New Guinea Flatworm, one of the Worst Known Invasive Species, Found in US

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Aamna Mohdin

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698 New Guinea Flatworm, one of the Worst Known Invasive Species, Found in US
Makiri Sei. CC-BY 4.0

The New Guinea flatworm’s detrimental impact on biodiversity has earned it a spot on the '100 worst invasive alien species' list, and unfortunately for the U.S., the pesky critter has now found its way to the mainland. Although it's only popped up in Florida so far, its highly invasive nature means it could represent a serious threat to other states, too.

The New Guinea flatworm, Platydemus manokwari, reproduces quickly and is mainly spread by humans. Once they arrive in a new territory, the flatworm is able to quickly adapt and predate on local snails and other invertebrates. The slippery species is 50 millimeters (2 inches) long and 5 millimeters (0.2 inches) wide. Its back is a black olive color with a thin medial pale line, and it has a pale white belly where the mouth is located.


The adults are able to survive in pots or on plants. In Florida, where gardens are often attended by professionals, movements of the flatworm from garden to garden – together with soil, compost, rooted plants, potted plants and garden waste – will likely disseminate the invasive species further. Researchers suggest that this has already happened, since the species has been found in several gardens in Miami.

 New Guinea Flatworm in Florida. Makiri Sei. CC- BY 4.0

“The discovery of this species in mainland America should be considered very bad news for biodiversity,” Professor Jean-Lou Justine, from the Institute of Systematics, Evolution, Biodiversity, who led the study, tells IFLScience. “It is a predator of land snails and of many other soil invertebrates. It can endanger existing populations of rare and threatened endemic snails.”

The study, published in the journal PeerJ, reports that the flatworm has been found in several additional countries and territories: Singapore, New Caledonia, an additional island in French Polynesia, Wallis and Futuna, the Solomon Islands, Puerto Rico and, of course, America. The flatworm has now been found in a total of 22 countries and territories around the world.


Map of New Guinea Flatworm around the world. Justine et. al.

Up until now, the flatworm mostly infected islands and the spread between islands was limited. The presence of the flatworm in mainland U.S. suggests it’s no longer subjected to these restrictions. Researchers warn that the invasive species could spread from Florida to other mainland states, which threatens not only the U.S., but also the West Indies and the rest of the Americas.

“The selection of the species to be included in the ‘100 of the worst’ list was based on both their serious impact on biodiversity, and on their illustration of important issues surrounding biological invasions,” says Dr. Piero Genovesi, senior conservation officer at the Institute for Environmental Protection and Research.

He tells IFLScience that the impact of the New Guinea flatworm on biodiversity should not be underestimated. Land snails, a particularly important group globally with a very high number of species, are incredibly vulnerable.


“This group has the highest rates of extinctions. It is important to remember that land snails are a major component of the entire faunal diversity of North America, and a key element of the ecosystems of this region,” he adds.

Justine is shocked that little has been done to tackle the flatworm. Though some experiments have shown that adult flatworms can be killed by high temperatures, the economic cost of such a measure has never been evaluated.

“It is crucial that we enforce more effective biosecurity policies, for example treating soils and machineries before moving them around the world, but also regulating more effectively the intentional movement of species for biocontrol or for other reasons,” Genovesi tells IFLScience.

“A more effective control of the movements of goods should be seen as an investment, as we know that invasive species cause huge economic losses globally as well as major health problems,” he adds.


  • tag
  • biodiversity,

  • conservation,

  • invasive species,

  • new guinea flatworm,

  • land snails