They're Like Cicadas

Consider the humble cicada. An insect that spends most of its life underground that goes unnoticed most of the time while it harmlessly feeds on buried detritus. That is until they reach their breeding season. And at this time they suddenly emerge from the soil in the millions and deliver an auditory assault upon the world.

Make your snails like cicadas. They live most of their inactive lives underground in caves or complex subterranean tunnels. Every year a slow but consistent number might emerge from their tunnels for a brief time. Not enough to be a serious threat but still enough to make hunting them a sport. However most hunters and farmers are understandably unwilling to venture deep into the dark, complex labyrinth of tunnels to eliminate every snail within. So when found snail tunnel entrance are usually just sealed shut. This gradual culling keeps their numbers in check but since most remain safely underground hunting will not seriously threaten them with extinction.

Thus most of the time the only danger the snails pose is the occasional tunnel collapse creating a sinkhole on the surface. However, if they are not regularly culled when on the surface their numbers will gradually increase over the years. Then, perhaps to breed, or perhaps because their regular underground food source is depleted, they emerge in mass. Hundreds or thousands of giant snails suddenly appear overnight and rapidly strip the land of anything edible. Farmers cannot kill such a large number quickly enough to protect their crops and famine follows not long after.

Indian Cobras

"Rewards are given to those who meet a giant snail hunting quota."

During the British Raj a bounty was placed on cobras to reduce their population. The cobra population exploded because people were intentionally breeding cobras to collect the bounty! So too your snails.

They probably COULD be hunted to extinction... except for the breeders. You see, Snail-bounties can pay enough to permit it to be a full-time, relatively undangerous job. Provided you can hunt enough snails. How to you guarantee enough snails? Why simple, you breed them yourself! Of course this is Very Much Illegal but when you make "maybe my family will have enough food for the winter maybe we won't" levels of money there's a very strong incentive to skirt the law and get yourself a nice payout. Of course the snails are a bit dangerous so you make sure your creche is in some OTHER town's woods/marsh/cave system whatever. Then when they're hatched/the right size you open the pen, slaughter them all, and collect your bounty! Not your problem if your pen is poorly made and some break out, they'll just eat the OTHER guy's crops!

So you end up with a constant low-level "there might be a plague of giant snails/the odd snail eating my field."

Additionally, snail-killing could be a mark of control. "DUKE SNAILSMITER KEEPS THE SMALLFOLK SAFE!" The herald cries, while Duke Snailsmiter ensures there is a constant flow of snails across his realm so his valiant knights can smite them and earn the continued fealty (and taxes) of the peasantry.

They breed like rabbits

Snails already breed very fast, they lay hundreds of eggs at a time. They are also hermaphroditic so any two can fertilize each other, hence there can be many many clutches. Once a bunch of eggs are laid, the snail doesn't stick around, it moves on. The slime goes away after a few hours so unless you are really really lucky you can't track down the nests. These snails also have a nasty habit of laying eggs in murky water, so you can't tell if you got all the eggs or if you missed some. It's also very hard to find out if a snail has laid eggs in the local pond.

The immortal snail

You said it yourself, they hide in their shells which are tough to break. Their slime is toxic so dogs are naturally afraid of them, hence farmers must make the difficult choice between sleeping and knowing snails may creep in (maybe breaking their fence) every night. Sure you can kill them with fire, but are you really gonna light a fire in the middle of your crop field? You can poison crops and make traps, but not only are there way too many to kill, you have to remember which crops are safe when harvest time rolls around. Medieval peasants worked from sunrise to sunset so if fences were reasonably effective at keeping out the snails they probably wouldn't care enough to go on full-blown extermination campaigns.

If a lone snail does get into a field they can call a bunch of friends to flip it over and pitchfork it, turning it into a delicious meal. This is unlikely to be cost-efficient against a large-scale infestation, but that's what knights are for, right?

[Edit] They would likely grow fairly slowly, but food would be abundant because they would have entire forests to sustain them. The small ones will be practically unnoticeable in dense foliage, while the medium-sized ones might have an instinct to hide in thick forest or in trees to keep away from predators. Once they reach giant size, nothing in their natural habitat can really hunt them, so they lose this instinct and venture out of the forest. It is through these ventures that the snails stumble upon a few virtual paradises, overflowing with food, with only a couple small wolves and hairless apes guarding them.

Even peasants can successfully ward them off with farming tools and some cunning

Peasants do not have the time and resources to invest in a systematic hunt down of the snails, they are too busy working their fields to pay taxes to the nobles, while nobles do not see them as a worth prize for they effort, since there are real wars to be fought which can bring land and fame, not just an empty shell.

They fight the symptoms, so to say, without putting effort of treating the root cause.

Snails migrate from areas people don't live

Natural Habitat of the snails

Vast areas of unfarmable land exist such as marshes, jungle, and swamps. Snails in this story are well adapted to these areas and are decent swimmers similar to how turtles are slow on land but fast in the water.

Hunting them down in their natural habitat exposes humans to the things that hunt snails

Hunting snails down in these areas is almost impossible; partly because they'd have to deal with the things that hunt these giant snails; insects and other large predators. Snails are protected from insects via their toxic skin and large predators via their shell.

Migration via waterways

When snails migrate from these unfarmable / uninhabitable areas (primarily via the ample water routes) they gorge themselves on nutritious crops that humans grow. To make maters worse, these jungles / marshes could be found at upstream locations of human settlements, making "drift snails" a common phenomenon.

Other thoughts

When humans are threatening a snail it will retreat into a nearby water source such as a nearby pond. They might also only leave these ponds on rare occasion, so guarding them is uneconomical.

"They're Like Cicadas"

[Idly ponders Klyis's suggestion]

[Huh!]

How about like locusts instead?

They are normally just ordinary snails.

Small, harmless, a danger only to the odd lettuce leaf.. and so ubiquitous as to be impossible to eradicate even if you tried.. but every once in a while given the right environmental triggers like the grasshoppers that morph into locusts when seasonal variations in climate are just right to trigger it a generation will develop a different morphology than normal and grow to alarming proportions.

They don't have to swarm like locusts if you don't want them to 🤗

Those hit with this transformation might live a season or years depending on your preference.

They reproduce in both forms, if conditions aren't right giants produce normal snail offspring.

Like locusts there may be decades between occurrences when the climate was just right.

[The environmental trigger for this doesn't have to be climate related .. it could be population crowding, over predation, maybe an abundance of a particular plant or crop .. take your pick]

People may not even realise the two types are the same species.

They may not know what triggers it.

why the giant snails can't be hunted to extinction or at least reduced to a manageable level despite extensive hunting?

Because, a medieval-like europe is thinly settled, so one might handwave the possibility of extinction away, by the fact that there aren't enough people around to hunt the snails to extinction.

Case in point: the spread of the plague in Europe.

The linked map shows the spread of the plague, with regions of Europe never being really affected, e.g., what's now modern East Germany, Poland and the northern regions of Slovakia and the Czech Republic.

Thus, thinly populated regions may serve as a reservoir from which The Snails may periodically emerge.

Spreading of the en:Black Death in Europe between 1347 and 1351

Manuscript miniatures, featuring snails.

https://manuscriptminiatures.com/search?tag=2131#results

I am surprised, that no one posted pictures so far. As far as I could see, at least.

All rights belong to their respective holders. I have downloaded and uploaded one image to avoid the potential issue of links ceasing to work in the future.

Reproduction

Stop sniggering, you at the back!

Giant snails sound like they would be extremely vulnerable to predators, even with their tough shells. So, to compensate, they must reproduce very, very quickly. They are hermaphrodites, (like normal snails) and when they mate, both individuals are impregnated. If they can’t find a mate, they impregnate themselves. Then they lay millions of eggs, which they bury just under the surface of the soil, making them virtually impossible to find. The eggs hatch into what appear to be regular-sized snails. And they constantly reproduce.

Furthermore, this makes it very easy for them to adapt to human methods of extermination. Creatures that reproduce like this tend to evolve more quickly; just look up how insects become resistant to pesticides. Poison the snails? If just one in a thousand has a mutation that gives it immunity, then it will pass on that mutation to all its thousands of offspring, who will find that they have a lot more space and resources now that all the non-immune snails are dead. Historically, humans have favoured poisons for exterminating unwanted species, because having to slaughter every animal by hand is way too labour-intensive.

Consider all the thousands of different brands of bug spray, bug zappers and deterrents you can buy at the store. Now consider why you’re still having to swat dozens of flies a day in the summer.

Driving the snails to extinction is simply more trouble than it’s worth. Like rats or cockroaches, the best you can do is reduce their numbers, but even this requires constant effort.

Well, speaking generally the reason pests of this sort are difficult to eradicate in the real world comes down to sheer, overwhelming numbers. Say these snails don't have defined breeding grounds: i.e., they mate and lay their eggs indiscriminately in any suitable area. Say they lay hundreds or thousands of eggs at a time, a common evolutionary strategy for creatures whose young are easy prey. Say they begin breeding when they are still comparatively small, even though they continue growing across their life span, so that killing the huge ones doesn't really impact their breeding population. Under those conditions it would be virtually impossible to eradicate them even with modern technology; they are everywhere and nowhere, with the next generation of eggs hidden in patches of moss or under the leaves of ferns, ready to fill the void as humans kill off the largest.

Sheesh, this question comes up every 30 or 40 years. Don't you search the Archives before posting?

The snails are a keystone species -- the ecosystem falls apart without them.

About once a century some poorly educated Duke decides to eliminate the snails on their lands. All the snails on their lands, salting all their nests, ripping up all the places where they build their dens. And everything is fine for 5 or 10 years. People rejoice that their crops and herds are safe.

Then the problems start.

Maybe the first thing people notice is that some of the best fishing ponds start choking off with algae.

Then there are no more skycherry and blacknut saplings in the forests -- their seeds only germinate after they have been eaten by a snail.

The forests themselves change -- fallen trees everywhere, drifts of leaf litter, fewer profitable places to hunt mushrooms, fewer patches of berry bushes, more rabbits and fewer deer to hunt.

The healers and mages start coming up short of critical herbs -- who knew that the giant snails were giant pollinators?

And then, the final indignity, the mosquitoes. So many mosquitoes. Giant mosquitoes.

People have even resorted to capturing and importing giant snails from other lands to get rid of the mosquitoes, but it takes 50 years for the snails to breed and adapt to the local lands.

Don't. Let the lessons of the past guide you, and hunt and harass the snails, but never kill ALL of them. We need them.

spontaneous generation

if this is a medieval world that can operate on a medieval understanding of how the world works, rather than having to conform to a modern view, then the snails can arise spontaneously from the earth, as at least some people in some parts of the middle ages believed many small animals did. they can't be hunted to extinction because even if none are left, more will still appear.

Why the giant snails can't be hunted to extinction?

As with many things it comes down to class.

The peasants do hunt the giant snails to extinction on their land.

But the snails breed in the private land and forests of the King and other nobles where the peasants cannot go on pain of death.

Some outlaws (in the style of Robin Hood) do poach giant snails in the King's forest, but it's not enough to drive them to extinction. And the nobles care more about oppressing the peasants than exterminating the snails.

Snails are much easier/safer/cheaper to deter than to kill

Rich farmers or lords surround their areas with some kind of equivalent of diatomaceous earth (that works as a boundary to deter real-life smaller snails) or uncomfortable dry powder.

Arrows and spears are the safe way of attacking giant snails, but are quite ineffective in actually killing them (one would say the giant snails are nearly invulnerable to piercing damage). It does make them turn away, though.

In fact, many real-life snails and slugs use what are called Love Darts as part of their courtship ritual. They essentially get impaled by these darts, but they continue along their merry way. This might be analogous to getting a tongue piercing (without asking for it).

Peasants might find that

1. snails will turn away when a few arrows are shot at them, and consider that part of the cost of farming
2. snails might 'accidentally' mistake a significant-sized spear attack as being a Love Dart from another snail, so may initiate courtship (which involves sending out its own Love Dart, and biting - also real-life snail behaviour)

so peasants prefer to deter the giant snails, as few would risk their life getting close enough to properly kill them.

Reproduction is somewhat self-balancing

Most snails and slugs are hermaphrodites, so your hypothetical giant snails may switch reproduction strategies when numbers are low (laying more eggs instead of expending energy on the prowl to mate with male behaviour).

This may mean they essentially, as a population, become introverted for a few years, then come out en-masse again.

And I would second other ideas that they can reproduce a lot, and the eggs may be able to last for a while in hiding - or their young may camouflage as innocuous terrestrial species, before developing their poison and more dangerous behaviour in adulthood.

Secondary functions

The snails do have a positive side, even if they are starving the common people.

• Their shell might be considered a super-material to make shields/armour from.
• Maybe their slime allows rare herbs to grow in their wake.
• They could expertly clean up corpses and sick animals.
• They are rumoured to be the weakness of immortals, killing them with a touch.
• They are worth money.

And while the peasants would like no less than exterminating all snails, organized militia/snail hunters will leave small groups of young and unhatched snails because of those reasons (and future profit).

Why would giant snails remain a constant problem for Medieval Europe?

Because they were weaponized! Some communities breed them on purpose, and then release the young snails to the fields and ponds of their enemies. And somehow, everyone is the enemy of someone else. They did not yet fully implemented the change from "An eye for an eye" to "Love thy enemies".

What would be the solution to this?

Well, a solution is to stop breeding them as weapons (or as anything else, for that matter). But things can be kept under some control by installing special anti-snail fences. These fences do not need to be very tall, but they need to be painful for the snails. Many materials can be just laid on the ground in thick stripes: broken glass, broken stones, broken pottery, thorny branches (from roses, forest fruits...), or just splintered plain wood. Even the broken shells of dead snails.

Even if they succeed to get over one fence, the other fences will slow down (and even repel) the invasion. It is better to split bigger plots of land into smaller plots, and then surround each smaller plot by such fences.

Embrace thy enemy

Instead of seeing the snails as enemies, use them as work animals. Even though they are very slow, they have enough power to aid in several activities. Pulling carts is nothing for them. Cleaning the land for agriculture is also possible, even if ploughing dry land is too much for the poor creatures. They can be steered with (literally) the proverbial carrot (or any other vegetable). Even a child could do that safely - as long as they do not touch the critters.

#Were-Snails

The giant snails are actually just regular snails that have been affected by the full moon. Full moons don't affect immature snails, but the process is one way - that is once a mature snail is affected by a full moon it does not revert to normal size the next day (or ever).

A regular sized snail moves about .03mph that's still about 5 miles a week. Snails take anywhere from one to ten weeks to mature depending on the exact variety but that means by the time they are affected by the moon they could be as much as 70 miles from where they spawned, making it nigh impossible to determine.

Finally it's worth noting that while they transform on the full moon, since they stay transformed, the ones that transform in the wilderness may still take days or weeks to arrive at locations where they'll need to be dealt with. As such it may be considered superstition that they appear on full moons, but "learned" people might debate over why it's so hard to find their spawning pools, and why there are never juvenile snails found.

The snails have many uses for people

Since you bring out knights battle them regularly, I assume they treat it as a game or a training exercise, while gaining toxic material for their weapons or catapult and trebuchet ammunition. The shell could also be worked into plate armor, as a few people have also said. Maybe these giant snails are also a prized delicacy, like escargot, if theres a way to remove or neutralize the toxin. Agriculturally, the snail’s slime could be used as a pesticide, or their corpses as fertilizer.

They might be used to produce some exotic paint or dye color, for the nobility’s fabrics, like royal purple from sea snails as a similar example.

In summary, there are a lot of hypothetical reasons these giant snails are keep around rather than driven to extinction. In addition, they might also breed in the forest where the king only allows the peasant to hunt small game, so the wider population can’t do anything about them, unless the snails come onto their (lord’s) land.

They lay from 1000 to 10000 eggs once every 3 weeks. Their original habitat was and still is in remote areas near or on the mountains. There they have plenty of vegetation to feed of. They develop and grow very fast and their lifespan is only a year or two (their genetic conditions are the cause of fast growing and the short lifespan).

Their natural predator was the dragon (it also lived at the mountains). Other predators stay away from the snails because they are poisonous but also because of their mucus which is also poisonous, very slippery and have a bad odor. The dragons were immune to their poison, and they like to hunt them by throwing fire at them. But the dragons were hunted down and killed by the knights. There is a rumor that someone saw some dragons still alive, but it's just a rumor. But it would be nice if a valiant knight would go for a quest to find at least some dragon eggs.

Humans usually stay away from them because of their mucus. So they don't venture into their layers to destroy them especially because of the very bad odor.

So the lack of natural predators caused the population to grow out of control. Therefore they move down more often from the mountains to the villages.