The cane toad (Rhinella marina), an amphibian native to Central and South America, has become an infamous invasive species across Australia’s northern landscapes. The cane toad is officially recognized as a key threatening process under the Environment Protection and Biodiversity Conservation Act 1999, highlighting the severe ecological disruption it has caused since its introduction.
Australia’s Sugar Cane Crisis
Queensland’s burgeoning sugar cane industry faced a severe agricultural problem from the late 1800s due to destructive cane grubs. These grubs were identified as the larval stage of native beetles, notably the grey-backed cane beetle (Dermolepida albohirtum) and French’s beetle (Lepidiota frenchi). The larvae would burrow underground and consume the roots of the sugar cane plants, causing widespread plant death or severely stunting their growth.
This persistent pest problem led to substantial economic losses for sugar cane growers, with annual damages reaching millions of dollars. In response to intense pressure from the industry, the Queensland government established the Bureau of Sugar Experiment Stations (BSES) in 1900. By the 1930s, an urgent need for an effective biological control method to protect the struggling sugar cane crops became clear.
The Introduction of the Cane Toad
The consideration of the cane toad as a solution was influenced by a 1932 paper presented by Raquel Dexter at a conference in Puerto Rico. Her research highlighted the apparent success of the cane toad in controlling beetle infestations in Puerto Rican sugar cane fields. This led to the toad’s introduction from Puerto Rico to Hawaii for similar pest control.
Driven by the promising results elsewhere, Reginald Mungomery, an entomologist with the Bureau of Sugar Experiment Stations (BSES), traveled to Hawaii in June 1935. His mission was to collect cane toads for Australia. Mungomery captured 102 toads and transported them to the Meringa experimental farm near Gordonvale in far north Queensland on June 22, 1935. Upon arrival, the surviving toads were placed in a breeding enclosure. By August 1935, the toads had reproduced, and on August 19, an initial release of 2,400 toadlets occurred at various sites around Gordonvale. The expectation was that these amphibians would provide a natural and efficient biological control against the devastating cane beetles.
The Unintended Invasion Begins
The cane toads proved largely ineffective in controlling the target cane beetles. Adult cane beetles feed on higher parts of sugar cane stalks, out of reach for the ground-dwelling toads. Furthermore, the most damaging stage of the beetle, the larvae, lived underground feeding on roots, making them inaccessible to the toads.
The toads, however, found the Australian environment highly favorable for their own proliferation. They possess an impressive reproductive capacity, with a single female capable of laying up to 35,000 eggs in one clutch, sometimes producing two clutches annually. Their generalist diet meant they consumed a wide range of native insects and other small prey, rather than selectively targeting the cane beetles.
Behind their heads, cane toads have parotoid glands that secrete a potent, milky toxin. This toxin is highly poisonous to many native Australian predators, such as monitor lizards, northern quolls, and snakes, leading to significant population declines in these species that attempted to consume the toads. With an abundance of food, a lack of natural predators, and high reproductive rates, the cane toad population rapidly expanded and began spreading from its initial release points, quickly becoming a substantial ecological problem.