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. Young seedling plants are eaten as well as mature plants. They frequently cause damage to glasshouse (greenhouse) and garden plants, and may be especially injurious in mushroom houses. Occasionally, they may congregate in large numbers in basements, on walls, doorways, and along walkways, making these areas unsightly. Slugs may be found when the ground thaws in the spring until it freezes in the fall. Wet conditions are ideal for slug development.
Probably the best description of a slug is that it is a snail without a shell. They vary in size depending upon the species and measure from 1/4 to seven inches long. They secrete a characteristic slime (mucus) which they leave behind as they move around. These slime trails are silvery in appearance upon drying and is a common diagnostic character used to identify the presence of slugs. The color of slugs also varies with species, ranging from a dark black-brown to an orange color. When an actual slug is found their soft slimy bodies and extensible eye stalks give the creature its 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 a mottled gray to black in color. It is usually less than one inch long. The leopard slug is the largest, commonly reaching four to five inches in length. It has characteristic black spots on its upper surface. The dusky slug is intermediate in size, being one to three inches long, and can range from a gray to a bright orange in color.
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.
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 may be up to 100, but average 20 to 30. Young adult slugs apparently lay fewer eggs than older ones.
Though slug eggs may be found outdoors during any month of the year, most of the eggs 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 can be laid from 30 to 40 days after a successful mating.
Eggs are generally laid on or near the soil surface, but are usually 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 egg development will take place varies with the species of slug but is in the general range of 32 to 42 degrees F. At the minimum temperature, as long as 100 days may be required for the eggs to develop. 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 which is regained rapidly when moist conditions return.
In a temperate climate, slugs usually live one year outdoors. In greenhouses, many adult slugs may live for more than one year.
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, wet newspaper, etc. are sometimes placed in the problem area for slugs to use a shelter. These slugs are then collected and destroyed. Protective barrier rings of coal tar, soot, ash, lime and other caustic substances were old suggestions and occasionally are used today.
Strategy 1: Cultural Control - Reduce Favorable Habitat - Since slugs require moist soil in which to lay their eggs and cool, moist, sheltered sites in order to hide during the day, try to open up 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 three inches and a uniform layer of one 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 which is constantly turned for more rapid compost formation. Likewise, remove plant remains and leaves in the fall.
Strategy 2: 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 surprise one in the number of slugs that fall into the cups and drown. This method is helpful but still has a lot to be desired. In fact, commercial slug baiting stations can be purchased for the same purpose.
Strategy 3: Commercial Slug Baits - Slug baits are probably the most consistent and efficient method of slug control. Several commercially available baits or pellets are available which contain a molluscicide, a poison which kills snails and slugs. Since these poisons may be toxic to pets, fish and humans, carefully use the products as directed on the labels. For a current listing of slug bait pesticides, please refer to OSU See Bulletin 504.
Revised from the original FactSheet compiled by Richard L. Miller, Extension Entomologist Emeritus and Alan W. Smith, Graduate Extension Research Associate, 1988.
NOTE: Disclaimer - This publication may contain 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 and follow all current label directions for the specific pesticide being used. 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.
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