What Do They Look Like?
Slugs are mollusks without shells. They travel on a large, muscular ‘foot’, leaving a trail of slime behind. The most destructive slugs in this region are non-native species that have become established: European black slugs
are very large slugs, reaching over 12 centimetres long. As well as black, they can also be reddish brown or even yellowish-orange in colour. Grey garden slugs
are much smaller (up to four centimetres long), mottled grey or light brown and are very common.
Native west coast Banana
slugs are olive green with a few to many black spots. While they can damage plants in moist, woodland gardens, they are usually not a problem in open areas and city gardens. If you find one, gently move it out of your garden to a wooded area.
What Does Slug Damage Look Like?
Slugs are valuable decomposers of organic material in the environment, but they also chew large, ragged holes in tender leaves and flowers. They can demolish small plants and new shoots of hostas, dahlias and other ornamentals. They chew on flowers of spring bulbs and can also climb up taller plants to chew leaves. Look for traces of silvery slime on chewed leaves to identify whether slugs caused the damage.
Slug Life Cycle
Slugs lay clusters of 40 or more round, translucent eggs in moist places under stones, boards or decaying plant material. Newly hatched slugs are tiny, but voracious. Slugs take months to mature and can live for two years. Slugs mainly feed at night and on wet, cloudy days. During the day, they retreat to dark, moist sites under plants, decaying leaves, pots and other objects.
When Are Slugs A Problem?
Slugs can be quite damaging to small plants, particularly in the spring and in wet weather. When plants are large and growing well, damage from slugs is less noticeable and plants quickly replace foliage. Slugs are slowed down by hot, dry conditions and are generally less damaging later in the summer.
How Can I Prevent Damage?
- Keep conditions as dry as possible. Water plants early in the morning so that the soil surface has time to dry by evening. Use drip irrigation or seep hoses to water plants. This keeps the soil between plants dry and inhibits the movement of slugs. It saves water too!
- Pull mulches well away from emerging shoots and seedlings until plants have grown large enough to withstand slugs.
- Plant extra seeds or seedlings to ensure a normal crop after losses from slugs.
- Protect slug predators. Ground beetles and other insects feed on slugs as do garter snakes, ducks, geese and toads.
- Avoid using insecticides and try to provide some naturalized areas of longer grasses and rockery, favoured by garter snakes.
- For ornamental gardens, choose plants that slugs avoid, such as herbs with leathery, strongly scented leaves and plants with furry leaves or tough needles.
What Can I Do To Control Slugs?
- Look for and destroy slugs when they are actively feeding at dawn or dusk or in wet weather.
- Lure slugs to hiding places under boards, pieces of plastic, even grapefruit halves laid on the soil. Check underneath during the day and kill any slugs hiding there.
- Repel slugs with metal barriers. Slugs avoid copper and zinc so you can use strips of these metals around planters, greenhouse benches and trunks of shrubs to repel them. Copper tapes or strips for slug barriers are sold at some garden centres.
- Gritty materials, such as sawdust, crushed eggshells, wood ashes, lime or diatomaceous earth, when spread around plants can repel slugs as long as the materials stay dry. They don’t work well in wet weather or where there are a lot of slugs.
- Attract slugs to fermenting liquids where they drown. Some people use beer, but you can get the same effect with a mixture of water, yeast and a little sugar.
- Use a commercial slug trap or make your own from a plastic cottage cheese container with a lid. Cut a couple of access holes in the sides of the container and bury it in the soil so that the bottom edges of the holes are at the soil surface. Pour the beer or the yeast mix into the container. Containers should be covered or set with the lip at least two centimetres above the soil level to avoid catching ground beetles, which are valuable slug predators.
- Sprinkle low-toxicity commercial slug bait around plants. The bait contains iron compounds (the product label lists ferric phosphate) and is available from garden centres. When slugs eat iron it interferes with their ability to make slime and they die. Spread small amounts of bait around plants and renew after rain.
Tips For A Healthy Garden
© Image courtesy of City Green
- Enrich the soil once or twice a year with compost or other organic fertilizers.
- Choose plants adapted to the conditions of sun or shade, moisture and soil acidity. If necessary, correct the drainage and acidity to suit the plants.
- Plant native plants, which are adapted to the local climate. Most are easy to care for and have few pest problems.
- Before buying plants, make sure they are healthy and free of diseases and insect pests.
- Water deeply, but infrequently, to encourage deep rooting.
- Cover the soil between plants and under shrubs with organic mulches. This insulates the soil, keeps in moisture and suppresses weeds.
- Protect and attract native beneficial insect, birds and other animals.