Achatina fulica (mollusc)
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Achatina fulica is considered one of the worst snail pests of tropic and subtropic regions. While their small size limits the quantity of plant material consumed per animal the aggregated nature of the infestations can lead to severe damage in infested plants (Raut & Barker 2002). The process of naturalisation may ameliorate the impacts of this invasive species. Mead (1979a) expressed the opinion that “...the phenomenon of decline in populations of Achatina fulica appears to be inevitable”.
Agricultural: In tropical agriculture the cost of A. fulica is fourfold. First there is the loss of crop yield caused by herbivory. Secondly, damage may be caused by the spread of disease through the transmission of plant pathogens. Thirdly, there is the cost associated with the control of the pest and, finally, there are the opportunities lost with enforced changes in agricultural practice such as limiting crops to be grown in a region to those resistant to snail infestation (Raut & Barker 2002).
For a list of “Economically important plants recorded as being subject to losses through damage by Achatina fulica Bowdich (Achatinidae) in regions outside of Africa” please see the full Impacts document. Irrespective of crop the seedling or nursery stage is the most vulnerable stage. In more mature plants the nature of the damage varies with the species, sometimes involving defoliation and in others involving damage to the stems, flowers or fruits (Raut & Barker 2002).
Economic/Livelihoods: In the US state of Florida it has been estimated that A. fulica would have caused an annual loss of USD 11 million in 1969 if its population had not been controlled (USDA 1982). In India it attained serious pest status, particularly in 1946/1947, when it appeared in epidemic proportions in Orissa and caused severe damage to vegetable crops and rice paddies (Pallewatta et al. 2002).
Disease Transmission: A. fulica distributes in its faeces spores of Phytophthora palmivora in Ghana; P. palmivora is the cause of black pod disease of cacao (Theobroma cacao); the oomycete which also infects black pepper, coconut, papaya and vanilla (Raut & Barker 2002). A. fulica spreads P. colocasiae in taro and P. parasitica in aubergine (Solanum melongena) and tangerine (Citrus reticulata) (Mead 1961 1979a, Turner 1964 1967, Muniappan 1983, Schotman 1989).
Ecosystem Change:Costs to the natural environment may include (Raut & Barker 2002) herbivory; altered nutrient cycling associated with large volumes of plant material that pass through the achatinid gut; adverse effects on indigenous gastropods that may arise through competition; and indirect adverse effects on indigenous gastropods that may arise through control of the snail (eg: biological control with the rosy wolfsnail (Euglandina rosea) or use of chemical pesticides applied against achatinids.
Human nuisance: A. fulica are also a general nuisance when found near human habitations and can be hazardous to drivers, causing cars to skid. Their decaying bodies release a bad odor and the calcium carbonate in their shells neutralises acid soils, altering soil properties and the types of plants that can grow in the soil (Mead 1961).
Human health: In many Asian, Pacific and American societies A. fulica may play a role in the transmission of the metastrongylus causative agents of eosinophilic meningoencephalitis (Angiostrongulus cantonensis and A. costaricensis).
Some island systems appear to be resistant to invasion by A. fulica. The low abundance of A. fulica on some Pacific atolls may be due to the sandy soils and predation by hermit crabs (Coenobita perlatus and Birgus latro) (Schotman 1989, in Raut & Barker 2002). The omnivorous crab Gecarcoidea natalis provides biotic resistance to invasion by A. fulica on Christmas Island (Lake and O’Dowd 1991, in Raut & Barker 2002).
Meyer and Shiels (2009) hypothesise that reduction or eradication of Rattus rattus populations may cause an ecological release of some nonindigenous snail species where these groups coexist. As such, effective restoration for native snails and plants may not be realised after R. rattus removal in forest ecosystems as a consequence of the complex interactions that currently exist among rats, nonindigenous snails, and the rest of the food web.
Preventative Measures: As there is a high risk of Achatina fulica being spread via trade routes there is potential to prevent its spread through international quarantine and surveillance activities. Small incipient populations of A. fulica have been eradicated at various times from California, USA; Florida, USA; Queensland, Australia; Fiji; Samoa; Vanuatu and Wake Island (Abbott 1949, Mead 1961 1979a, Colman 1977 1978, Muniappan 1982, Waterhouse & Norris 1987, Watson 1985, in Raut & Barker 2002). Control costs can range from USD 60 000 dollars for a 7-month procedure, to over USD 700 000 dollars for the eradication in Florida (Muniappan et al. 1986, Smith and Fowler 2003). For the few species in which spontaneous collapse has been repeatedly observed such as A. fulica, the possibility of such an event is warranted as a potential rationale for a do-nothing approach to management (Simberloff & Gibbons 2004).
Physical Control: Collection and destruction of the snails and their eggs has been reported to be effective in Guam, Hawaii, Japan and Sri Lanka, Australia, USA (Peterson 1957c, Mead 1961 1979a, Olson 1973, Colman 1977, in Raut & Barker 2002). Physical barriers that prevent movement of snails include the use of a strip of bare soil around the crop, a fence that consists of a screen of corrugated tin or security wire mesh.
Chemical control: Metaldehyde and/or calcium arsenate were used in early attempts to control A. fulica. A number of new molluscicidal chemicals are now available. The principal toxic effect of metaldehyde is through stimulation of the mucous glands, which cause excessive sliming, leading to death by dehydration; metaldehyde is toxic to slugs and snails both by ingestion and absorption by the ‘foot’ of the mollusk (Prasad et al. 2004). Sodium chloride (common table salt) is an effective dehydrating agent (Prasad et al. 2004). Various molluscicides like metaldehyde are non-selective, thus their use has a chance of endangering the survival of non-target snails, including endemic fauna (Prasad et al. 2004). Please see section 2.1.3 of Barker and Watts (2002) for information on the application of molluscicides.
There is much interest in naturally occurring chemicals as molluscicides. Panigrahi and Raut (1994, in Raut & Barker 2002) have demonstrated that an extract of the fruit of Thevetia peruviana has activity against A. fulica. Prasad and colleagues (2004) found natural softwood cutting fences made of alligator apple (Annona glabra) acted as snail repellents to protect the nursery beds.
Biological Control: rosy wolfsnail (Euglandina rosea) has been introduced throughout much of the introduced range of A. fulica in “biological control programmes” (Mead 1961, Tillier & Clarke 1983, Murray et al., 1988, in Gerlach 2001). The failure of these programmes and the devastating effect that E. rosea has had on many indigenous species is well known (Tillier & Clarke 1983, Clarke, Murray & Johnson 1984, Hadfield 1986, Murray et al. 1988, Cowie 1992, Pearce-Kelly, Clarke & Mace 1994, Coote et al. 1999 2000, in Gerlach 2001). Generalist predators such as E. rosea, Gonaxis quadrilateralis and Platydemus manokwari continue to be dispersed to new areas in misguided attempts to control this invasive gastropod.
Achatina fulica has a remarkably broad range of host plants on which it feeds. Young A. fulica appear to prefer soft textured banana (Musa), bean (Beta vulgaris) and marigold (Tagetes patula). As the snail matures its dietary preferences broaden to include a larger variety of plants, including brinjal (Solanum melongena), cabbage and cauliflower (Brassica oleracea v. capitata and botrytis), lady’s finger (Abelmoschus esculentus), sponge gourd (Luffa cylindrica), pumpkin (Cucurbita pepo), papaya (Carica papaya), cucumber (Cucumis sativus) and peas (Pisum sativum) (Raut & Ghara 1989).
Updates on management information with support from the Overseas Territories Environmental Programme (OTEP) project XOT603, a joint project with the Cayman Islands Government - Department of Environment