Slugs: Interesting Facts, Mucus Slime, and Pest Control
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, and after careful observation and study, a person may decide that slugs 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. There are several pest control methods for slugs that are effective, safe for the environment, and in some cases, humane as well.
The Body of a Slug
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. The tentacles wave gently through the air as the slug moves, sensing the environment as it travels. The tentacles can be regrown if they're lost.
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.
The anus and the genital opening are located underneath the right side of the mantle. These openings aren’t located 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.
Slugs don’t have a single structure that can be called a brain, but they do have ganglia distributed around their body. A ganglion (the singular of ganglia) is often described as a “knot” of nerves. The ganglia of a slug are connected to each other, forming a nerve network.
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 slugs even 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 slug 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.
A slug contains both male and female reproductive organs and is therefore known as a hermaphrodite. During mating, two slugs entwine, exchange sperm, and then separate. The leopard slug has a very unusual and impressive mating ritual. A pair of slugs climb a tree or shrub and then lower themselves towards the ground on a string of mucus. Mating happens in midair before the slugs continue their journey to the ground. Not only do the slugs entwine, but their reproductive organs do, too.
Once sperm have been transferred from one slug to another, the sperm fertilize the eggs inside the slug’s body. A few to several hundred eggs are laid, depending on the species of slug. 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 slug doesn’t guard the eggs once they're laid. The eggs stay dormant until the environment is suitable for them to hatch.
In Bornea, 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 slug. The dart enters the second slug and injects a hormone that increases the chance of successful reproduction.
The Amazing Courtship of Leopard Slugs
Composition and Nature of 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 slug slime, it's easier to let the slime dry and then rub our hands together. The slime will form little balls that are easy to remove.
A Leopard Slug or Spotted Garden Slug
Functions of Slug Slime
The soft slug body 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 dessication. Even so, slugs are usually seen in damp environments rather than dry ones and are most active at night. Many slugs spend a lot of time underground.
Slug slime also plays a vital role in locomotion. The lower surface of a slug’s body contains many slime-secreting glands. The slime released by these glands allows the slug 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 slime 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 slugs different chemicals appear in the slime during the mating season. The chemicals in the slime sometimes attract predator slugs, which is unfortunate for the prey.
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.
Slug are often thought of as animals with dull colours. Brightly coloured 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 colour 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 picks up a banana slug would feel their mouth go numb and might drop the slug unharmed.
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.
Giant Pink Slugs of Australia
Although it’s understandable that farmers and gardeners would want to wage war on the slugs 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 slugs, however, such as in cases where important plants are being killed. There are many methods of slug control, some of which are better than others. Both natural and chemical methods can be useful.
The kindest method to remove slugs and the safest method for the environment, wildlife, pets, and children is to collect the slugs by hand and transport them to another place. This is the method that I use.
Attracting the slugs to a particular area would make the job of collecting them easier. Although I've never tried the technique, 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 colour 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. Some slugs are attracted by the odour of the beer, climb into the tub, and then drown. Tubs should be cleared of dead slugs 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 slug, 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 slug to die.
Red triangle slugs (Triboniophorus graeffei) have only two tentacles instead of four, a variable background colour, and a distinctive mark on their back.
Creating an Abrasive Barrier
Creating a physically repellant 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.
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 is another option for controlling slugs, however. Copper tape or foil wrapped around plant containers, trunks, or other items repels slugs 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.
How to Get Rid of Slugs
Two pesticides are commonly used to kill slugs. Both can be very 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 slugs eat the pellets, which kill them. The poisoned slugs stop feeding, hide, and eventually die. Even though the chemical is considered to be safe for children and pets, I would still keep the bag of iron phosphate pellets out of their reach. Many chemicals are safe in small quantities but not safe in large amounts.
The second chemical pesticide that is commonly used for slugs is metaldehyde. This is much more toxic than iron phosphate. The National Library of Medicine lists some horrible symptoms of metaldehyde poisoning in humans. 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 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 slugs may enable scientists to create new materials and devices with useful applications. While slugs can certainly be pests at times, I think that their behaviour is interesting to observe.
- Slug biology from the University of Alaska Fairbanks
- The 2010 discovery of a slug with love darts from The Guardian
- 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 the Bielzia coerulans from the University of Gottingen
- Slug biology and management from the University of California Integrated Pest Management Program
- More slug facts and removal ideas from the University of Minnesota
- Iron phosphate for slug control from the Florida Department of Agriculture
- Metaldehyde dangers from the National Library of Medicine
Questions & Answers
© 2011 Linda Crampton