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A Snail Tale

While you worry about enough trucks for shipping and whether there is inventory to match sales, the snails are creeping in. They move in silently on plants, in trucks, on airplanes and ships. They are brought in surreptitiously for food, entertainment, and biological control. Unfortunately most of us are unaware of their potential for harm, real disaster in some situations. Exotic snails are getting a toe-hold into the Pacific Northwest and the only thing between them and global domination is a vigilant public.

When do exotic snails move from menu item to menace? Many of us are familiar with escargot and how the brown garden snail, Cantareus aspersus, worked its way into our diet, then into our landscapes. Allegedly brought into San Francisco as a culinary item, this slippery fiend has escaped into the wild, foraging as it goes. Nurseries in Oregon are well aware of the harm caused by just one brown garden snail showing up - quarantine and the associated chemical applications and misery of such status. But there are other snails of which we should all become aware. They can do real harm to nursery stock, grass seed fields, vineyards, tree fruits, small fruits, and grain crops. Worse yet, they can carry diseases and parasites detrimental to native snails and animals as well as people. The rat lung worm, Angiostrongylus cantonensis, which causes eosinophilic meningitis, a potentially lethal human disease, can be transported by snail mucus on leaves or snail meat.

In the last 10 years, there have been approximately 12 thousand mollusk interceptions at US ports, airports, and border crossings. These interceptions have involved 490 terrestrial and freshwater taxa. Interestingly, one of the most common routes of entry for snails is on imported tiles (around 2000 interceptions). Cargo containers on ships are also common means for snails to sail into new locations. The Department of Homeland Security - Customs and Border Patrol spends considerable time inspecting these containers, inside and out, for freeloading snails. There have been 14 exotic snail species found and 121 interceptions during routine inspections at the Port of Portland in the last decade.

Snails are mollusks, more specifically, gastropods. This article is concerned with terrestrial or pulmonate species of snails, mainly in the order Stylommatophora. Much of the concern of alien snail establishment is concentrated on several families of snails, in particular snails which originate from locales with Mediterranean climates. Some of the most problematic species are in the families Helicidae, Hygromiidae, and Succineidae. These snails may easily adapt to the mild conditions presented in the Pacific Northwest. When hot or cold temperatures arrive they withdraw into their shells. They seal the opening with a thin layer of hardened mucus and calcium called an epiphragm. This tactic, termed aestivation during hot weather and hibernation or diapause when induced by short photoperiods and cold, allows them to siesta until conditions are more favorable, namely warm, moist and humid.


The following are some of the snails that APHIS considers of primary concern for introduction:

Brown Garden Snail
Brown garden snail, Cantareus aspersus (formerly Helix aspersa and Cryptomphalus aspersus) a member of the family Helicidae, originates from Britain, western Europe, and along borders of the Mediterranean and Black Seas. There have been introductions of this species into Argentina, the Atlantic Islands, Australia, Chile, Haiti, Mexico, New Zealand, and South Africa. In the United States, it is reported from California north to British Columbia, Canada, in most southeastern states and along the east coast north to New Jersey. This snail is reported to cost the state of California 7-10 million dollars every year. Currently, C. aspersus presents the greatest risk to nurseries given its status as a quarantine pest and its establishment in sites around Oregon and Washington. The brown garden snail feeds on a wide range of host material and can commonly be found climbing into trees and shrubs. Closely related snails of concern are the European apple snail, H. pomatia, and the green snail, H. aperta.

White garden snail
Theba pisana. This is another high risk snail for introduction into the Pacific Northwest. It has established populations in San Diego, California, just stops away for ships visiting west coast ports. The white garden snail, also a Helicid, is thought to be potentially one of the worst foreign snails as an agricultural pest. It is also the most frequently intercepted exotic snail. Theba has remarkable abilities to increase in population once established and can be particularly destructive to ornamental plants and various types of trees. One citrus tree in California was covered with more than 3000 Theba pisana snails. It can move 55 meters in one month. The white garden snail is native to the Mediterranean countries and Great Britain. It generally is found in coastal habitats. It has been introduced into Australia, the Atlantic islands, South Africa, and the United States. California is thought to be the only state with established populations.

Milk Snail
Sometimes called the milk snail, Otala lactea ( formerly Eobania) this snail originates from Spain and South Africa. It has been introduced to SE United States, Bermuda, and Cuba. O. vermiculata, another potential pest has been introduced into Louisiana and Texas.

Vineyard snail
Cernuella virgata, sometimes called the vineyard snail, is a snail in the family Hygromiidae. Like the other Hygromiid snails it tends to aestivate atop various structures, from plants to fence posts. This habit of aestivating in Hygromiid snails, often on grains, causes significant economic losses due to crop contamination. In 1984, barley shipped from South Australia was rejected by quarantine authorities in Chile because it was contaminated with living C. virgata. It is reported that this rejection cost the Australian Barley Board A$1.3 million in compensation payments. The barley grown for malt was downgraded to feed with a cost comparison of A$130/ton down to A$30/ton. This snail will also climb and feed on new growth causing considerable damage to vines, shrubs, and trees. Another species of concern in this genus is C. cisalpina which has established in coastal Virginia to North Carolina. Several other genera of Hygromiid snails are also considered actionable species by APHIS, requiring holding and treating. These include Candidula intersecta, Monacha cartusiana, Trochoidea elegans, and Xerotricha conspurcala.

Giant African snails
The Giant African Snails (GAS), Achatina fulica and A. achatina, are an interesting threat as they are often imported illegally in the pet and shell trades and sometimes used in educational settings. It is illegal to possess these large attractive snails and several school children have been left in tears as government agents repossess their pets. They can be voracious pests in tropical and subtropical areas, can switch host plants readily, and may also carry the parasites, A. cantonensis and A. costaricensis. Recently over 170 of these snails were turned in or seized in an APHIS/Wisconsin Department of Agriculture operation in Wisconsin. More information about these snails can be found at the APHIS website: http://www.aphis.usda.gov/ppq/ep/gas.html


Ambersnails
Succinea spp. and other species in the family Succineidae such as Succinea dominicensis and Calcisuccinea dominicensis have been determined to be "actionable" snails. Their introduction is considered a significant threat to fruit and horticultural crops. One pathway into the US has been from Haiti and the Dominican Republic. S. dominicensis and C. dominicensis increase rapidly in a greenhouse or nursery, resulting in severe losses. They can also become established outdoors under favorable conditions.

Decollate snails
Decollate snails, Rumina decollata, have been reared and released as biological control agents to control brown garden and other snails. They are native to the Mediterranean and have been in the US since the 1820's and in southern California since the 1950s. They are commercially available and have been used rather commonly in citrus orchards in California. They can harm native snails and are also plant feeders themselves. Decollate snails are prohibited from shipment to the Pacific Northwest but have managed to slip in at times.

While exotic snails of are great concern, native snails play a vital role in natural systems including decomposition and are an integral part of the food chain. In the forests of the Netherlands, acid rain removed calcium in runoff, reducing snail populations thereby dropping bird populations. We can learn about and appreciate these native snails. When it comes to alien snails, though, xenophobia might be helpful. An alert citizenry can help secure our borders from an invasion of pillaging hordes of grazing gastropods.


Name that snail
How does one identify a snail? The snail shell is the predominant character used for identification. The size and shape of snails shells can vary widely from elongate to globose to discoidal. Identifiers note whether they are higher than wide or visa versa. The coils may turn in a right handed (dextral) or left handed (sinistral) direction. The number of whorls are often counted and the type of suture between can vary. The opening of the shell is called the aperture and it is surrounded by a lip which can be straight or curved or turned back. The shape of the aperture is used for identification as well. The coloration and sculpturing of the shell is also important.


What to do if you think you might have an exotic snail, would like to turn one in, or would like to help with the Oregon Mollusk Survey?
Contact:
USDA-APHIS-PPQ
6135 NE 80th Ave., Suite A-5
Portland, OR 97218-4033
(503) 326-2814


Acknowledgments: The author would like to thank Mark Hitchcox and David Robinson with USDA-APHIS-PPQ for their generous support, including information and images, for this article.

References:
Airey, W.J., I.F. Henderson, J. A. Pickett, G.C. Scott, J.W. Stephenson, and C.M Woodcock. Novel Chemical Approaches to Mullusc Control. In Henderson pp. 301-307.

Baker, G.H. 1989. Damage, Population Dynamics, Movement and Control of Pest Helicid Snails in Southern Autstralia. In Henderson. pp. 175-185.

Barker, G.M. 2002. Molluscs as Crop Pests. CABI Publishing. Oxon, UK. 468 pp.

Burch, J.B. 1960. Some snails and slugs of quarantine significance to the United States. U.S. Dept. Agr. Res. Ser. 82: 1-70.

Capinera, J.L. 2001. Handbook of Vegetable Pests. Academic Press, San Diego. 729 pp.

Gordon, David George. 1994. Field Guide to the Slug. Western Society of Malacologists, Sasquatch Books, Seattle, WA. 48 pp.

Graveland, J. and R. van der Wal. 1996. Decline in Snail Abundance Due to Soil Acidification Causes Eggshell Defects in Forest Passerines. Oecologia Vol 105, 351-360.

Mead AR. 1971. Helicid land mollusks introduced into North America. The Biologist 53: 104-111.

Quarles, William. 1997. Slugs and Snails in Gardens and Fields. Common Sense Pest Control. Vol. 13, no. 1, 5-15.

 

Website editor:
Robin Rosetta
Page last modified 8/11/04
 

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