Slugs are simply snails (mollusks) without shells. These slimy creatures live in and on the ground and have big appetites for a wide variety of plants found around the home, yards, and gardens. Young seedling plants are eaten as well as mature plants. They frequently cause damage to plants in flower beds and home vegetable gardens. They also commonly feed on turfgrasses, although little damage is observed. It’s just unpleasant to walk on these slimy animals in the morning or at night! Occasionally, they may congregate in large numbers in basements, on walls, doorways, and along walkways, making these areas unsightly. Slugs can be found when the ground thaws in the spring until it freezes in the fall. Wet conditions are ideal for slug development.
Description of Slugs
Probably the best description of a slug is that it is a snail without a shell. Slugs vary in size depending upon the species and, in Ohio, measure from 1/4 to 5 inches in length. They secrete a characteristic slime (mucus) that is left behind as they move. These slime trails are silvery in appearance upon drying and are a common diagnostic character used to identify the presence of slugs. The color of slugs also varies with species, ranging from dark black-brown to orange. Their soft, slimy bodies and extensible eye stalks give slugs their characteristic appearance.
The most common slugs found in Ohio landscapes are the gray garden slug, the leopard slug, and the dusky slug. The gray garden slug is the most common and is generally mottled gray to black in color. It is usually less than 1 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 bright orange.
The eggs appear as perfectly round, gelatinous spheres filled with a watery substance. They range in size from 1/8 to 1/4 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.
The Development of Slugs
All slugs lay eggs. Each species requires a different length of time for the development of its eggs and the maturing of its young. The number of eggs laid at one time by one slug can be as high as 100, but averages about 25. Young adult slugs lay fewer eggs than older ones.
Though slug eggs can be found outdoors during any month of the year, most of them are laid in the spring and early summer. Most species overwinter as adults or nearly mature young. In the spring, eggs are laid in moist areas and the new slugs normally reach maturity by fall. During periods of particularly warm and wet climatic conditions, the rate at which the slugs develop may allow for eggs to be laid in mid-summer, thus making possible a second generation. Mating usually takes place from August until mid-October, and eggs are laid from 30 to 40 days after a successful mating.
Eggs are generally laid on or near the soil surface and are usually deposited in places of concealment, such as underneath mulch, dead leaves, rocks, flowerpots, 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 egg development takes place varies with the species of slug but is in the general range of 32 to 42 degrees Fahrenheit. The minimum temperature allows the eggs to develop if the temperature is maintained for 100 days. At higher temperatures, development is usually completed in ten days to three weeks.
As soon as slugs hatch, they are active and begin to crawl or feed if the temperature and humidity are right. They are mainly nocturnal and remain motionless and concealed until nightfall provides suitable conditions for activity.
The rate of growth of immature slugs depends mostly on the type and amount of food available. Dry conditions usually result in a loss of weight that is regained rapidly when moist conditions return.
In a temperate climate, slugs usually live one year outdoors. In greenhouses, many adult slugs live for more than one year.
Commercial slug control products were first made during the last decade of the 1800s. Home remedies were probably used even earlier. Flat boards, cabbage leaves, rocks, wet newspaper, etc. are sometimes placed in the problem area for slugs to use as shelter. These slugs are then collected and destroyed. Protective barrier rings of coal tar, soot, ash, lime, and other caustic substances were used in the past and are occasionally used today.
Control of slugs is best accomplished by using an Integrated Pest Management (IPM) strategy. Integrated Pest Management relies on more than just pesticide application for control and includes knowledge of the pest’s lifecycle, habitat, feeding times, and feeding locations. An IPM plan is constructed using multiple strategies to control the pest.
Scouting—Identification of slug presence and species through scouting of is a critical component of your IPM plan. If slugs have been an issue in the past, early and regular scouting should be performed so that major damage can be avoided. Slugs are easier to detect earlier in the day when the soil is cool and moist with dew. Scouting can also be done at night with a flashlight when slugs are most active.
Cultural Control: Reducing Favorable Habitat—Since slugs require moist soil in which to lay their eggs and cool, moist, sheltered sites to hide in during the day, try to open the garden and landscape to more sun and air penetration. Often, gardens and plants have been over mulched. Mulch should not be applied thicker than 3 inches, and a uniform layer of 1 inch is desirable. This provides some protection from rapid drying to the plants but does not retain excessive soil moisture. Since slugs often like wilted or decaying plant material, do not place weed remains or fresh grass clippings in the garden. Place these into a composting area that is constantly turned for more rapid compost formation. Likewise, remove plant debris and leaves from the garden or landscape in the fall.
Cultural Control: Trapping—The use of beer, near-beer, or any fermenting food (such as a mixture of sugar, yeast, and water) put in cups in the ground will attract a surprising number of slugs that then fall into the cups and drown. This method is helpful but leaves a lot to be desired. In fact, commercial slug baiting stations can be purchased for the same purpose. Trapping can, however, be used as a scouting tool to determine when slugs are active.
Chemical Control: Using Commercial Slug Baits—Slug baits are probably the most consistent and efficient method of slug control and a critical part of your IPM plan. Several commercially available baits or pellets that contain a molluscicide, a poison which kills snails and slugs, are available. Since these poisons may be toxic to pets, fish, and humans, carefully use the products as directed on their labels. Many of these baits readily degrade if there is frequent irrigation or rainfall, so reapplications may be necessary to satisfactorily reduce slug populations. Both organic and non-organic slug-bait pesticide options are available and effective.
Timing Pesticide Applications—Make sure to apply slug bait at times that will maximize its impact. For example, apply when plants are young to minimize feeding damage to tender seedlings. Consider a fall application of slug bait to decrease the number of overwintering adults or to catch a second yearly population if environmental conditions facilitated a second generation.
NOTE: This publication contains pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide. It is always the pesticide applicator's responsibility, by law, to read, understand, and follow all current label directions for the specific pesticide being used. The label is the law. Due to constantly changing labels and product registrations, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned. The author and Ohio State University Extension assume no liability resulting from the use of these recommendations.
This fact sheet was originally written by David J. Shetlar Ph.D., Professor Emeritus, Department of Entomology, The Ohio State University.