In 1955, Hawaiʻi released a predatory snail to control giant African snails; it climbed into native forests and helped wipe out rare tree snails

An effort to control an agricultural pest in Hawaiʻi produced an unexpected ecological consequence: the introduction of a predatory snail contributed to the decline of native Hawaiian tree snails. The consequences unfolded over decades and became part of a broader pattern of native land-snail extinctions across the islands.

A pest that got out of hand

The problem began with the giant African snail, a large plant-eating species that had arrived in Hawaiʻi decades earlier, likely as a garden ornamental or through commercial trade. By the 1950s, it had become a major pest of agriculture and ornamental horticulture throughout the islands. The species had become sufficiently widespread and damaging to agriculture to prompt efforts at biological control.


In 1955, Hawaiʻi imported a predatory snail to control giant African snails; it spread into native forests and helped drive rare tree snails toward extinction.

The wolfsnail had other plans

The biological-control strategy failed to produce the intended result. Rosy wolfsnails were largely indifferent toward the giant African snail and showed little interest in consuming it in a natural setting. After the rosy wolfsnails were released, the giant African snail remained established in Hawaiʻi.

Instead, the rosy wolfsnail spread into the mountainous forests. Hawaiʻi is home to a diverse group of native land snails, including small, colorful arboreal species that evolved in isolation and occupy ecological niches found nowhere else in the world. Some of these snails were historically important culturally and were used in lei. Earlier surveys had documented abundant and apparently stable native snail populations in the forests, but subsequent surveys recorded rapid declines after the wolfsnail became established. However, when the wolfsnail entered the Hawaiian archipelago, these native populations collapsed quickly.

Evidence of wolfsnail predation

One biologist who spent decades studying Hawaiian tree snails, Michael Hadfield of theUniversity of Hawaiʻi at Mānoa, described watching robust snail populations in the mountains just collapse. All that was left were hollow shells. Rosy wolfsnails do not typically consume the shell; instead, they extend a long feeding structure through the shell opening and extract the soft body of their prey.

The presence of intact, hollow shells can therefore provide evidence consistent with wolfsnail predation. By contrast, rat predation can leave shells visibly damaged or crushed because rats must break the shell to reach the animal inside. These differences in shell condition can help researchers distinguish between the two forms of predation.

The scale of what happened

Conservationists have documented dozens of Hawaiian land-snail extinctions, although estimates vary depending on the taxonomic group and the criteria used to classify species as extinct. For the most part, the extinctions have been due to the predatory acts of invasive species, among which the rosy wolfsnail is only one perpetrator. Also responsible are habitat devastation from farming and construction, other invasive predatory animal species like rats, invasive plant species, and a warming climate. At least 60 percent of Hawaiʻi’s roughly 750 native land-snail species are considered extinct, and dozens of the survivors are federally listed as endangered.

Biologists have described the scale of these losses as an ‘extinction avalanche,’ reflecting the rapid accumulation of species declines driven by multiple interacting threats. The pattern also reflects a broader problem affecting isolated Pacific islands. Over fifty percent of all described animal extinctions in the past 500 years have been of mollusks, and most of these were land snails on isolated Pacific islands succumbing to precisely this kind of diet of introduced predators and vanished habitat.

One snail named George

The case of the Oʻahu tree snail Achatinella apexfulva illustrates the broader decline at the level of a single species. The species had once been common on Oʻahu, and its shells were sometimes used as jewelry. In 1989, Hadfield established a captive-breeding program to help preserve Hawaiian tree snails. The last known wild individuals of A. apexfulva were brought into captivity in 1997 as part of efforts to preserve the species.

George, the last surviving A. apexfulva, was born in captivity in the early 2000s. The captive population eventually dwindled until George was the only known living member of the species. He died on January 1, 2019, marking the extinction of A. apexfulva.

Why this story still matters

The case does not demonstrate that biological control is inherently ineffective, but it illustrates the difficulty of predicting how an introduced predator will behave in a new habitat, particularly in an isolated ecosystem such as Hawaiʻi.