Slugging It Out
By Kate Rogers Gessert
Illustration By Hal Mayforth
Horticulture 8-1988

I garden in slug country. Here in the Pacific Northwest, every gardening family has its favorite slug story. People who are otherwise gentle and mild mannered will describe-at first with embarrassment, then with increasing relish-their favorite means of slug extermination. They stab slugs with shish-kebob skewers and dissolve them in bags of salt. They drown them in beer. They offer boards and grapefruit rinds for the slugs to hide beneath and overturn these by day, stamping on the slugs or cutting them in half with trowels and clippers. Which of these slime thirsty methods should you use? Some, background on the lives of slugs will help you understand how to deal with these all too common garden pests.


Slugs are especially plentiful in regions with mild winters and long, damp growing seasons. There are many different species of various sizes (one-quarter to six inches) and colors (gray, brown, ocher, black, and spotted). Most of the species that attack garden plants are not native to North America but have been accidentally imported from Europe.


Like snails, clams, and oysters, slugs are mollusks, but they lack shells. To protect their vulnerable bodies from sun and air, they forage at night and on wet, gray days, spending sunny daylight hours in the shelter of garden debris, rocks, boards, and soil crevices. Well-kept gardens generally have fewer slugs, but many hideouts remain: mulches, dense groundcovers, compost piles, undersides of pots and planters. A slug will travel up to 100 feet for a meal, gliding along on its "stomach-foot," which secretes a sticky mucus that smooths irs path. The next day you can trace where they've been by following the silvery trail they've left behind.


Slugs are hermaphrodites. They take part in elaborate courtship rituals and fertilize each other's eggs. About a month after mating, they lay clusters of small, pearllike eggs in the same moist, protected places in which they hide. An average cluster consists of 20 or 30 eggs, with older, larger slugs laying more. In northern temperate climates the major mating season for most slug species begins in August and continues until freezing weather. During a long autumn slugs can manage three or more egg layings. The eggs hatch within a month of laying if weather conditions are warm and moist; they take longer if the weather is hot and dry, or too cold. Many slugs overwinter as eggs, emerge in warm spring weather, and begin feeding voraciously. In milder climates and in greenhouses, slugs feed and lay eggs year round.


Slugs eat with a tongue-like structure called a radula that is covered with thousands of tiny barbs. They use this radula to macerate plant tissue before drawing it inside. With it they

can devour whole rows of seedlings; they also eat rounded holes in plant leaves, crowns, and roots. (There are several species that specialize in eating underground plant parts).


In my garden I grow a wide variety of flowers, vegetables, herbs, and fruits. I have found that slugs prefer buds of crocuses, Iris reticulata, and daffodils; spring shoots of hostas, delphiniums, dahlias, and pansies; and seedlings of spinach, lettuce, beans, and the entire cabbage family. Given a chance they eat ripe strawberries and tomatoes, carrots and potatoes.


Fortunately, there are hundreds of plants that slugs do not eat, and some of their favorites become less palatable once they grow larger, their leaves tougher. Best of all, slugs become dormant in hot, dry weather, although they revive enthusiastically in fall, in time to mow down the fall crop of seedlings.


A number of slug poisons exist in the form of pellets, granules,, and sprays. The active ingredients are usually metaldehyde or mesurol. Metaldehyde slug bait, especially in pellet form, is attractive and poisonous not only to slugs but also to cats, dogs, and small children. Wild birds and mammals that eat bait or poisoned slugs may also die. Enclosing the bait so it is inaccessible to all but slugs is an important safety measure, as is picking up poisoned slugs to keep birds from eating them.


Metaldehyde also comes in a liquid form called Deadline, which is squeezed out on the soil in small gray blobs. These blobs are thought to be less attractive to animals or children than pellets and therefore less dangerous. Deadline also has the advantage of being rain-resistant (the pellets are not). Metaldehyde is sometimes sold in combination with carbaryl in order to kill more pests at once. Mesurol granules are much more poisonous than metaldehyde (to both slugs and nontarget species); therefore they cannot be used near food plants. The same is true of metaldehyde-sodium fluosilicate-fluorine meal, a bait designed to kill slugs, cutworms, and strawberry-root weevils.


Because of the considerable toxicity of these products I don't use poison slug bait in my garden. There are too many children, pets, and visiting birds I wish to protect. As a result, I have experimented with many of the diverse and creative nonchemical options available.


One very straightforward way of killing slugs is to go out on spring and fall evenings with a flashlight and hunt them down as they dine. Dropping the slugs in a bag of salt or squashing them is better than sprinkling salt on them in situ, because the salt is bad for the soil.


You can also set out slug hiding places: boards and shingles, crumpled black plastic, flowerpots, half-grape-fruit rinds, banana peels, cabbage and lettuce leaves. Set these out at night and check under them in the heat of the day.


Perhaps the most famous method of slug control is the beer trap. Card-board milk cartons, plastic cottage-cheese or yogurt containers, and metal coffee cans make excellent traps. Cur holes in the sides of the trap several inches above the bottom, add an inch or two of beer, and sink the trap so the' holes are at soil level. Instead of using beer, some gardeners make their own bait, a mixture of one cup of water, one teaspoon of sugar, and one-fourth teaspoon of baker's yeast. In some experiments this has proved a stronger attractant than beer. Whatever attractant is used, slug traps need to be closed at the top to keep out rain, irrigation water, and thirsty dogs.


Slugs are greatly attracted by the scent. They crawl into it and drown. New slugs come and eat the dead ones, and they drown too. It's disgusting but effective. Every four or five days, dump everything out and add more attractant.


Some gardeners try to lure slugs away from succulent sprouts with nearby offerings of decaying plants. I find that a thick layer of mulch can be rolled back in the spring to expose large numbers of slugs.


Many gardeners believe slugs will be reluctant to drag their tender bodies across rough-textured substances. They surround their gardens with wood chips, hardwood sawdust, sharp builder's sand, cinders, and diatomaceous earth. However, slugs easily can cross these obstacles-and even the edge of a razor blade-their bodies protected by a layer of mucus.


The newest barrier, and one that does work, is copper slug fencing. These are three-inch-wide, flexible strips of lightweight copper slit half-way through like a coarse comb. These strips can be wrapped around containers and tree trunks or set in the soil around plants and garden beds with the slit tabs bent outward at right angles. The copper interacts with the slug's slime and apparently shocks their bodies when they touch it. Also, the bent-out tabs may be creating an angle that's difficult to crawl around. The copper strips cost about $10 for a 20-foot roll, but they last for many years.


A garden with copper battlements may look odd, but it's a better place than one full of poison slug bait and more enjoyable than hand-to-slug combat. The copper fencing is soft enough that its projecting tabs crumple harmlessly if people or animals run into them.


Natural predators of slugs include starlings, toads, frogs, snakes, skunks, opossums, ground beetles, black rove beetles, and firefly larvae. Chickens, especially bantams, are good at cleaning slug eggs out of unplanted areas. Ducks are quite fond of slugs, but they also eat young seedlings. One of my friends keeps her ducks fenced out of the garden, catches the slugs herself, and feeds them to the ducks.


Another friend employs the best predator of all: the school-age child. She hands each of her children a flash light and pays them a penny apiece for small slugs, two cents each for large ones. During damp seasons expenses mount up, but she is very satisfied with the results.