Negative impacts of invasive predators used as biological control agents against the pest snail Lissachatina fulica: the snail Euglandina ‘rosea’ and the flatworm Platydemus manokwari

Abstract

Since 1955 snails of the Euglandina rosea species complex and Platydemus manokwari flatworms were widely introduced in attempted biological control of giant African snails (Lissachatina fulica) but have been implicated in the mass extinction of Pacific island snails. We review the histories of the 60 introductions and their impacts on L. fulica and native snails. Since 1993 there have been unofficial releases of Euglandina within island groups. Only three official P. manokwari releases took place, but new populations are being recorded at an increasing rate, probably because of accidental introduction. Claims that these predators controlled L. fulica cannot be substantiated; in some cases pest snail declines coincided with predator arrival but concomitant declines occurred elsewhere in the absence of the predator and the declines in some cases were only temporary. In the Hawaiian Islands, although there had been some earlier declines of native snails, the Euglandina impacts on native snails are clear with rapid decline of many endemic Hawaiian Achatinellinae following predator arrival. In the Society Islands, Partulidae tree snail populations remained stable until Euglandina introduction, when declines were extremely rapid with an exact correspondence between predator arrival and tree snail decline. Platydemus manokwari invasion coincides with native snail declines on some islands, notably the Ogasawara Islands of Japan, and its invasion of Florida has led to mass mortality of Liguus spp. tree snails. We conclude that Euglandina and P. manokwari are not effective biocontrol agents, but do have major negative effects on native snail faunas. These predatory snails and flatworms are generalist predators and as such are not suitable for biological control.

The tree snail on Rota Island, Northern Mariana Islands, long identified as Partula gibba (Partulidae), is a different species

“…Across the Pacific, partulid species have been driven to extinction by introduced predators, most notably North American carnivorous snail species in the genus Euglandina, and the New Guinea flatworm Platydemus manokwari (Clarke, Murray and Johnson 1984;Murray et al 1988;Hopper and Smith 1992;Bauman 1996;Coote et al 1999;Cowie and Cook 2001;Régnier, Fontaine and Bouchet 2009;Pelep and Hadfield 2011;Meyer et al 2017;Sischo and Hadfield 2017;Hadfield 2020;Gerlach et al 2020). This has been particularly true in the Mariana Islands where half of the described partulid species are thought to be extinct, and the remaining species are imperiled across their ranges.…”

Biological Control of Pest Non-Marine Molluscs: A Pacific Perspective on Risks to Non-Target Organisms

“…The sad story of non-target impacts of previous biocontrol programs using the snails Euglandina spp. and the flatworm Platydemus manokwari targeting the land snail pest Lissachatina fulica has been recently reviewed in detail by Gerlach et al [ 20 ]. Therefore, we will provide just a brief overview of these two failed biocontrol programs and include a lesser-known historical biocontrol introduction to establish the context for our critical view of using generalist biocontrol agents to control non-marine molluscs.…”

“…Of those released, at least three, Gonaxis kibweziensis , G. quadrilateralis , and Euglandina “ rosea ”, have become established [ 28 , 29 ]. These snails, especially Euglandina “ rosea ”, have subsequently been introduced to numerous Pacific islands and beyond as part of efforts to control L. fulica and other snail pests [ 20 , 26 ].…”

“…Despite the lack of evidence for effectiveness of this control agent, agricultural authorities continued to introduce Euglandina spp. to additional island groups into the 1990s [ 20 ]. While failing to control L. fulica [ 7 , 8 , 12 , 17 , 33 , 34 , 35 ], Euglandina spp.…”

“…Accordingly, any proposal for the use of biological control agents targeting pest species of non-marine molluscs must be evaluated in the light of lessons learned from that long and unhappy history, as well as a modern understanding of the ecological and evolutionary complexity of species interactions. Our view of control programs targeting pest non-marine molluscs is shaped by studies of the systematics and conservation of the non-marine molluscs that have been the collateral damage of those programs, particularly on Pacific islands (e.g., [8][9][10][11][12][13][14][15][16][17][18][19][20]).…”