Land slugs are terrestrial gastropod mollusks, essentially snails without a large, external coiled shell. Slugs play a dual role in the ecosystem, acting primarily as decomposers that recycle decaying plant matter, fungi, and carrion back into the soil. However, their broad diet and ability to consume living vegetation also establish them as common agricultural and garden pests.
The Physiological Imperative: Why Slugs Need Moisture
The primary driver of slug behavior is the need for moisture, as their bodies are highly susceptible to desiccation. Lacking the protective, watertight shell of their snail ancestors, the slug’s soft tissues consist mostly of water, which evaporates rapidly in dry or sunny conditions. This vulnerability forces them to seek cool, damp environments to maintain necessary hydration levels for survival.
Locomotion is also linked to their need for water due to the production of pedal mucus, or slime. This slime acts as both a lubricant and an adhesive, allowing the slug to glide across varied surfaces and climb vertically. Since moving involves continuously secreting this water-based mucus, the slug constantly depletes its internal water reserves, requiring frequent replenishment from a moist environment.
When Slugs Emerge: Optimal Conditions and Timing
Slugs emerge from their daytime hiding spots when external conditions provide the moisture and temperature they require. They are nocturnal, preferring to move and feed during the night when temperatures are lower and humidity is higher, minimizing water loss. Activity generally occurs when the temperature is between 5°C and 20°C, with optimal activity often observed around 17°C.
Emergence is typically triggered by heavy rainfall, especially following a dry period, as this saturates the soil and surface environment. During the day, slugs retreat deep into the soil, under mulch, rocks, debris, or into crevices, where the air is consistently damp. They only venture out in daylight hours if the sky is overcast or the ground is heavily wet.
Life on the Surface: Feeding and Reproduction
Once slugs emerge, their activity focuses on two primary biological functions: feeding and reproduction. As generalist feeders, slugs consume a wide array of organic materials, including decaying plant matter, algae, fungi, and young plant seedlings. They use a rasping organ called a radula—a ribbon-like structure covered in microscopic teeth—to scrape and grind their food.
Slugs are hermaphrodites, meaning each individual possesses both male and female reproductive organs. While they usually mate with another slug to exchange genetic material, some species can self-fertilize if a partner is unavailable. Following mating, slugs lay clusters of small, pearly, gelatinous eggs in protected, moist locations like under wood, in soil crevices, or beneath debris. Eggs hatch in a few weeks or overwinter until favorable, moist conditions return.
Managing Slugs in Residential Areas
Managing slug populations can be achieved through non-toxic, cultural practices that modify the environment. Since slugs seek damp, dark shelter during the day, removing hiding spots such as loose debris, excess mulch, and low-lying boards reduces their population density. Adjusting watering habits is also effective; watering plants early in the morning allows the soil surface to dry out by evening, making it less hospitable for nocturnal slug activity.
Simple barriers and traps offer control without the use of chemical pellets. Abrasive materials like crushed eggshells or diatomaceous earth can be sprinkled around susceptible plants. These materials make movement difficult and dehydrate the slug’s skin, though they lose effectiveness when wet. Setting out shallow containers of beer buried to the rim serves as a fermentation trap. Handpicking them two hours after sunset with a flashlight is a highly effective manual removal method.