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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
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
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,
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
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.
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.
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
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, 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 Mark Hitchcox, a pest survey specialist, at:
6135 NE 80th Ave., Suite A-5
Portland, OR 97218-4033
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.
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