Above, left: The great gray slug, also known as the leopard slug because of the spots and streaks on its mantle, at home in Bonnefont garden. These nocturnal garden pests are not normally seen during daylight hours. This specimen was spied early one morning, and posed to have his-her portrait taken (slugs are hermaphrodites). Center: When fully extended, the adult slug can reach an impressive length. Right: A single strategically placed slug trap baited with beer will attract quite a few slugs. Photographs by Corey Eilhardt.
The great gray slug (Limax maximus) is also commonly known as the leopard slug because of the characteristic dark spotting on the mantle which covers the upper part of its body; the lower part of its body, known as the foot, is often streaked or striped. Leopard slugs vary in color from brownish green to gray, with whitish undersides. Adult slugs range from 4 to 8 inches in length.
Native to Europe, Limax maximus was discovered to have been accidentally introduced to the United States in the nineteenth century. It was found in a Philadelphia cellar by George Washington Tryon, Jr., a malacologist (zoologist specializing in the study of mollusks) at the Academy of Natural Sciences. This gastropod is now a common horticultural pest on both the east and west coasts of the United States, and is the largest of the European slugs that has been introduced here.
Bonnefont garden has long been home to a large population of leopard slugs; a night-time film shoot in the spring of 2008 revealed hundreds of slugs on the prowl. Last May, we found the characteristic irregular holes made by the rasping mouthparts of the slug on the leaves of our mandrake (Mandragora officinarum), one of the most prized plants in our collection. (Mandrake is a member of the notoriously poisonous nightshade family, but slugs seem quite immune to the alkaloids contained in the plant. See my post “The Nightshades,” Friday, November 7, 2008.) We resorted to a traditional method of slug control, and set a beer trap near the mandrake: the very large culprit was drowned overnight. Although we’ve had some sporadic damage on the leaves of our rhubarb, collards, and sea kale over the season (slugs have definite plant preferences, and rhubarb and members of the cabbage family are among their favorites), we didn’t notice any marked increase in slug presence until last week. When we found a large number of small holes in the leaves of our clary sage (Salvia sclarea) last week, we were forced to take action.
Limax maximus is a nocturnal feeder. Leopard slugs are solitary and do not congregate; each individual roams the garden or greenhouse at night in search of sustenance, returning early in the morning to whatever dark, damp crevice it calls home. The slimy trails left by slugs may be evident on the garden pavement as well as on the plants themselves. (Here at The Cloisters, the slugs seem to favor the moister recesses of the brick raised beds, but slugs will also shelter under stones, logs, or leaf litter.)
August is mating season for leopard slugs, who have a peculiar and protracted method of propagating themselves while suspended in the air:
A new crop of very small, pale, unmarked slugs will emerge early next spring from eggs produced late this summer. Since the slugs are hermaphrodites, both individuals in a mating pair are capable of producing eggs, which are deposited in masses of one to two hundred. It takes up to two years for newly hatched slugs to reach sexual maturity, and each slug has a lifespan of two and a half to three years.
Last Wednesday evening, we set out five traps, digging shallow holes into the shadiest and moistest areas in the garden, where slugs where most likely to be active, as well as in the vicinity of the damaged plants. We inserted plastic drinking cups into the holes, their rims level with the soil, and filled each cup about three-quarters of the way with stout, enriched with a little extra yeast. (It is the yeast which strongly attracts the slugs, and stale beer is as good for the purpose as fresh.) We found a total of twenty-seven adults and a number of very small immature slugs in the traps the next morning. We emptied the traps, added two more, and repeated the experiment on Thursday evening. Another thirteen adults and many more juveniles were collected the next day.
It’s not likely that we have eliminated all the slugs from the garden, nor is it necessary or desirable to do so. We are trying to achieve a balance and to protect the plants from an unacceptable level of damage, not to eradicate the slugs. While they may be pests as herbivores, leopard slugs are also detrivores, consuming fungi, algae, and decaying matter in the garden, as well as carnivores, preying on other species of slugs. Except in cases where there are very large populations in the garden, they may do as much good as harm (for a discussion of this point, see The Living World of Molluscs website.)
Death by drowning while drunk seems less cruel an end than death by dehydration—another traditional method of control is to salt the slugs, which shrivels them up immediately. Some gardeners are concerned when they find bees or other insects they didn’t intend to lure in their slug traps, but since we put the traps out at the end of the day and retrieve them early in the morning, we’ve caught nothing but slugs.