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EVEN SCIENTISTS sometimes think that snails and their relations are
a bit of a joke, hardly worthy of study. Yet these animals, members of the
gastropod family, comprise one of the world’s most abundant, varied and
economically important groups of animals. In evolutionary terms, they are
a great success. They have slid over the Earth’s surface for 600 million
years, witnessing the first land vertebrates evolve and the dinosaurs come
and go. We could profit from their protein-rich bodies if we could overcome
our prejudice about eating them. Yet in their undomesticated form, slugs
and snails can be serious pests, damaging crops and spreading parasitic
diseases such as schistosomiasis.

Despite the widespread use of molluscicides, these creatures create
an enormous financial drain on farmers. In Honduras, a half of the land
planted to beans in 1974 was abandoned by 1979, because of damage by slugs.
A single shipment of barley, rejected because it contained land snails,
cost the Australian Barley Board A$1.3 million in compensation payments.
In 1986, nearly half of the maize crop in southwest France was damaged by
slugs.

The economic, and human, impact of schistosomiasis is incalculable.
Caused by a parasitic worm that multiplies in the nine or ten species of
snail that can act as vectors, it makes its human carriers lethargic, anaemic
and unable to work. In the 1960s, schistosomiasis seemed to be almost under
control, but now it is making a comeback.

Today, agriculture often changes the environment in ways that enable
gastropods to thrive, and so compete with people for crops. Native species
are bad enough, but worse still are the depredations of alien species introduced
through human greed or folly. When would-be snail farmers smuggled Pomacea
lineata into Taiwan, they thought it would be like its edible relative in
South America, P. urseus, only bigger. But P. lineata has flesh tasting
of petrol. It escaped into rice paddies in 1980 and is now seriously damaging
rice crops. In Taiwan, the snails have already devastated 3.5 per cent of
the rice paddies.

Amateur aquarists are another source of ‘illegal immigrants’, a by-product
of the international trade in weed for tropical aquaria. Suppliers all over
the world ship exotic aquatic plants to Hong Kong, which acts as the distribution
hub for most aquarists in the West. When hobbyists clean out their tanks,
alien snails make a bid for freedom in a new environment. Via this route,
a freshwater snail from Brazil, Biomphalaria straminea, has reached enormous
densities in Hong Kong. Because this snail can act as a host to the parasitic
worm that causes schistosomiasis, it seems only a matter of time before
the snails introduce the disease to the people of the colony.

Aquarists, or simply travellers who inadvertently carry snails’ eggs
on their shoes or in trouser turn-ups, have also spread a North American
freshwater snail, Lymnaea columella, around the world. This snail can carry
liver fluke, a parasitic helminth worm that particularly affects sheep and
cattle. Since 1927, the snail has swept across Europe, South Africa (1944),
New Zealand (1969) and Australia (1975), displacing native species and changing
the epidemiology of liver fluke across the world.

Enormous densities of snail species such as Theba from southern Europe
are now endemic in South Australia. These animals not only feed on crops
such as cereals, peas and beans, but reach such high densities that they
physically foul the crop. Each wheat plant may carry four or five snails
the size of your thumb; at harvest, the wheat is heavily contaminated by
rapidly decaying snail meat. Farmers try to rake them out before harvest,
but often with little success. Farmers in South Australia judge barley to
be of high quality when it contains no more than five snail shells per litre
of grain. They have to leave substantial areas of crop unharvested, and
struggle with squashed snails that clog the harvesting machinery.

Native gastropods can also be serious pests. Slugs, usually the common
field slug Deroceras reticulatum, a small grey creature, seriously damage
crops throughout the world, especially wheat, potatoes and beans. Researchers
have charted their impact on strawberries in Quebec, soya bean in the US,
beans in Central America and potatoes in Britain.

To date, few people have tried to breed varieties of crops that are
resistant to slugs, but several studies suggest promising approaches. Researchers
at the University of Newcastle, for instance, have found that King Edward
potatoes are most resistant to slugs, apparently because they contain enzymes
that readily oxidise phenolics to bitter-tasting quinones. Unfortunately,
this also means that the potatoes darken readily when cut in air, and so
are disliked by chip manufacturers and many consumers.

On the other hand, studies in Belgium have found that slugs especially
like to eat rape plants, perhaps because they contain little glucosinolate,
a chemical that slugs dislike, compared with many other crops. Other research
has shown that once you’ve got slugs in your field, you’re in trouble. Wheat
that follows rape in a crop rotation is particularly prone to damage by
slugs. Slugs may also damage crops indirectly; their faeces and slime can
spread clover mosaic virus and stem nematode.

So what can we do to control these beasts, given that a new generation
of resistant crops is not on the horizon? Gardeners, working on a small
scale, can take some preventive measures. Slugs often focus their attention
on plants stressed by lack of water or nutrients, or by being transplanted.
An over-tidy gardener makes matters worse by giving them no choice of food
other than pristine but temporarily stressed young plants. Snails are less
of a pest than slugs in most gardens because they consume more rotting than
live plant material. Gardeners might try establishing piles of decomposing
plant material to lure snails away from the garden proper. Lilies planted
next to walls provide ideal snail ‘roosts’; the animals eat the decomposing
older leaves without doing any harm. Other, ‘organic’ approaches to the
control of slugs and snails can also ease the problem .

Most farmers and gardeners trying to kill land-dwelling molluscs turn
to baited slug poisons. These are usually a bran pellet containing the molluscicides
methiocarb (produced by Bayer) or metaldehyde (produced by Lonza). Controversy
rages over which is better. Research that might help to resolve the debate
comes from Bill Bailey and his colleagues at the University of Manchester.
They have developed an acoustic pellet for studying how slugs and snails
feed. They attach a slug pellet to a gramophone pick-up to record the sound
of the creature’s toothed ‘tongue’, or radula, as it rasps the pellet. They
film the bugged slug at the same time.

Both these slug poisons are carbamates, neurotoxins that act on the
junction, or synapse, between neurons. The compounds interfere with the
neurotransmitters that normally convey nerve impulses across the synapse.
Bailey and his colleagues have found that the poisons disrupt the ‘central
pattern generator’, the part of the brain charged with making the mouth
move in a coordinated fashion as the animal feeds. Exposure to the molluscicides
makes these nerve cells fire irregularly and finally stop.

Bailey and his colleagues found that metaldehyde baits cause the slug
to stop feeding earlier than methiocarb, so reducing the chance of it taking
in a lethal dose of poison. But as Peter Newell of Queen Mary College in
London points out, the frequency of the dose is important. Farmers leave
slug pellets out on the field, repeatedly challenging the slug population.
In these circumstances, the apparent advantage of methicarb over metaldehyde
disappears, and both baits are equally effective. Yet neither is perfect:
because both these neurotoxins interfere with feeding behaviour, a snail’s
uptake of the poisons is self-limiting. Furthermore, slugs are not the only
consumers of molluscicides; some evidence suggests that methiocarb is more
toxic to a variety of species including worms, carabid beetles, fish and
mammals.

Researchers at the Institute of Arable Crop Research at Rothamsted are
exploring another compound, based on aluminium, which seemed to avoid some
of these problems. But it may have a limited future as we learn more about
the hazards of aluminium in the environment: some researchers argue that
acid rain kills trees by mobilising aluminium in the soil, while others
suggest that Alzheimer’s disease is more prevalent in areas where levels
of aluminium are high. Another team at Rothamsted has investigated a wide
range of compounds in the search for one that discouraged slugs, but only
the antifeedant (+)fenchone appears to have any prospect of success. Plants
coated with the compound might gain some protection, because it interferes
with a slug’s digestion and makes it stop eating.

To use molluscicides efficiently, we need to know when pests are active,
out in the open and vulnerable. Barbara Dainton, working at Cambridge during
the Second World War, found that the rate at which the temperature changes,
down as well as up, largely governs what snails do. When it becomes rapidly
warmer, snails go out to eat, but when temperature falls rapidly they go
and hide. Investigations at the University of Newcastle have shown that
snails are happiest when it is warm, humid and dark.

Researchers at Long Ashton Research Station near Bristol have been able
to predict when slugs are least likely to damage newly sown crops: they
recommend compacted soil and deep sowing for wheat. Not surprisingly, the
more slugs there are, the more likely the damage; the catch is that it can
be difficult to count them. Infrared video cameras, baiting or trapping
over a defined area all have drawbacks. For instance, the ‘defined area
trap’ isolates a large core of soil in situ; slugs are attracted to the
surface by a damp piece of sacking and then extracted by hand and counted.
But as researchers in Pennsylvania have discovered, when the sacking freezes
solid in winter, it is impossible to extract them.

The slug problem appears intractable, but perhaps we need approaches
more sophisticated than the mere application of poisoned bait. D. Prior
of Northern Arizona University at Flagstaff found that dehydrated slugs
adhere closely to wet substrates. They actively soak up water through specially
adapted areas of the foot. This soaking behaviour seems to rely on the activity
of a particular nerve cell; could the chemical secreted by this neuron,
which seems to make the slugs want to bathe, be mixed with a molluscicide
to formulate a novel control agent? Biological control is another promising
line of attack. At the University of Wales at Cardiff, biologists have used
the carabid beetle Abax to control slugs in enclosed areas: the beetles
are avid consumers of the gastropods. At the University of Avignon, French
researchers suggest that parasitoid sciomyzid flies might act as control
agents. Flies lay their eggs inside slugs; once the eggs hatch into larvae,
they eat their host alive. Biologists in southern Australia are also hoping
to exploit these flies as a means of controlling snails. In New Zealand,
‘mob-stocking’ sheep at very high densities for a few days before sowing
can reduce slug populations by 90 per cent. The sheep eat everything in
sight and trample the ground so thoroughly that few gastropods survive.

But gastropods do more than just damage crops. They can also kill people.
Schistosomiasis, or bilharzia, carried by freshwater snails, is endemic
in the Third World. Methiocarb and methaldehyde, even if safe, are relatively
insoluble and so ineffective against these gastropods. Some 200 million
people now have the disease, and it is spreading as new irrigation projects
encourage the snails. In the Blue Nile Health project in the Sudan, for
instance, researchers from the Danish Bilharzia Laboratory found that 80
per cent of 15-year-olds are infected.

The Chinese made intense efforts to eliminate the amphibious disease-carrying
snail Oncomelania during the Cultural Revolution. But now new dams and irrigation
schemes are planned for the Yangtse, and no one seems to be paying much
attention to the risk of spreading schistosomiasis.

One way to fight the snails is to engineer water systems in the tropics
to be fast-flowing to dislodge them. But many calculations of the flow rates
required are based on data on the detachment of Biomphalaria glabrata, a
still-water snail from South America that is often used in research simply
because it is easy to culture. African snails are more difficult to culture;
but our research at Canterbury has shown that some African species detach
much less rapidly than Biomphalaria. So flow rates built into the systems
might not dislodge the snails. This could mean that, throughout the world,
millions of dollars have been spent on irrigation systems that spread snails.

Research on the control of schistosomiasis continues in only a few places.
Molluscicides made out of local plants, such as Endod extracted from the
palm Phytoalacca, are valuable approaches, but they are not the whole answer
(see ‘Schistosomiasis: the Zimbabwe experience,’ New ÐÓ°ÉÔ­´´, 1 October
1987). Researchers have long searched for some way to suppress the disease
by activating our immune defences against the parasite, but so far with
little success. Only integrated control measures can hold the disease back.

Again, novel solutions may be possible. For example, the ecological
principle of competitive exclusion of one species by another might be a
basis for control. So far, most research on competition among snails has
focused on Helisoma duryi. This is a marvellously fertile, non-vector water
snail from Florida that multiplies furiously in the laboratory. In trials,
Henry Madsen from the Danish Bilharzia Laboratory cultured this snail and
introduced it to several sites in Africa hoping that it would eliminate
local vectors, notably Biomphalaria pfeifferi. Unfortunately, Helisoma performed
poorly in African field conditions; it was outcompeted by snails that have
spent millions of years evolving in those habitats. Nevertheless, such an
approach might work if backed with the application of a safe chemical that
shifted the environment in the non-vector’s favour. Specially bred snails
might then outcompete the vectors.

Such novel approaches remain fanciful through the lack of basic research
on these troublesome gastropods. Although slugs and snails represent an
enormous financial burden for health services and farmers, industrial concerns
are reluctant to fund research into the new methods of control. One reason
is that the developing world, most afflicted by schistosomiasis, does not
have the money to pay for solutions. In the West, farmers buy the molluscicides
available and put up with losses of up to 20 per cent of their crop. As
the technical directors of three major pharmaceutical companies said, independently:
‘We’ve got products . . . why should we look for anything else?’

The meal beneath your feet – why not eat slugs?

* * *

PEOPLE often overlook the productivity of snails and slugs because they
are mostly small and well camouflaged. But put together, they make a substantial
contribution to many ecosystems. For example, in a stream in Canterbury,
Kent, the mass of aquatic snails can reach 1 kilogram per square metre (with
fish amounting to a mere 20 to 200 grams per square metre); the snails have
the second highest biomass in the ecosystem, after bacteria.

Such abundance demonstrates that slugs and snails could be a substantial
source of food. In 1885, V. M. Holt wrote in his book Why Not Eat Insects?:
‘I have known two gardeners who were in the constant habit of picking up
and swallowing any small slugs they happened to see. One gave as his reason
for so doing, that he thought his chest was weak; the other that he liked
them: both honest enough reasons.’

A street market in Canton can have 10 different species of edible mollusc
on display, and the French each eat as much as 5 kilograms of snails per
head a year. Snail meat is an increasingly important source of protein for
the rural poor in Africa, where beef, pork, chicken and fish are often priced
out of reach of the average citizen. Unencumbered by fat, bone or hair,
snails provide more protein, weight for weight, than pork or goat. Some
snails grow quickly too: the African land snail Achatina reaches a kilogram
in weight in 10 months. Aquatic snails too are often edible. In Trinidad
and Guyana, street traders sell the large aquatic gastropod Pomacea urseus.

But to become an important food in the West, we need better ways of
growing snails such as the common garden variety in Britain, Helix aspersa.
The French have pioneered methods of rearing thousands at a time in ‘batch’
cultures where all the snails reach maturity at the same time. Batch methods
now allow 94 per cent of juvenile snails to grow through their most vulnerable
stages into edible adults.

Heliciculture is also becoming an accepted agricultural enterprise in
Britain, where there are more than 120 commercial growers. Should you wish
to keep a snail in your home or school, house it in a flower pot filled
with moist compost, topped with lettuce and covered by an inverted plastic
beer glass, wiped out daily.

Dr Georges Dussart is a senior lecturer in biology at Christ Church
College, Canterbury, Kent, and a member of the Durrell Institute of Conservation
and Ecology at the University of Kent. This article is based in part on
papers presented at the conference Slugs and Snails in World Agriculture,
held at the University of Surrey in April 1989 and organised by the British
Crop Protection Council and the Malacological Society of London.