Slugs are soft-bodied gastropods closely related to snails but lacking an external protective shell. An infestation occurs when environmental conditions allow the population to surge, leading to widespread damage in a garden, especially to tender seedlings and leafy greens. Understanding the three elements—moisture, shelter, and food—that facilitate this population boom is the first step in managing them.
Environmental Requirements for Survival
The lack of a shell means a slug’s survival depends entirely on maintaining a high level of body moisture, making humidity and water availability the most significant factors driving an infestation. Slugs produce a layer of protective mucus, but this process rapidly depletes body water, forcing them to seek out damp conditions continuously. They thrive in environments with high soil moisture and humidity, often becoming most active during the spring and fall when mild, damp weather is prevalent.
The ideal temperature for maximum activity and reproduction for many common slug species, such as the grey field slug, is around 17°C (63°F). Activity continues in a wider range, generally from 5°C to 20°C (41°F to 68°F), but prolonged hot, dry weather forces them into a dormant state to avoid drying out. Extended periods of rain or heavy irrigation directly correlate with population increases, as the perpetually damp soil and air allow slugs to feed and reproduce without interruption. A single slug can lay up to 500 gelatinous eggs in moist soil crevices over a year, which hatch quickly in warm, damp conditions.
Factors That Provide Ideal Shelter
While ambient moisture is necessary for survival, the presence of physical structures that provide daytime refuge is what allows a slug population to persist and multiply. Since slugs are largely nocturnal, they must retreat during the day to avoid the sun and heat that would quickly dry out their soft bodies. These daytime havens must remain cool and damp, effectively trapping the moisture slugs require.
Common garden practices often inadvertently create this perfect shelter, such as the heavy application of organic mulches like wood chips, straw, or grass clippings. These materials act like a sponge, insulating the soil and maintaining a consistently moist, dark environment directly over the soil surface. Other structural elements like loose boards, stones, large plant debris, and dense ground covers offer similar protection, allowing slugs to rest undisturbed. Even poorly ventilated cold frames or containers can trap humidity and create a localized microclimate favorable to slug habitation.
Primary Food and Attractant Sources
The final component necessary for an infestation is a constant, palatable food supply that sustains a large population and keeps them concentrated in the garden. Slugs are not picky eaters, but they are particularly drawn to plants with soft, tender tissues, which are easily consumed using their file-like mouthpart, the radula. Seedlings and young transplants are especially vulnerable, as are leafy vegetables like lettuce, basil, and cabbage, and ornamentals with delicate foliage, such as hostas.
Beyond living plants, slugs are also major consumers of decaying organic matter, making compost piles, rotting leaves, and crop residues highly attractive food sources. This generalist feeding habit means that a messy, debris-filled garden provides a constant buffet. Slugs are strongly attracted to fermenting materials, particularly the smell of yeast. This attraction is why simple traps using beer or a yeast-and-water mixture are effective, as the odor draws slugs into the area.