1. What Do Slugs Eat in the Wild, and When Are They Active?
Slugs hide under stones or garden mulch during the day and are mostly active at night. They are vulnerable to high temperatures and dryness [2]. This may be why they are hard to spot in gardens during the day.
Because they are treated as agricultural pests, people often think slugs only eat vegetables. However, a document from Yamaguchi Prefecture describes slugs as omnivores that eat fresh plants, fallen leaves, and animal matter [2]. Another source notes that in the wild, slugs often eat fallen leaves and have a simple diet [1].
The Iberian slug, an invasive species, eats vegetables, citrus fruits, and decaying wood. It also preys on clusters of moth eggs. Its origin is the Iberian Peninsula, and it likely entered Japan in the late 1950s, though the exact year is unknown [3]. Is there a secret to their mouth structure that allows them to eat such a variety of foods?
Seeing slugs raid gardens at night is not the whole story of their diet. When observing them, remember they are part of a larger ecosystem.
2. How Does a Slug's Radula, Its Toothed Tongue, Help It Eat?
Slugs have a radula in their mouth, which is a tongue-like structure covered in tiny teeth. The Kagoshima Prefectural Museum explains that slugs use this to scrape food rather than chew it [4]. The eating method is more like scraping off material than biting.
The same source describes the Japanese species Yamana-me-kuji, which grows over 15 cm (about 6 inches) long when stretched out. It is nocturnal and eats plants, mushrooms, and animal carcasses, making it an omnivore. In captivity, they reportedly enjoy carrots [4]. Since the number of teeth varies greatly by species, this article simply describes them as having rows of file-like teeth.
So, what happens to the food after it passes through the slug’s body? Let us look at what happens with mushrooms in a specific study.
3. What Do Slug Droppings Show About the Fungi Slugs Eat?
A study on a species of forest slug found DNA from basidiomycetes (common mushrooms) and ascomycetes in its droppings. This shows they eat a wide variety of fungi. The study reported about 100 million spores per dropping, 100–200 times the number in earlier fly studies [5]. Note that this count is per instance, not per day.
Researchers tested germination on four types of mushrooms. Spores from Pleurotus and honey fungus retained their ability to germinate after passing through the slug’s digestive tract. Spores from Hohenbuehelia germinated faster than the control group [5]. This means some spores can still sprout even after being eaten.
These results suggest slugs may play a role in transporting mushroom spores. However, this is based on one study of one species, so the full extent of their role in the entire forest is not yet known. By the way, fallen leaves and dead wood are hard materials. Is the slug itself breaking them down?
4. Which Gut Bacteria Help Slugs Break Down Hard Plant Matter?
The European slug *Arion ater* eats living plants, rotting plants, animal carcasses, and fungi. A study of the genes in its gut bacteria found over 3,383 genes associated with enzymes involved in cellulose and lignin breakdown [6].
It is important to note that these 3,383 genes do not belong to the slug itself. They belong to the community of bacteria living in the slug’s gut. This suggests that slugs have bacteria inside them that help break down hard plant material. The ability to eat tough fallen leaves may partly depend on these gut bacteria.
So far, we have looked at slugs as the ones doing the eating. But are there animals that eat slugs?
5. Which Slugs Eat Other Animals, and Which Are Eaten by Leeches?
Some slugs specialize in carnivorous diets. The *Testacella* slug, which has a shell, is said to catch earthworms deep in the soil [7]. This refers to a group of slugs native to Europe, not Japanese slugs.
In Japan, the invasive spotted slug (10 to 15 cm long) was first confirmed in Ibaraki Prefecture in 2006 and in Hokkaido in 2012. A press release from Hokkaido University reported that photos taken by citizens suggested native large leeches (*Haemopis sanguisuga*) might be preying on these invasive slugs [8]. This is only a possibility shown by photos; it is not yet confirmed that this happens widely.
Slugs appear to be both eaters of leaves and mushrooms, and also prey for other animals. How can we observe these slugs in our daily lives?
6. How Can You Look for Slugs Under Stones Safely?
You can try looking for slugs during the day in a safe place, like a home garden, by gently lifting stones or looking under fallen leaves. Since slugs are weak against heat and dryness, put the stones back immediately if you find them [2]. Do not touch them with bare hands, and wash your hands if you do. If you do not find any, it might just be due to the weather or location.
After spotting a slug, check if there are fallen leaves or mushrooms nearby. This is not proof of what they are eating, but it helps you imagine their wild diet.
If you want to read the research, the full text of the spore study is available on PubMed Central (PMC8855016) [5]. It is an English paper, but looking at the numbers in the figures can help you understand how the count of 100 million spores per dropping was determined.