Invasive Slugs

Module 2 – Slug Biology

  • This module covers the basics of slug biology and identifies anatomy important for identification

Slug and Snail Biology

Terrestrial gastropods are biologically interesting and complex. This is only a brief summary which should help in evaluating ways to find or control them. Both the Terrestrial Mollusk Tool and OSU have some great pages on slug and snail biology for further research.

Slugs and snails evolved from marine and aquatic mollusks. They have evolved means to control their moisture, through a shell, tough skin, and mucus. Obviously, slugs have dropped the shell, but that change has happened several times in evolutionary history. What this means is that most slugs are more closely related to a snail, sometime an aquatic one, than to another slug. Not all slugs will have the same preferences, or the same susceptibilities to molluscicides. But what is important to all terrestrial gastropods is maintaining moisture.

Most species are also nocturnal. The highest times of slug activity is at early night fall, and late dawn, and there is actually a decrease in activity in the middle of the night. These times happen to often coincide with the highest humidity which allows the slugs and snails more liberty in foraging as the conditions are more favorable.

Slugs and snails are hermaphrodites. This means they have both male and female reproductive organs. Behavior around mating is complex and there is lots of variation between species. Some species can self-fertilize with no problems. Others will only as a last resort and show very little reproductive success when they do, but all have the potential for self reproduction.

Most slugs and snails are generalists when it comes to feeding. They do seem to prefer vegetation and fungi that are beginning to rot or turn, over healthy vibrant plants. Many species also show a preference for yeasty things, like beer or bread dough. One of the most effective slug baits is 2-6 day old bread dough, and many of the commercial bait molluscicides mimic this.


For identification purposes some familiarity of slugs general anatomy is helpful. Important features are listed below.

Mantle – A fleshy, membranous covering of the anterior portion of the body of a mollusc. It secretes the materials that form the shell.

Pneumostome – This is the breathing hole on the right side of the mantle of molluscs. This allows air to pass through to the lung for gas exchange.

Foot – The muscular organ on the undersurface of the body of a mollusc upon which the animal rests or uses to crawl.

Keel – Also known as the carina. This is a longitudinal ridge that runs dorsally along the apex of the tail of the animal.

Continue to Module 3