Anyone who has watched their hosta foliage turn “lacy” has experienced the work of slugs. Slugs, a common plant pest in wet weather, are snails without shells. They are mollusks, which means they are related to oysters, octopi, and clams.
Slugs secrete a slimy substance to help them move about. They need moisture to create this “slime”, so they are highly dependent on soil moisture. When slime trails dry up they are sometimes visible as shiny lines. Slugs feed at night when humidity is high, so the best time to see them is by checking your plants in the dark with a flashlight. Slugs hide during the day under mulch, in debris, and in short plants, such as ground covers.
Most slugs overwinter as eggs in debris at the soil surface. Eggs are round, colorless, gelatinous spheres that appear cloudy just before hatching. They hatch in spring and the young slugs begin feeding immediately. Adults lay from 20 to 100 eggs at a time throughout the growing season, causing overlapping generations. Most slugs live for one season outdoors, but adult slugs may survive mild winters.
Slugs can be destructive to many plants in the landscape, including annuals, perennials, bulbs, ground covers, trees, and shrubs. They prefer succulent foliage found on seedlings and herbaceous plants, and fruit lying on the ground. One of their favorite plants is hosta. Slug damage on leaves appears as irregularly shaped holes or tattered edges. Insects also eat leaf margins, but large holes in leaves are more indicative of slug feeding.
A combination of strategies may be necessary to manage slugs. They can be handpicked and placed in a jar with soapy water. Temporary traps, consisting of rolled, wet newspaper and flat boards placed near damaged plants, provide sheltered hiding sites during the day. Check under the boards and in the newspapers in the morning. Slugs in these traps can then be collected and destroyed.
Habitat modification may be effective in reducing populations. Hiding places, such as excessive mulch piles, weeds, and other organic debris should be eliminated. Vegetable plants should be staked to keep fruit off the ground. Avoid watering late in the day because the moist conditions that are created will be conducive to slug movement. Using drip irrigation to water directly onto soil under plants may help reduce slug activity.
Thin strips of copper bands placed around the bases of shrubs and trees can repel slugs by giving them a slight electric shock when their bodies touch the copper. Some gardeners successfully pour beer into a shallow pan and place it in infested areas. Slugs are attracted to the yeast, fall into the pan and drown. The traps should be emptied occasionally.
Firefly larvae, birds, and ground beetles are natural slug predators, but they may not exist in sufficient numbers to control a serious slug problem. Slugs also die in dry weather because they are so dependent on moisture.
Insecticides are ineffective against slugs because they are not insects. Registered commercial baits fatal to slugs are available. Metaldehyde breaks down quickly in direct sunlight and moisture. Iron phosphate bait is safe to use around pets and wildlife and is effective for a longer time than metaldehyde. Each of these materials is less effective during hot, dry weather when there is less slug activity. Irrigate before applying these baits.
Refer to University of Illinois Extension’s “Pest Management for the Home Landscape” for a complete listing of chemical recommendations. Use pesticides safely and wisely; read and follow label directions.
The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.