Three Annoying Garden Dwelling Slugs: Old World Invaders of New World and Old World Gardens

Slugs are soft, slimy creatures that patronize gardens with a proclivity for voracious consumption that belies their tiny size.

Slugs are classed in the Phylum Mollusca, which is one of approximately forty phyla (Greek: φῦλον, phylon, "race, stock"), or divisions, in the animal kingdom (Animalia). Mollusks include land and marine invertebrates, such as clams (class Bivalvia), octopi (order Octopoda), oysters (class Bivalvia), squid (order Teuthida), land and sea slugs, and land and sea snails. Within Phylum Mollusca, slugs belong to the class Gastropoda (Greek: γαστήρ, gastér, "stomach" + πούς, poús,
"foot").

As gastropods, slugs -- along with cowries (genus Cypraea) and snails -- achieve mobility through a ventral, or abdominal, muscular foot and see and sense through separate stalks protruding from their heads. The uppermost, longer pair of stalks end in eyes and are known as ocular tentacles. Sensory function is contained in the lower, smaller pair, which are called peduncular tentacles.

The characteristic flap of skin located behind a slug's head is called the mantle. The pneumostome (Greek: πνεῦμα, pneuma, "wind, spirit" + στόμα, stoma, "mouth, opening") is a large respiratory hole in the mantle that opens and closes as a breathing apparatus in slugs.

Depending upon the species, slugs have a length varying from one-fourth of an inch to over 7 inches (0.6 - 17.7 centimeters). Also varying with species, slugs range in color from dark black-brown to orange.

Geographical distribution

Slugs are found throughout the world's temperate and tropical regions.

Habitat: coolness, darkness, moisture

Slugs need to maintain moisture in their environment. Coolness and darkness facilitate moistness.

• Slugs inhabit natural habitats such as forests, grasslands, and river edges.
• They also thrive in habitats, such as urban gardens, garden centers, plant nurseries, and agricultural enterprises, which result from anthropogenic disturbance, that is, resulting from the influence of humans on nature.
• In woodlands, slugs seek shelter under bark, logs, and leafy debris.
• In urban and residential areas, slugs prefer boards or planks, mulch, or garden paraphernalia such as flower pots.

As hermaphrodites, slugs contain both female and male sex organs. Thus, slugs are able to self-fertilize their own eggs or to produce eggs through mating.

Except for some mucus gland pores, all body openings are located near or in the head. In proximity thereforeare the breathing pore (pneumostome), the genital opening, the anus, and the opening of the kidney. Eggs are expelled through the genital opening.

Most eggs are laid between spring and early summer. Females lay an average of 20 to 30 eggs per batch, with number of batches variable across individuals. A grey field slug (Limax maximus) prolifically deposits up to 500 eggs in one year.

Although laid on or near the surface of the soil, eggs preferentially are concealed by such covers as boards, leaves, mulch, or rocks.

Moistness, which is necessary for slugs, is also imperative for their eggs. Filled with watery material, slug eggs, which are round and gelatinous, have diameters of one-eighth to one-fourth of an inch (0.3 to 0.6 centimeters). While often mirroring the colors of their surroundings on their colorless surface, eggs sometimes become beclouded as hatching nears.

Waking hours: nightly forays

Hatchlings are immediately mobile, but remain concealed during the day. Slugs are nocturnal and become active only between dusk and dawn.

Lifespan

The lifespan, which generally ranges from one to six years, varies according to species and according to habitat. Life cycles may be annual, confined to one year, or biennial, stretching over two years, or even plurennial, lasting beyond two years.

Also, an outdoor slug faces more hazards from predators than a greenhouse slug.

Iridescent paths

As slugs move, they secrete slime, which protects its sensitive ventral, or belly, area from harm by coating its path. These silvery, iridescent trails also guide slugs back and forth between favored food sources in their nightly forays. Additionally, many potential predators are disinclined to struggle with slippery prey that also have a slimy taste.

Summer dormancy

During dry, hot spells, slugs slather themselves with mucus and then aestivate (Latin: aestivus, "summer-like") by immobilizing themselves through burial in mulch or soil or through attachment to or under rocks and stones.

Synecology (Greek: σύν, syn, “with” + oικoλoγία, ecologia, “house” + “study”) identifies organisms which regularly coexist with slugs in their habitats. Regular cohabitants include natural predators, which represent the carnivorous trophic (Greek: τροφή, trophē, "food, feeding") level in the food chain.

Slugs as prey

Predators that attack slugs -- and sometimes consume them by dusting their sliminess with a coating of dirt -- include:

• amphibians such as frogs (order Anura) and salamanders (order Caudata);
• birds, such as red-winged blackbirds (Agelaius phoeniceus), owls (order Strigiformes), rooks (Corvus frugilegus), seagulls (family Laridae), song thrushes (Turdus philomelos), and starlings (family Sturnidae);
• insects, such as ground beetles (family Carabidae) and devil's coach-horse beetle (Ocypus olens); and
• mammals such as badgers (family Mustelidae), foxes (family Canidae), and hedgehogs (subfamily Erinaceinae).

Slugs as predators

Slugs have rasping mouthparts, which scrape plant tissue and leave gaping, irregular holes in bulbs, flowers, fruits, leaves, and stems.

Although feeding often is focused just beneath the soil surface or at ground level, some slugs are motivated to expand their feeding range by climbing.

New World slugs tend to inhabit woodland environments where their dietary habits are focused on carrion, fungi, lichen, and uncultivated plants.

Contrarily, invasive slugs usually constitute the bane of the agricultural, floricultural, and horticultural environments. Their dietary habits are destructive to flowering and leafy garden plants and to agricultural crops such as artichokes, carrots, celery, corn, cucumbers, leeks, onions, potatoes, tobacco, and tomatoes.

Common species now invading New World and Old World gardens and farmland are the dusky slug, the grey garden slug, and the leopard slug.

Arion subfuscus

Commonly known as dusky slug, this northern and western European native of the Arionidae family haunts forest edges and residential areas but is not enamored of agricultural fields.

Intermediate in size at 1 to 3 inches (2.54 to 7.62 centimeters) in length, dusky slugs vary in color from brown or grey to bright orange.

Deroceras reticulatum

Commonly known as grey garden slug or field slug or milky slug, this Old World slug of the Agriolimacidae family is native to the western Palearctic (Greek: πάλαι, palai, “long ago” + ἄρκτος, arktos, “Ursa Major") region, which encompasses the northern part of the Old World and comprises North Africa, Europe, and Asia. Thoroughly adaptable to any moist environment, grey garden slugs are now located in most parts of the world.

Grey garden slugs may reach a length of almost 2 inches (5 centimeters).

Their creamy or grey bodies are usually marked with dark reticulations, that is, a pattern resembling the interlacing lines of nets.

Unintimidated by heights, grey garden slugs indulge in climbing during their forays.

Limax maximus

Commonly known as leopard slug or great grey slug or giant garden slug, this Old World native of the Limacidae family has colonized the New World as well as Australia and New Zealand.

Leopard slugs reach lengths of just under 8 inches (20 centimeters).

Dark bands on the tail often fragment into spots on their pale brown bodies. Dark bands never form on the mantle, which instead displays marbling or spotting. Their antennae, or tentacles, are red brown.

John William Taylor (1845-1931), Leeds printer, amateur limacologist (Latin: limax, "slug"), and co-founder in 1876 of the Conchological Society of Great Britain and Ireland and the Journal of Conchology, the oldest continuing publication worldwide on molluscs, noted:

"The varieties of Limax maximus are grouped in three series: according to the ground colour of the body; the character of the dark markings; and to accommodate examples in which these two factors exist together in unusually striking combinations."

Nevertheless, he cautioned that his list was by no means exhaustive because an "infinite number of sub-varieties" arises from the association of "various ground tints . . . with any of the different markings . . ." (Monograph, pp. 40-41)

With their keen sense of smell and their determined trekking, slugs track the locations of their desired diet of plants. Gardens serve as open-air markets for slugs. Surely they appreciate finding all their favorite plants in one convenient eatery.

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Ulm-Ringingen, Baden-Württemberg, southwestern Germany: Holger Krisp (Holleday), CC BY 3.0, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Arion-subfuscus-braune-wegschnecke.jpg

"Flipped and relabeled image so that pneumostome is on right side of body.": Billlion, CC BY-SA 3.0, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Slug_parts.png

Lakewood, Jefferson County, north central Colorado: Joseph Berger, Bugwood.org, CC BY 3.0 US, via Insect Images @ https://www.insectimages.org/browse/detail.cfm?imgnum=5386116; CC BY 3.0 US, via Wikimedia Commons @ https://es.wikipedia.org/wiki/Archivo:Deroceras_reticulatum.png

photo taken under awning of unused sandbox: Lokilech, CC BY-SA 3.0, via Wikimedia Commons @ http://en.wikipedia.org/wiki/File:Tigerschnegel_Einablage.jpg

found under stone in garden in Weimar, Thuringia, central Germany: Lipedia, CC BY 3.0, via Wikimedia Commons @ http://commons.wikimedia.org/wiki/File:Slug_eggs_and_baby.jpg

John W. Taylor, Monograph of the Land & Freshwater Mollusca of the British isles (1907), Figure 56, page 37: Not in copyright, via Internet Archive @ https://archive.org/details/monographoflandf814tayl/page/37/mode/1up; via Biodiversity Heritage Library @ https://www.biodiversitylibrary.org/page/13171826

northern Germany: Soebe, CC BY-SA 3.0, via Wikimedia Commons @ https://en.wikipedia.org/wiki/File:Carabus_auratus_with_prey.jpg

French malacologist Jacques Philippe Raymond Draparnaud (June 3, 1772–1804) gave scientific name to dusky slug in 1805.: Gary Bernon, USDA APHIS, Bugwood.org, CC BY 3.0 US, via Forestry Images @ https://www.forestryimages.org/browse/detail.cfm?imgnum=1192005

Lakewood, Jefferson County, north central Colorado: Joseph Berger, Bugwood.org, CC BY 3.0 US, via Forestry Images @ https://www.forestryimages.org/browse/detail.cfm?imgnum=5386118

Carl Linnaeus (May 23, 1707–January 10, 1778) assigned scientific name to leopard slug in 1758.: Noj Han (nojhan), CC BY-SA 2.0, via Flickr @ https://www.flickr.com/photos/nojhan/2202913365/

John W. Taylor, Monograph of the Land & Freshwater Mollusca of the British isles (1907), Plate VI, opposite page 46: Not in copyright, via Internet Archive @ https://archive.org/stream/monographoflandf814tayl#page/n84/mode/1up; Not in copyright, via Biodiversity Heritage Library @ https://www.biodiversitylibrary.org/page/13171836

November 17, 2010-April 18, 2011 exhibit, MoMA Lobby, 53rd Street exit: Charles16e, CC BY 2.0, via Flickr @ https://www.flickr.com/photos/carlmikoy/5527925880/

John W. Taylor, Monograph of the Land & Freshwater Mollusca of the British isles (1907), Figure 57, page 37: Not in copyright, via Internet Archive; Not in copyright, via Biodiversity Heritage Library @ https://www.biodiversitylibrary.org/page/13171826

Die Gartenlaube, 1897, no. 27, page 449: Public Domain, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Die_Gartenlaube_(1897)_b_449.jpg; via Wikisource @ https://de.wikisource.org/wiki/Die_Gartenlaube_(1897)/Heft_27

Adams, Lionel Ernest. The Collector's Manual of British Land and Freshwater Shells. Containing Figures and Descriptions of Every Species, An Account of Their Habits and Localities, Hints on Preserving and Arranging, Etc.; the Names and Descriptions of All the Varieties and Synoptical Tables Showing the Differences of Species Hard to Identify. Illustrated by Gerald W. Adams and the Author. London: George Bell and Sons, 1884.

• Available via Biodiversity Heritage Library at: http://www.biodiversitylibrary.org/item/90699

Taylor, John W. Monograph of the Land & Freshwater Mollusca of the British Isles. Testacellidae. Limacidae. Arionidae. With the Assistance of W. Denison Roebuck, F.L.S., the Late Charles Ashford, and Other Well-Known Conchologists. Parts 8-14. Leeds: Taylor Brothers, Publishers, 1907.

• Available via Biodiversity Heritage Library at: https://www.biodiversitylibrary.org/page/13171762
• Available via Internet Archive at: http://archive.org/details/monographoflandf814tayl

Thomas, Anna K., Rory J. McDonnell, Timothy D. Paine, and James D. Harwood. A Field Guide to the Slugs of Kentucky. Publication SR-103. Lexington KY: College of Agriculture, 2010