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Slugs: Interesting Facts, Mucus Slime, and Pest Control

Linda Crampton is a writer and teacher with an honors degree in biology. She loves to study nature and write about living things.

Slugs in Human Lives

Slugs get little respect from most people. They are usually thought of as slimy, very unappealing creatures and annoying garden and agricultural pests. Not all slugs are pests, however. After careful observation and study, a person may decide that they are actually interesting animals. Some people even keep them as pets.

Slugs produce copious amounts of slime. Many people think that the slime is gross, but it has some impressive properties. Researchers are investigating these properties to see if they can be helpful for us.

Should it be necessary to remove slugs from an area in order to protect plants, some type of pest control will be necessary. Removal of the animals can be safe for the environment and humane, depending on how it's done.

External Features of a Slug

Tentacles

A slug has a soft, elongated body. The head has two pairs of tentacles, which can be retracted. The top tentacles are longer. They have eyes at their tips that can detect light, but the eyes can’t form an image. The lower tentacles are sensitive to smell. Both pairs of tentacles are also sensitive to touch. They wave gently through the air as the slug moves, sensing the environment as it travels. The tentacles can be regrown if they're lost.

Mantle

Behind the head is a fleshy lobe known as the mantle. The mantle has an opening called the pneumostome, which leads to the slug’s single lung and is used for breathing. The pneumostome is usually on the right side of the mantle.

Tail and Foot

The part of the body behind the mantle is known as the tail. The bottom of the body is called the foot. Muscle contractions in the foot and its secretion of mucus enable the animal to move. More details about the mechanism of movement are provided below.

Torsion

The animal's anus and genital opening are located underneath the mantle. These openings aren’t found near the rear of the body as they are in many animals because of a phenomenon called torsion. Torsion takes place in the larval stage of the slug's development. The visceral mass of the animal contains its internal organs and is twisted 180 degrees during torsion.

Nervous System

A nerve cell (or neuron) consists of a cell body containing the nucleus and other organelles and a long extension from the cell body called an axon. A typical human neuron is shown in the illustration below. The most common type of neuron in invertebrates is similar to the one shown in the picture but not identical, as I explain below the illustration.

Slugs have ganglia and nerves distributed around their body. A ganglion (the singular of ganglia) is a collection of neuron cell bodies. The axons of the neurons extend from the ganglion, forming a nerve.

The nerve from a ganglion connects to another ganglion within a slug's body. This enables a network to form. A group of six connected ganglia in the head region is sometimes referred to as the brain of the animal. Slugs don't have the large, complex, and highly specialized brain that we do, but their nervous system is efficient and enables them to survive.

A multipolar human neuron (The axon may be much longer in relation to the cell body than shown.)

A multipolar human neuron (The axon may be much longer in relation to the cell body than shown.)

A multipolar neuron has many dendrites, which conduct the nerve impulse towards the cell body, and a single axon, which conducts the impulse away from the cell body. Invertebrate neurons are often unipolar. They have no dendrites, but they do have an axon. The axon forms branches as it approaches the cell body of the next neuron.

Diet and Life

Slugs eat fresh or decaying plant parts and fungi. They may also eat insects, worms, carrion, animal droppings, kitchen scraps, and pet food. Some species eat other slugs.

A slug’s mouth contains a structure called a radula, which is covered by rows of tiny and sharp teeth. The teeth are used for cutting and scraping or for grabbing hold of active prey like worms. There can be up to 27,000 teeth on the radula.

Slugs are most active at night when their surroundings are wet. They may be active in winter in mild climates but hibernate if it gets too cold. Some species die at the end of a season, living for only a few months, but others can live for six or seven years and need one or two years to mature.

Reproduction

A slug contains both male and female reproductive organs and is therefore known as a hermaphrodite. During mating, two animals entwine, exchange sperm, and then separate. The leopard slug has a very unusual and impressive mating ritual. A male and female climb a tree or shrub and then lower themselves towards the ground on a string of mucus. Mating happens in midair before the animals continue their journey to the ground. Not only do the animals entwine, but their reproductive organs do, too. The video below shows the courtship of the slugs.

Once sperm have been transferred from one slug to another, the sperm fertilize the eggs inside the animals' bodies. A few to several hundred eggs are laid, depending on the species. The eggs are generally white or transparent and are deposited in sheltered areas, such as in soil or under leaves or logs. Several batches of eggs may be produced in a year. The adult doesn’t guard the eggs once they're laid. The eggs stay dormant until the environment is suitable for them to hatch.

In Borneo, a green and yellow slug called Ibycus rachelae has been found to use “love darts’ when it's preparing to mate. The darts are needle or harpoon-shaped structures made of calcium carbonate. A slug releases a dart when it contacts another member of its species. The dart enters the second animal and injects a hormone that increases the chance of successful reproduction.

Composition and Nature of the Slime

Slug slime contains water, mucus, and salts. Mucus is made of mucins, which are proteins with attached carbohydrates. They are able to form sticky, moisture-trapping gels when they're added to water. Slug slime is said to be hygroscopic due to its ability to absorb water. It also has the ability to change its consistency when pressure is applied and has elastic properties.

Fresh slug slime is hard to wash off our skin due to its stickiness and hygroscopic nature. Although it may be tempting to immediately reach for soap and water if we're covered with the slime, it's easier to let the material dry and then rub our hands together. The slime will form little balls that are easy to remove.

Functions of Slug Slime

The soft body of a slug dries out quickly if it's not protected. Slugs deal with this dilemma by secreting copious amounts of slime from skin glands, which keeps the skin moist and acts as a barrier against desiccation. Even so, the animals are usually seen in damp environments rather than dry ones and are most active at night. Many slugs spend a lot of time underground.

The slime also plays a vital role in locomotion. The lower surface of a slug’s body contains many slime-secreting glands. The material released by these glands allows the animal to stick to surfaces—even vertical ones—as it moves by a series of muscular waves in the body. This movement is known as adhesive locomotion. A slug sticks part of its body to the ground with its slime, uses its muscles to move its body forward, and then pulls its body away from the adhesion. More slime is released and the process is repeated. The slime also helps to prevent injury when a slug travels over rough surfaces containing stones or sticks.

A trail of glistening slime remains after a slug has passed through an area. The trail contains chemicals that can be detected by other slugs, indicating where the trail-layer has gone. This can be especially useful if a slug wants to find a mate. In some species, different chemicals appear in the slime during the mating season. The chemicals sometimes attract predator slugs, which is unfortunate for the prey.

Research and Potential Applications

Scientists and engineers working in the area of robotics are very interested in the relative roles of slug slime and muscles in controlling movement. The engineers are creating experimental biomimetic robots—ones that operate according to principles discovered in animals—based on their research.

Researchers are also studying the properties of slug slime with the aim of creating a similar material for human use. The slime has the unusual characteristic of changing its consistency as a slug moves over it. Its very adhesive nature allows slugs to move over a wide variety of textures at a wide variety of angles, even while hanging in an inverted position in some cases. The material is inspiring scientists in their effort to create a new type of surgical adhesive.

Banana slugs may be bright yellow, but the species in my area is greenish yellow with dark blotches.

Banana slugs may be bright yellow, but the species in my area is greenish yellow with dark blotches.

Slug are often thought of as animals with dull colors. Brightly colored yellow, pink, and blue slugs exist, however.

Unusual Slugs: Banana and Pink

Three species in the genus Ariolimax are referred to as banana slugs. They are interesting and attractive animals. The animals are bright yellow to greenish yellow in color and sometimes have black blotches. They live in the Pacific Coast region of North America from Alaska to California.

The banana slug is the second largest slug in the world and may reach a length of almost ten inches, although most adults are six to eight inches in length. (The largest slug in the world is Limax cinereoniger, which is found in Europe and may reach a length of nearly twelve inches.) The slime of a banana slug contains an anesthetic. A predator that grabs hold of the animal would feel their mouth go numb and might drop the slug without harming it.

On Mount Kaputar in Australia is the strangest slug so far discovered—a bright, neon pink species known as Triboniophorus aff. graeffei. As far as is known, the species reaches a length of up to eight inches. It's thought to be related to the red triangle slug, which is found elsewhere in Australia, although it's not identical to its relative. The pink slug lives only on an isolated mountain top but is locally abundant.

Pest Control

Although it’s understandable that farmers and gardeners would want to wage war on the animals destroying their plants, many types of slugs do not attack domestic plants. Pest species can create a lot of damage, but unless slugs are interfering with human lives in some way there is no need to kill them.

Sometimes it is necessary to get rid of the animals, however, such as in cases where important plants are being killed. Many methods of control exist, some of which are better than others. Both natural and chemical methods can be useful.

Hand Picking to Get Rid of Slugs

The kindest way to remove slugs and the safest method for the environment, wildlife, pets, and children is to collect the animals by hand and transport them to another place. This is the method that I use. It works well for me. If I had a major problem with the animals I might need to use other control methods, however. I've described some of these methods below. I haven't used them myself, but they seem to be worth trying.

Attracting the slugs to a particular area would make the job of collecting them easier. Placing the rind of a grapefruit on the soil at night reportedly attracts the animals. People also report success when they create a shallow pit covered with a board to keep the inside of the pit humid. Slugs are said to be attracted to the pit because of the moisture and enter it through the gap left for them.

Carpathian blue slugs (Bielzia coerulans) are yellow with as juveniles. They become a beautiful light to dark blue color as adults. The animals are found in the Carpathian Mountains in Eastern Europe.

Drowning and Salting

A popular way to trap and kill slugs is to put a small quantity of beer in a container such an empty yogurt tub and then bury the tub in soil with just the rim exposed. The animals are said to be attracted by the odor of the beer. They may climb into the tub and drown. Tubs should be cleared of dead animals every day. Water containing sugar and yeast is said to have the same effect as beer.

Sprinkling salt on a slug also kills it. The salt draws water out of the animal, causing dehydration. This method of removing slugs is not the best from a gardener's point of view, since it increases the salt content of the soil. In addition, it's almost certainly a very unpleasant way for the animals to die.

Red triangle slugs (Triboniophorus graeffei) have only two tentacles instead of four, a variable background color, and a distinctive mark on their back.

Creating an Abrasive Barrier

Creating a physically repellent and abrasive barrier around plants might help to control slugs. Abrasive materials that may work include coffee grounds, broken egg shells, and diatomaceous earth. A large amount of barrier material arranged in a deep and wide band may be required in order to be effective, however. The University of California Integrated Pest Management Program recommends that the barrier be one inch high and three inches wide.

The caffeine in coffee grounds may act as a neurotoxin for slugs, perhaps increasing the effectiveness of a coffee barrier. Egg shells must be clean and dry before they're used. In addition, the inner membrane of the shell must be removed. Some people say that they have found coffee grounds and egg shells helpful for slug control while others say that these materials are useless. The University of Minnesota says that diatomaceous earth is moderately useful and that it "is most effective when used in dry conditions and has little effect when it absorbs moisture".

There are several problems with any abrasive barrier. The first one is the necessity to create a thick barrier in an attempt to overcome the slug's protective slime. The second is that in damp weather, when slugs are most active, the barrier material may be absorbed by the wet soil. In addition, even if it's not absorbed, the material may become ineffective once it absorbs moisture. Another problem is that some barrier materials may alter the soil's properties. Egg shells raise the soil's pH, for example.

Deroceras reticulatum, the grey field slug,  or the grey garden slug

Deroceras reticulatum, the grey field slug, or the grey garden slug

Creating a Copper Barrier

The best way for a gardener to discover whether a particular abrasive barrier is helpful is to make it while following the recommendations for its creation. There may be another option for controlling slugs, however. Copper tape or foil wrapped around plant containers, trunks, or other items is said to repel the animals and may be a better choice for a barrier than an abrasive material. The exact mechanism of the repulsion isn't known, but it's thought that the copper and components of slug slime interact to give a slug an electric shock.

Chemical Pesticides

Two pesticides are commonly used to kill slugs. Both can be effective. Iron phosphate has very low toxicity for children and pets. It's sometimes classified as nontoxic. In fact, it's used as a human mineral supplement. The EPA (Environmental Protection Agency) classifies iron phosphate as GRAS, or Generally Recognized as Safe.

Iron phosphate pellets containing tasty food for slugs as well as the pesticide are applied as bait around plants. The animals eat the pellets, which kill them. The poisoned animals stop feeding, hide, and eventually die. Even though the chemical is considered to be safe for children and pets, it's important to keep the bag of iron phosphate pellets out of their reach. Many chemicals are safe in small quantities but not in large amounts.

The second chemical pesticide that is commonly used for slugs is metaldehyde. This is much more toxic than iron phosphate and can produce some horrible symptoms in humans, as described in the Extoxnet reference below. The chemical is also very dangerous for dogs and cats. If they eat the pesticide they may die unless they are treated very soon. The chemical is dangerous for wildlife as well.

At one time, metaldehyde was going to be banned from outdoor use in the UK in spring 2020 due to its dangers. Objections to the ban were raised, however. The case was taken to court, the ban was overruled, and the pesticide is still in use.

Limax cinereoniger is the largest slug on the world.

Limax cinereoniger is the largest slug on the world.

The Importance of Slugs

Many slug species play useful roles in the environment. They break down and recycle plant and animal material in the soil. They also provide food for some birds, frogs, snakes, and even mammals such as raccoons. Studying the sticky mucus and movement mechanism of the animals may enable scientists to create new materials and devices with useful applications. While slugs can certainly be pests at times, I think that their behavior is interesting to observe.

References

  • Snail and slug biology and management from the University of California Integrated Pest Management Program
  • The discovery of a slug with love darts from The Guardian (The photo of the animal is the first item in the gallery.)
  • Information about slug slime and surgical glue from NPR (National Public Radio)
  • Facts about banana slug slime from Scientific American
  • A report about pink slugs in Australia from National Geographic
  • Information about Bielzia coerulans from the University of Gottingen
  • More slug facts and pest removal ideas from the University of Minnesota
  • Iron phosphate for slug control from the Florida Department of Agriculture
  • Metaldehyde dangers from the Extoxnet (Extension Toxicology Network from Cornell University and other institutions)
  • Metaldehyde ban from the Government of the UK
  • Metaldehyde slug pellet ban overturned from Farmers Weekly

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

Questions & Answers

Question: Is slug slime harmful to humans in any way?

Answer: According to researchers, slug slime itself is harmless. It’s still a good idea to wash our hands after handling a slug in case the slime has picked up something harmful from the environment. There is one situation in which the slime may be dangerous. In some places, slugs and snails may be infected by a parasite called the rat lungworm (Angiostrongylus costaricensis). The parasite may enter the slime and then enter humans if they ingest the slime, causing illness. You should do some research to see if the rat lungworm has been found where you live.

Question: Is the lungworm dangerous to humans?

Answer: Slugs are one of the intermediate hosts of the rat lungworm, or Angiostrongylus cantonensis. Humans can get infected by the lungworm by eating slugs or snails. The CDC (Centers for Disease Control and Prevention) says that this happens only “under unusual circumstances” and that the infection generally causes only minor problems. It should be noted that the CDC is a United States organization, however. If you don’t live in North America, you should investigate the lungworm situation in your country. The parasite is more common in some parts of the world than in North America and might have different effects. In addition, some individuals may be more susceptible to its effects than others.

The CDC website gives a lot more information about the parasite. I’ve given a link to the site below. It would be a good idea to read the information on the site if you are concerned about the lungworm.

https://www.cdc.gov/parasites/angiostrongylus/gen_...

© 2011 Linda Crampton

Comments

Linda Crampton (author) from British Columbia, Canada on September 09, 2014:

Hi, bravewarrior. Thank you very much for the visit and the comment! I generally leave slugs alone, too. If I do need to move them I pick them up and transport them someone else. I think they are very interesting animals, although I can understand why they could be very annoying for someone growing lots of plants.

Shauna L Bowling from Central Florida on September 09, 2014:

This is fascinating! I never thought I'd want to read about slugs, much less watch a mating ritual, but this article is very educational. I know slugs are good for the garden. When I see them, I just leave them alone.

Linda Crampton (author) from British Columbia, Canada on August 09, 2011:

Thank you for the comment, andrebreynolds.

andrebreynolds on August 09, 2011:

Great.. interesting.

Linda Crampton (author) from British Columbia, Canada on May 31, 2011:

Thank you very much for the comment and ratings, Peggy W.

Peggy Woods from Houston, Texas on May 31, 2011:

Very informative hub about slugs and slug slime. I learned much from you today in reading this hub. The videos (especially the first one) were also good. Rated useful and up.

Linda Crampton (author) from British Columbia, Canada on May 27, 2011:

Thank you for the comment and rating, Prasetio. I hope that you have a nice weekend too!

prasetio30 from malang-indonesia on May 27, 2011:

Very entertaining hub. Thanks for writing and share with us. I'll show this to my student. I learn much from you. Rated up! Have a nice weekend...

Prasetio

Linda Crampton (author) from British Columbia, Canada on May 27, 2011:

Thank you for the comment and vote, Docmo. I agree with you - every creature is interesting in some way!

Mohan Kumar from UK on May 27, 2011:

What a great information filled journey through the world of slugs! Really enjoyed the biological info and the insight into slugdom. There is so much wonder in each little critter. Thanks for sharing. Voted up!

Linda Crampton (author) from British Columbia, Canada on May 24, 2011:

You’re so right, Tina - knowledge of the enemy is important! Although I know that some slugs can be people’s enemies, other slugs play important roles in the environment. I don’t find it hard to like the slug species that leave garden plants alone, and I even find the ones that do attack gardens interesting -- and very annoying at the same time! Thank you very much for the vote.

Christina Lornemark from Sweden on May 24, 2011:

The best way to meet your enemy is to get knowledge about them so I read this with great interest. And even if some slugs are my enemies in the garden I can´t help to admire them. They are still animals that have a purpose. You have done a great job writing a hub about this slimy creatures! And I can even like some of them:)

Voted up, useful

Tina

Linda Crampton (author) from British Columbia, Canada on May 24, 2011:

Thank you very much for the comment, kashmir56. Slime trails are good for slugs but not for us!

Thomas Silvia from Massachusetts on May 24, 2011:

Hi AliciaC, thanks for this very interesting and fascinating hub,i do hate it when they leave their slime trails through my garden.

Linda Crampton (author) from British Columbia, Canada on May 24, 2011:

Hi, Chatkath. I guess slugs have a lot of work to do if they are going to improve their public relations! Thanks for the visit and comment.

Kathy from California on May 24, 2011:

Slugs are constant little slimy visitors at my front step (cat food most likely) but I never have cared for them much and I am a die hard animal lover. (are they an animal?)

This hub is most interesting, lots that I did not know about the little creatures! :-)

Linda Crampton (author) from British Columbia, Canada on May 23, 2011:

I sympathize with your slug problem, Fossillady. Thank you for reading my hub, which was kind of you, considering your local slugs are such a nuisance and you don't really want to think about them!

Kathi from Saugatuck Michigan on May 23, 2011:

Alicia, I have such a slug problem in my landscape. I have discovered there are certain plants they stay away from, while others, they can destroy in no time. I never really wanted to know anymore about them because they are such a nuisance to me, but i read about them anyway. They have some interesting features, i have to admit. Take Care

Linda Crampton (author) from British Columbia, Canada on May 23, 2011:

Hi, b. Malin. I’ve been interested in slugs since I was a child, too. If I find any of them around my garden plants I pick them up and transport them somewhere else. I can’t bring myself to kill them!

b. Malin on May 23, 2011:

I played with "slugs" as a child. This was a Wonderful Hub Alicia and kinda fun to read...and they are quite the little Hero's in the garden as well...Though their Sex Lives leaves a lot to be desired! Ha, Ha.

Linda Crampton (author) from British Columbia, Canada on May 23, 2011:

Thank you very much for the visit, A.A. Zavala.

Linda Crampton (author) from British Columbia, Canada on May 23, 2011:

Hi, Peter. I've always liked slugs too! Thank you for your comment.

Augustine A Zavala from Texas on May 23, 2011:

Fascinating hub, as always. Thank you for sharing.

Peter Dickinson from South East Asia on May 23, 2011:

Always been a slug fan...thank you for your interesting and informatitive article.