Slugs are terrestrial gastropod mollusks that lack the external, coiled shell of their snail relatives. They are found across diverse ecosystems, playing a significant role as part of the decomposer community. Their diet is highly varied and opportunistic, allowing them to exploit a wide range of food sources depending on availability in their environment. This flexibility contributes to the recycling of nutrients within the soil.
Primary Diet of Decaying Matter and Plants
The bulk of a slug’s diet consists of detritus, which is decaying plant matter such as dead leaves and rotting vegetation. This consumption of organic waste material is an important ecological function, helping break down complex substances and returning valuable nutrients to the soil. Slugs are often more important as decomposers than as plant herbivores, consuming materials that would otherwise accumulate.
When feeding on living plants, slugs display a clear preference for young, tender tissues, which are easier to digest. Newly sprouted seedlings, soft leaves, and ripe fruits lying on the ground are common targets. Plants like lettuce, basil, and hostas are frequently damaged because their soft structure provides little resistance.
Slugs do not chew their food; instead, they possess a specialized feeding organ called a radula. The radula is a ribbon-like structure covered in thousands of microscopic, chitinous teeth that the slug uses like a file. This rasping action scrapes away plant tissue.
The result of this scraping is the characteristic damage slugs leave behind: large, irregularly shaped holes with smooth edges. Because slugs are susceptible to desiccation, their feeding activity is almost exclusively nocturnal or occurs during cool, moist conditions. They hide in damp, dark places during the day and emerge at night to feed.
Opportunistic Feeding and Protein Sources
While plant matter forms the foundation of their diet, slugs are omnivores and scavengers that actively seek out non-plant foods for protein. This protein is necessary for metabolic activities, tissue growth, and successful reproduction. For reproductive slugs, a high-protein diet increases the number of egg clutches and the viability of the eggs produced.
Fungi are a common and important part of the slug diet, a behavior known as fungivory. Slugs consume various mushrooms, molds, and slime molds, which are rich sources of nutrients. This feeding habit also helps disperse fungal spores through their digestive tract.
Slugs also scavenge on dead animal matter, or carrion, and consume small, slow-moving invertebrates. They eat earthworm castings, insect eggs, and other slugs, particularly when food resources are scarce. Consuming this animal matter ensures they meet the protein requirements necessary for maintaining their soft bodies and for the production of reproductive structures.
Plant Characteristics Slugs Prefer or Avoid
A slug’s food choice is strongly influenced by the physical and chemical defenses of a plant. They are attracted to plants that are easy to consume and offer a high nutritional reward. Plants with a smooth texture and high water content are preferred because they require less effort to file down with the radula.
High concentrations of nitrogen and sugar also make a plant more palatable, which is why fast-growing seedlings and leaves exposed to high light levels are often targeted. The increased photosynthetic rate in sun-exposed leaves leads to a higher sugar content, making them a sweeter, more desirable food source.
Conversely, certain plant characteristics serve as effective deterrents. Plants with rough, fuzzy, or hairy leaves (pubescence) are avoided because the texture hinders the slug’s movement and feeding. Strong aromatic chemicals, like the volatile oils found in herbs such as mint, rosemary, and lavender, can also repel slugs.
Some plants contain natural toxins that make them unpalatable or poisonous. Other plants may contain high levels of latex or defensive compounds that activate anti-herbivore defenses, causing slugs to lose their preference for the foliage.