Landscape and Greenhouse - Slugs

Slugs continue to be a problem in landscapes with the cool wet weather. The following is some information from the Ohio State University.

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 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.

Control Hints

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.

Reprinted in part from "Slugs and Their Management" HYG-2010-95, by David J. Shetlar, the Ohio State University.