The Skinny On Slugs

Up until early June we were dry, very dry, and for gardeners that’s not a good thing. Except that when it’s dry one garden pest is nearly absent. But then it began to rain, then rain some more, and within days they showed up on hostas, day lilies, lettuce and a host of other garden goodies. The slugs, those absolutely gross of the grossest garden pests, had returned. And now, under that flagstone, the rotting log and the deceptively protective mulch, the tiny spherical eggs and the slimy elders of these boneless mollusks were dropping in. Those unwanted garden dinner guests and evening marauders are visiting, and if you don’t have a yard of ducks, chickens or a ton of toads, there is a host of products and techniques that can keep these slimies under control.

Slugs are simply snails without shells. In spite of the email I once received (“Should I use malathion or diazanon on my slugs?”), these are not insects, and insecticides should not be used on them, ever! They are classified as gastropods and are more closely related to mussels and clams than aphids or beetles. These slimy creatures live in and on the ground and have voracious appetites for a wide variety of plants found around the home garden. Young seedling plants are eaten as well as mature plants, and if you have a greenhouse or cold frame the damage can be a year-round problem. Occasionally they may congregate in large numbers in basements, well pits, on walls, doorways and along walkways, making these areas mildly unsightly to overbearingly disgusting. These slugs can be found outdoors from the time the ground thaws until it freezes again, and as you may have noticed … wet conditions are prime for their development and propagation.

Slugs vary in size from just a ¼ of an inch to 7 or 8 inches long. They secrete a characteristic slime or mucus which they leave behind as they move around. In fact, it’s the slime that clues us into slug damage, as many gardeners confuse slug damage with caterpillar damage. Another clue is that slugs generally eat from the center of the leaf out, while caterpillars and beetles chew from leaf margins inward. The color of slugs also varies with the species, ranging from dark black-brown to an orange color. Some are also striped, while others are spotted. When the slug is actually found, their soft slimy bodies and extensible eye stalks give the creature its characteristic appearance.

Our most common slugs are the gray garden slug, the leopard slug and the dusky slug. The gray garden slug is the most common and is generally a mottled gray to black in color. It is usually less than an inch long. The leopard slug is the largest, commonly reaching 4 to 5 inches in length. It has characteristic black spots on its upper surface. The dusky slug is intermediate in size, being 1 to 3 inches long, and can range from gray to a bright orange in color.

The eggs of slugs appear as perfectly round gelatinous spheres filled with a watery substance. They range in size from 1/8 to an eighth to a ¼ of an inch in diameter. They are usually colorless, often reflecting the color of their surroundings, but they may become cloudy just before hatching. Baby slugs resemble adults but are smaller and may not be as fully colored.

All slugs lay eggs and each species requires a different length of time for development from egg to slug. Each slug may lay as many as 100 eggs and the older slugs lay more eggs than the younger ones. The eggs are generally laid on or near the soil surface, but are mostly deposited in places of concealment such as underneath mulch, dead leaves, rocks, flower pots, trash and boards. Particularly preferred are spots where the nature of the cover keeps the surroundings relatively cool and moist.

The minimum temperature at which the eggs will develop varies but is usually between 32 and 43 degrees Fahrenheit. At this temperature, though, it can take as long as three and a half months for development, while at higher temperatures it can take as little as 10 days. As soon as the eggs hatch, the slugs begin to crawl and feed. They are mainly nocturnal, as sunlight will dry them out, so during the day they tend to be concealed and motionless.

There are a number of strategies for controlling slugs and even getting rid of them. What most gardeners don’t realize, though, is that there are some natural predators of slugs in most of our gardens, and these include a number of beetles, frogs, toads, possums, skunks, garter snakes and best of all … ducks. One summer when we rescued a half-dozen ducklings whose mother was killed on Montauk Highway. The ducklings spent the summer growing up in our garden, and it was slugless (and just about bugless and weedless) all summer.

Formal slug control recommendations were first made during the last decade of the 19th century. Home remedies were probably used even earlier. Flat boards, cabbage leaves, rocks, pine needles and wet newspaper are sometimes placed in the problem area for slugs to use as a shelter. The slugs are then collected and destroyed. Protective barrier rings of coal tar, soot, ash, lime and other caustic substances were old suggestions, and some are still used today.

Slugs require moist soil in which to lay their eggs, and cool, moist, sheltered sites in order to hide during the day. Obviously opening the garden to more light and air penetration is a prime defense, but not always practical. Often when we find a garden that’s over-mulched we find high slug concentrations and problems. Mulch should not be applied thicker than 3 inches and a uniform layer of 1 inch is generally adequate and desirable. You should also know that slugs are hermaphrodites. One slug can do it all, as they have both male and female reproductive parts, so Noah needed only one slug. Controlling them early in the season is critical, as this reduces egg populations and thus slugs that can reproduce the following spring.

A second strategy is trapping. The use of beer, or any fermenting food (such as a mixture of sugar, yeast and water) put in shallow cups or pie plates buried to ground level will attract slugs that then drown in the brew. While effective, this method still leaves a lot to be desired. In fact, a number of commercial baiting stations or slug bars can be purchased for the same purpose and may make better traps.

Slug baits were once thought to be the most consistent and efficient method of slug control, but they are not without controversy, as they have resulted in some poisonings of wildlife, pets and possibly children when not properly used. Metaldehyde is the most commonly used molluscicide. It’s sold in pellets, flakes and liquids. This product can be used in flower gardens as well as in vegetable gardens. After feeding on it the slugs become immobilized and are either dried by the sun or they simply ooze to death from excessive water loss. Caution needs to be used, though, because if the poison is used during periods of high moisture (as in rain or irrigation) the slugs can often recover. Metaldehyde is best applied when the evening is warm and the forecast for the following day is for dry and sunny weather.

There are at least five other “natural” control methods that may appeal to many gardeners. Copper flashing cut into long thin strips or copper strips purchased in garden centers are available. Slugs and snails will not crawl over the copper barrier, as an electrical charge is created when the slime and the copper meet. Remember, though, that slugs inside the barrier will get stuck inside.

A second barrier method is to use diatomaceous earth, or DE. This crystalline dust made of pulverized diatoms is a deadly irritant to the slugs that they will make every effort to avoid. If they for some reason decide to cross a barrier line of the DE, the slug will simply be irritated to death. However, DE can also be fatal to earthworms, so it should be used only in places where you know there’s a slug problem. A third method is to use one of the new products that contain an iron phosphate (Sluggo, Escar-go and others). In some tests these products have been shown to be more effective than chemical treatments, but they must be reapplied after each rain event.

It also appears that a 2-percent caffeine solution will kill snails and slugs. In fact, according to a USDA research project several years ago in Hilo, Hawaii, this solution may kill more slugs than a standard dose of metaldehyde. The report noted that even “a concentration of 0.01 percent caffeine was associated with partial repellency to large slugs.” Instant coffee has about 0.05 percent caffeine and regular coffee is much stronger. Bran is also said to be an effective method of control. It can be left in small piles or rows and it’s ingested by the slugs. Once inside their bodies, the bran swells and expands and essentially bloats the slugs to death.

The timing of any baiting is critical; baiting is less effective during very hot, very dry or cold times of the year because snails and slugs are less active during these periods. Irrigate before applying a bait to promote slug activity and apply the bait in the late afternoon or evening. Application on a warm, humid evening is ideal. Apply bait in a narrow strip around sprinklers, close to walls and fences or in other moist and protected locations, or scatter it along areas that slugs cross to get from sheltered areas to the garden.

The best news is in the future, though. In Great Britain and many European countries, you can purchase a nematode known as Phasmarhabditis hermaphrodita (Nemaslug). This soil-dwelling nematode enters the slug’s body and reproduces. Over a period of about three weeks, the nema eats the slug from the inside out. It been shown to be as effective on both slugs and snails as most of the chemical treatments, but it’s a bit expensive and needs to be reapplied every four weeks. So far it’s not available in the U.S., but maybe someday. Keep growing.