Inseminated slugs don't lay their clusters of fertilized eggs right away; they store the sperm for later, delaying fertilization until the conditions are propitious for hatching offspring. When times get really tough, some species of slugs can fertilize themselves.
The slugs' versatile reproductive system makes them very hard to control. Most of the slugs that cause farmers and gardeners misery are from Europe; they came in on imported plants. The invaders you're most likely to see in Northern states, especially the cool damp Pacific Northwest, are Limax maximus, the big spotted slug of the dangling sex act; Arion subfuscus, a small brown or yellow spotted creature; and Agriolimax agrestis, the half-inch-long gray slug. Southerners will likely see slugs from South America.
The voracious Europeans and Latin Americans are outcompeting the natives. Like many invasive animals and plants, they adjust with ease to man-made environments like gardens, greenhouses, and fields of crops. Our native banana slug would be happy to stay in the forest chewing on lichen and mushrooms.
Here's an idea for limiting slug populations: sauté them. In a world of less food-producing land and more people, more of us could wind up swallowing our repulsion and consuming slugs. (Like their distant relatives the clams, slugs are rich in vitamin B-12.) Dried snails and slugs, hardened into a sort of gastropod jerky, are already being fed to farmed fish; sea slugs have long been on human menus in China. In Eastern Europe the Limax is prepared much like the French escargot: a vinegar rinse to remove the slime, followed by garlic sauce. (The banana slug isn't one for the table, despite its name. A creature that big, slow, and yellow had better be distasteful if it wants to survive.)
What slugs themselves consume ends up helping us. Along with beetles, earthworms, centipedes, and others, they're part of the great army of decomposers—breaking down plant material, fungi, lichen, and corpses and turning it all back into soil. (Not all slugs participate. Some, including the blind, white ghost slug, suck live earthworms into their grinding teeth. And some, sad to say, eat other slugs.)
Neuroscientists make special use of the slimy creatures, as experimental animals; elements of their simple nervous systems can be seen without a microscope. But is usefulness the point? A slug, like our more obviously attractive fellow creatures, is its own excuse for being. Researcher Alan Gelperin, who has spent decades studying slugs' memory and learning abilities, gives the creatures credit for weighing evidence and making decisions. Using primarily their sense of smell, they manage to answer some of the same questions that concern us primates: what to eat that's nutritious, whom to mate with, and how to avoid an untimely death. Gelperin urges, "Before you step on a slug, or sprinkle the poison, pause and consider the creature's marvelous complexity and place in the scheme of things."
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