Note the silvery slime trail left behind by the slug. Slime trails on plants and the ground are a sure sign of slug activity.
Doomed pansies show the scars of slug attack.
With the return of the sun late this week, the misery of a month of chilly moist weather was almost a memory until I examined my dahlias, hostas, and pansies and discovered the treachery levied on my blossoms by hordes of slugs. These non-insect invertebrates are mollusks, members of a large clan of creatures including aquatic species such as clams, octopi, and nudibranchs. Terrestrial mollusks like snails and slugs eat a wide variety of organic matter including vegetables and ornamental plants. On the up-side, slugs are important recyclers of organic matter. On the down-side, slugs are serious crop pests in my garden and worldwide.
Slugs are anatomically unique. Unlike their cousins the snails, slugs wear their shell internally under a flat shield called a mantle that covers their back. Their method of locomotion relies on laying down a trail of proteinaceous mucus produced by specialized glands near the front of a muscular organ called the foot. Undulating contractions of the foot propel the slug forward across a slippery trail of slime. Slime trails along garden paths or on vegetation are one of the important clues used to finger slugs as perpetrators of heinous garden crimes. Their modus operandi is a feeding apparatus called the radula. This ribbon-like tongue is lined with rows of tiny sharp teeth that rasp and scrape soft tissues from leaves, flowers, and fruit leaving behind shredded foliage and ragged holes. We all know that slugs feed mostly at night or during damp, overcast weather.
While gliding along on its slime trail, photoreceptors on the upper tentacles peer about and chemoreceptors on the short lower tentacles gather information on food and progress along the slug trail.
During my slug musings, I often wondered where slugs hang out on bright sunny days. Clearly, they were nowhere to be found at their feeding stations. At the front of my home near my flowerbed there is a large crack where the sidewalk has slipped an inch or two and separated from the doorstep. One sultry spring evening as dusk arrived, I was astonished to see dozens of large and small slugs emerge from beneath my front stoop and glide across the sidewalk into the flower bed where my pansies were fast disappearing. At first light the next morning, the migration reversed, and a stream of slugs flowed back beneath my front stoop. As invertebrates go, slugs sit pretty high up on scales of sensory function and invertebrate IQ. At the front end of the beast, two pairs of tentacles waggle about in seemingly uncoordinated gesticulations. The pair nearer the ground is used to detect odors such as the smell of tasty plants and the scent trail that leads slugs back to their hideout beneath my doorstep each morning. Waving in the air is a second pair of tentacles capable of sensing light, the eyes of a slug as they were. Unlike many of their less intellectual kin, slugs are able to learn and remember certain odors and flavors associated with their food.
Slugs often lay eggs in batches under wood or leaves.
Slugs have several other peculiarities including being hermaphrodites, which is the reality of being both male and female at the same time. While one might think this solves the problem of finding a date on the weekend, it turns out that slugs usually mate with another of their kind in a mutually beneficial and reciprocal ritual a bit too complicated and strangely intimate to be revealed in this episode.
This brings us to what to do with slugs that are eating your gardens to nubbins. Slug baits containing poison can be purchased and sprinkled or placed around your garden to eliminate these ravenous mollusks. Diatomaceous earth or other abrasive products like crushed oyster shells placed around plants may deter slugs from attempting to feed. One avid gardener told me a remedy that called for placing corn meal in a jar and turning the jar on its side to prevent entry of rain. Slugs are said to consume the corn meal which then swells in their gut and apparently clogs them to death. A quick search of the literature failed to turn up a study on this, but I find this particularly dastardly and a compacted slug would surely be curious to see. Slugs need moisture to survive and reducing irrigation and saturated mulch may limit slug populations. Slugs are attracted to fermenting products, and yes, the yeasty odor of beer can be used to lure slugs to an inebriated death in a beer trap. One old timer shared his method of placing a roofing slate on the shady edge of a flower garden and leaving it undisturbed for several days. As slugs sought daytime refuge beneath the cool moist slate, he would simply lift the slate, scrape them off, and annihilate them.
Watch as this large leopard slug cruise over the rocky boarder of my flower bed. The large hole on the side of the slug is called the pneumostome, the breathing port to the single lung inside sluggo.
My personal slug program is a bit more direct. On my way to work each morning, I grab a cup of freshly brewed coffee and greet the slugs as they return from their nocturnal debauchery of my flower bed and head for their redoubt beneath my stoop. With a small sharp stick, I divide their bodies in two and set their remains aside as a feast for the ants and other beneficial bugs in my garden. In the evening, I sit on my front stoop, sometimes with a yeasty beverage, and repeat the process of recycling slugs to the decomposition phase of my garden’s food web. As clever as they are, slugs seem to have no solution to my intervention and apparently lack resistance to this straightforward management approach.
References
Two interesting articles were used in preparation of this episode, “An analysis of associative learning in a terrestrial mollusc” by C. Sahley, J. Rudy and A. Gelperin, and “Analysis of associative learning in the terrestrial mollusc Limax maximus. II. Appetitive learning” by C. Sahley, K. Martin, and A. Gelperin.