Dealing with slugs and snails

These pests are not just the bane of the suburban gardener – they can wreak havoc with agricultural and horticultural crops, explains Paul Donovan.

Unlike a slug, a snail has a shell into which it retreats when threatened.
Photo: Paul Donovan

Slugs and snails are molluscs and related to squid, cuttlefish and octopuses. The only real difference between a slug and a snail is that the snail carries a spiral shell. Other than the presence of chewed plants, one of the tell-tale signs of slugs and snails is the mucus trail they leave behind. This is secreted by the muscular foot and aids movement.

Slugs and snails are hermaphrodites – each individual has both male and female sex organs, so any two individuals can mate and reproduce, and a single individual can even self-fertilise to reproduce. Eggs are laid in clusters in the soil, beneath leaves, or in other areas where the soil is damp. The eggs can lie dormant in the soil for months and hatch only when conditions are suitable.

Slugs and snails are nocturnal and feed on decaying matter, living plants, flowers, ground fruits such as strawberries and tomatoes, citrus fruits and even the young bark of trees. Seedlings can be stripped bare of their leaves.
There are a number of methods to control slugs and snails:

Mechanical: This may be laborious as it involves searching for them in their hiding places, but it can be effective. Search at night with the aid of a torch and kill the pests by dropping them into a bowl of soapy water.

Barriers: One of the most effective barriers is to ring the plants under attack with a copper screen. The copper reacts with the molluscs’ slime, creating an electrical charge that deters them from crossing. The screen should be at least 10cm high. The downside to copper is its value to thieves, so it is best used in an enclosed environment such as a greenhouse. A mound of dry ash 3cm high by 7cm wide, spread around the crop, will also act as an effective barrier. (Diatomaceous earth is not effective in slug and snail control.)

The problem with barriers is that, while they may prevent slugs and snails from climbing onto the plant, they do nothing to those already there. They should therefore be used in conjunction with other control systems.

Chemicals and baits
A range of effective chemicals and baits is available. Unfortunately, baits containing metaldehyde and carbaryl can be extremely toxic to earthworms and other surface predatory insects. It is also toxic to cats, dogs and birds. Should you use one of these baits, remember that they are more effective on a warm day with low humidity. It can kill a slug or snail within 24 hours.

Irrigate the crops before placing the bait to make the slugs and snails more active, but do not irrigate for two or three days after placement as this can reduce the bait’s effectiveness.This bait deteriorates in direct sunlight, so place it in the shade. An alternative is iron phosphate baits, which are safer for wildlife and domestic animals.

Predators
Monitor lizards, some snakes, toads, predatory beetles, birds and nematodes all feed on slugs and snails. If the plants are robust enough, chickens, ducks and geese can be used to keep slugs and snails under control.

Other methods
Scatter bran around seedlings and vulnerable plants. When ingested, it swells in the stomach of the slug or snail and kills it. Smear petroleum jelly around the rims of seed trays to prevent slugs or snails from climbing up them. Sunken traps containing beer are well-marketed. But you’ll be better off drinking the beer and filling the traps with sugary water and yeast instead; it will have the same effect. These traps have the disadvantage that they can kill beneficial insects too.

Paul Donovan, who is based in Botswana, advises farmers on crop pest control.