Snail and Slug Control in Harmony with Nature
For as long as I have been in the garden care business gardeners have asked me how to get rid of the snails and slugs. This season is no different; especially with the wet conditions this year. There are many methods, ranging from setting up mini-bars (beer served on a plate) to crunching snails under willing feet. Baits control adult snails but I am convinced that the more adults you kill the faster young ones make their appearance. The most common snails in the garden are the brown garden snail, dune and otala snail.
Brown garden snail
Biology of Snail
Snails comprise a muscular foot (sole), head and coiled mass located in the shell. They move by expanding and extracting the sole. Glands secrete mucus to help the snail move and to protect it against water loss. There are two pairs of tentacles on the snail’s head and its eyes are on the upper pair. Slugs do not have protective shells. Dune snails are mainly found in coastal areas and are smaller than the typical brown garden snail. They are white to light yellowish-brown with brown markings. The otala snail is found in the Cape Peninsula, is a creamy-brown color and appears flatter than the brown snail.
Snails and slugs attack many plants in the garden and are particularly fond of agapanthus and other soft, broad-leafed plants. They also tend to attack aloe species, vegetables such as lettuce, pepper, spinach, and tomato, as well as herbs such as rocket and mint. Snails and slugs will also find their way up to larger plants such as fruit trees and shrubs.
Damage and Economic Impact
Snails eat by rasping pieces of tissue off plants. Severe damage can be caused to young plants, especially seedlings just after they are transplanted. Older plants will have unsightly marks with slime trails showing on the leaf surface. Fruit of vegetables such as green pepper can be spoiled, while lettuce and other leafy crops can become unmarketable. Slugs can also leave behind holes on older plants and, as with snails, slime trails show on the surface of leaves and fruit.
When and Where
Snails and slugs are happiest in densely-planted, cool parts of the garden. They tend to gather under and inside plants for protection during the day. They prefer damp conditions and are mainly active at night and on cool, overcast days.
Methods of Control
Bio-pesticides: Organic formulations containing garlic and applied to plants and surrounding soil can assist in preventing snails and slugs from identifying host plants. Ferramol is the only certified organic snail bait available in South Africa (at the time of writing).
Biological: Ground beetles are associated with the control of snails and slugs. Parasitic nematodes, such as Phasmarhabditis bermaphrodita, also control slugs and are available commercially in Europe. Birds, snakes, rock lizards (leguaan) and various vertebrates prey on snails and slugs. Larvae of fireflies (glow worms) are known to feed on snails. In southern California, the predaceous decollate snail (Rumina decollata) has been released in citrus orchards to control the brown garden snail and is providing effective biological control. It feeds only on small snails, not full-sized ones. Because of the potential impact of the decollate snail on certain endangered mollusk species, use of this predator is being closely monitored. Physical or cultural practices: Collect by hand at night or on overcast days. Crushed eggshells sprinkled around susceptible plantings can help as snails and slugs dislike crossing sharp-edged objects.
Diatomaceous earth is a natural powdery substance that causes soft bodies like those of snails and slugs to dehydrate. Diatomaceous earth is mainly used on soil surfaces. Do not incorporate it deeply into the soil as it may harm earthworms.
Beer-baited traps can be used to trap and drown slugs and snails. However, they are not effective for the labor involved. Beer traps attract slugs and snails within an area of only a meter or so and must be refilled every few days to keep the liquid deep enough to drown the mollusks. It is the fermented product that attracts them and a sugar-water and yeast mixture can be used in place of beer. Traps should be buried at ground level, so the snails fall into them easily. Traps must have deep, vertical sides to keep the snails and slugs from crawling out and a cover to reduce evaporation.