Nature Notes

by Bob Thomas

Most people think slugs are icky, sticky, gooey critters without value to humans. Well, that description is more or less correct, but they are important components of our ecosystem.

Slugs are mollusks in the Order Gastropoda (stomach walkers). The moisture-loving animals are most active after rains and during the most humid months. They love the warm months and avoid the drying impact of sunlight, so they are most commonly encountered at night, taking refuge during the day in burrows, under pots, or in leaf litter. During extended droughts, they retreat, cover themselves with dense mucus, slow their metabolic activity, and chill. The Florida leatherleaf (Leidyula floridana) defies the rules by also being active on dry nights, at twilight, and even in winter months. My former professor, the late Dr. Harold Harry, reported finding spotted garden slugs (Limax flavus) frozen in ice during the winter in Ruston, Louisiana. When thawed, they seemed unphased by the experience.

In the United States, most of what we consider “our” slugs are introduced via either eggs or adults hitchhiking in potted plants. Of our common slug species, spotted garden slugs and three-banded garden slugs (Lehmannia valentiana) are native to temperate Europe, Florida leatherleafs are from the Caribbean and south Florida, black-velvet leatherleafs (Angustipes ameghini) are from the southeastern Brazil region, and Carolina mantleslugs (Philomycus caroliniana) are locally native. Most of our knowledge on these animals is from the work of the late Dr. Dee Dundee, University of New Orleans.

Slugs’ value is the role they play in breaking down detritus and making nutrients available again in nature. To many humans, this is far outweighed by their destructive impact on plants people value.

It is easy to distinguish slugs from their cousins, the snails, in that they lack an external shell. They do have an internal shell, as is true of many types of mollusks.

They appear to move with no discernible motion. However, they are producing a continuous secretion from their mucous glands on their bellies, and a rippling wave of muscle contractions allow them to slide along their slippery path. Knowing this, you will certainly see shiny, dried mucus pathways around your home, especially at night and in early morning. Slugs find one another by following intercepted slime trails, and predatory snails find their slug prey by doing the same. (N.B., mucus is the secretion of mucous glands).

Slugs are hermaphroditic, meaning each individual contains each set of sex organs. They have internal fertilization with the males having a penis. They mate by encircling one another with each inseminating the other, although some alternate being male or female. The Cupids of our slug community (mantleslugs, members of the genus Philomycus) enhance their reproduction by impaling their mate with a “love dart” (called a gypsobelum). Their thick, curved darts are assumed, based on research on certain snails, to bear a hormone-like substance that increases the viability of the darter’s sperm.

After mating, the “female” finds a moist retreat and lays a clutch of gelatinous eggs. The eggs are round to oval, colorless (they look a bit like a small, clear vitamin gel), and may have calcium particles dispersed throughout the inside. The developing embryos absorb the calcium bits to make their internal shells. As a result, their egg clusters may become clearer as they develop as the embryos consume the calcium.

Slugs have two pairs of tentacles. The upper tentacles are for vision and the lower are for olfaction.

As do snails, slugs feed with the use of a ribbon-like structure called a radula that resembles a wood rasp. Tiny denticles on the radula are used to scrape the surface of plants. The radula grows continuously, so as it wears, new growth keeps them in business

Slugs are eaten by a wide variety of predators. Some small non-venomous snakes (like our local earth snake, Virginia striatula, and brown snake, Storeria dekayi) love to eat them as do some birds, frogs, and beetles. Oddly, it is reported that ducks like them and chickens do not.

When faced with a predator, slugs may contract their bodies to become much smaller and protect their tentacles, and secrete copious amounts of mucus so they are more slippery and distasteful. Who wants a gooey mouthful of slug?

Slug mucus is hygroscopic (absorbing moisture from the air) and primarily prevents desiccation. It also protects the belly as they crawl over rough substrates. It is often said that slugs can safely crawl along a razor blade’s sharp edge, although I’m not sure why they would. Some species produce a strand of mucus and suspend themselves in order to move from one leaf to another or for copulation.

Everyone wants to know how to control slugs. You can try to control habitat, but that is difficult to do in warm, humid south Louisiana. One proactive step is to place shallow dishes of beer in slug-infested areas. They cannot resist a good cerveza, and usually drown, sometimes in huge numbers. I have had success by simply handpicking (actually using forceps) them and placing a jar full of slugs in the garbage can. There are commercial baits, but the non-chemical methods seem to work well.

The next time you see a slug, tell him/her “thanks” for being a detritivore, otherwise we would be up to our ears in slowly decaying plant material.

Thanks to the following slug experts for their help with identifications: Drs. Timothy Pearce, Carnegie Museum of Natural History, and Jim Wiley, Florida Extension Service. There are no checklists of slugs for Louisiana.

Black-velvet leatherleaf slug, Angustipes                                         Three-banded garden slug, Lehmannia
ameghini, was first reported in New Orleans                                    valentiana. Metairie, Louisiana. Our smallest
in 1960 by Dr. Dee Dundee. They are                                              slug can be quite common in gardens.
somewhat common on the Loyola University                                    Photo by Bob Thomas.
New Orleans campus.
Photo by Bob Thomas.

Florida leatherleaf slug, Leidyula floridana.                                     Spotted garden slug, Limax flavus. Metairie,
Metairie, Louisiana. This species was first                                       Louisiana. This is usually the largest garden
found in New Orleans in 1970. It is easy to                                     slug in our area, reaching about five inches
identify as the mantle covers the entire back.                                 in length.
Photo by Bob Thomas.                                                              Photo by Bob Thomas.

Carolina mantleslug, Philomycus caroliniana,                                  Slime trail on a sidewalk, probably by a
with tentacles withdrawn. Fountainbleu State                                 Florida leatherleaf.
Park, Louisiana. This native species is more                                   Photo by Bob Thomas.
common in the forest than in urban areas.
Photo by Bob Thomas.