ORGANIC SLUG AND SNAIL CONTROL

© Frances Michaels

If overnight newly planted seedlings are disappearing and there are large holes in your plants, the culprits are probably snails or slugs. Usually a close look at the plants will reveal the characteristic silvery trail. Snails and slugs are molluscs and need a moist environment to survive. They avoid the sun and come out mainly at night or on dark, cloudy days. They secrete mucous to move about and by using the same trail and sharing trails with other snails they save on mucous production. During cold or dry weather, snails can seal their shells, and remain dormant for several years.

Australia has 6,000 species of native snails, none of which cause problems to garden plants. Some of our native species are carnivorous, feeding on garden snails.

The common brown snail Helix aspera is an unwanted arrival from Europe. It can take from 4 months to 2 years to mature and may live 12 years. Snails are hermaphrodite and during the warmer months can lay several egg clusters. The clusters contain up to 100 white eggs, at a soil depth of 20-40 mm.

Effective control of snails and slugs in the long term needs a combination of cultural, biological and chemical methods rather than relying on a single solution.

BIOLOGICAL CONTROLS
Since all of our pest snails and slugs are introduced to Australia, the range of native biological controls is limited. Predators include some birds, including magpies, kookaburras, mudlarks and starlings. The appetite of birds for snails tends to vary greatly from area to area. Other predators include blue tongue lizards, rats, centipedes, frogs, predatory beetles, predatory snails and slugs. Chickens or ducks, particularly Khaki Campbell or Indian Runner, can provide effective, long-term control in orchards. Poultry are too destructive in gardens to be given unlimited access but in small mobile cage systems rotated over garden beds, they will clean up slugs and snails, fruit fly, 28-spotted ladybeetle and other pests that are attempting to hide out in the garden.

PHYSICAL AND CULTURAL CONTROLS
Start with a garden cleanup to reduce snails and slugs breeding sites. Wear thick gloves and gumboots to remove any old wooden boards and other garden rubbish. Check around the compost heap, inside stored pots and around drains and retaining walls. Have ready a bucket of soapy water to drown any you find or a bucket of just hot water if you keep poultry. Simply stepping on them may still allow mature eggs to hatch.

Handpicking, will over time, greatly reduce the number of snails; it is less effective for slugs. The best time is 2 hours after sunset by torchlight. Consider offering a small financial incentive to young members of the household to collect them.

Barriers can be used to protect vulnerable plants and young seedlings. Suitable materials include diatomaceous earth, crushed eggshells, lime, wood ash, wood shavings and sawdust but these are only effective when kept dry.

LEAST TOXIC CHEMICAL CONTROLS
Repellent sprays can be made at home from garlic or wormwood. Commercial garlic sprays are strong enough to kill slugs and young snails.

In 1997 a new snail bait Multiguard Slug and Snail Pellets was introduced based on iron. This product minimises the risk of harm to pets, poultry and wildlife and is very effective when compared to the following baits. It has the added advantage of breaking down in 1 to 4 weeks to become a soil micronutrient.

Other snail baits containing either metaldehyde (green) or methiocarb (blue) have been around for many years. Metaldehyde baits are preferable to methiocarb because they break down relatively quickly to form acetic acid (vinegar) and are less toxic. Metaldehyde by itself is a snail and slug attractant but it is usually mixed with wheatmeal and an added bitter substance to discourage pets and wildlife. Metaldehyde bait is also produced as a dolomite pellet with no attraction for pets and wildlife. If using baits, place them in a container to restrict access of pets and wildlife and to reduce leaching of chemicals into the soil. Use as few pellets as possible, to prevent resistance to the chemicals increasing in the snail and slug population. Don't use these baits at all if your poultry have access to the garden.

Slug & Snail Trap
 

Homemade traps such as inverted grapefruit halves, pots or wooden boards can be placed close to where the slugs and snails are harbouring. Check early each morning or they will become habitat instead! Beer, wine or any yeast product mixed with water, is an attractant; place it in a bowl, with the rim 1 or 2 cm above the ground, to drown them. Research has found sugar water (5% sugar solution) to be a highly effective slug attractant. Empty the traps every day into the compost or chook run.