genetic modification and gene drive technology to suppress populations. Scientists are using tools like CRISPR/Cas9 to create gene knockouts that disrupt the toad’s reproductive cycle or detoxification processes. One goal is to disable the gene that triggers metamorphosis, resulting in tadpoles that never grow into reproductive adults. Another approach involves creating adults with reduced toxin levels for use in aversion training programs.
Large-Scale Community Management Programs
The success of control strategies depends heavily on coordinated community and government management programs. Events like annual “toad musters” mobilize volunteers to collect thousands of toads, providing continuous removal and valuable data. These citizen science efforts are supported by educational campaigns that teach the public how to safely identify and humanely dispose of toads, including guidelines on backyard control like removing standing water.
A significant conservation strategy involves Conditioned Taste Aversion (CTA) training to protect native wildlife populations. Toads collected are used to create “sausages” containing a small, non-lethal dose of toad toxin combined with a nausea-inducing chemical. When native predators, such as quolls or goannas, eat these baits, they associate the taste of the cane toad with illness, teaching them to avoid eating the real, lethal toads.