Garden pest control - Slugs (Description and control instructions)

Slugs and Snails are molluscs of the class Gastropoda which literally translated is 'stomach foot'. Gastropods form the second largest class in the animal kingdom, the largest being insects. Most terrestrial snails and slugs belong to the subclass Pulmonata, order Stylommatophora. Of the 29 species in Britain a few are carniverous, but most live on living and decaying plant tissue; usually the the plants we like to look at or eat ourselves!. Instead of a shell like the snails, slugs have an internal horny plate covering their breathing cavity.

There is a range of colours and sizes of slug depending on the species. The Field Slug is 2cm and grey, while the Round Back Slug can be 5-10cm, black or brown and in some cases can have an orange colour on their foot. They feed by rasping ragged holes in flowers, leaves, stems, roots and seeds, with young plants the leaf feeding can kill. Below soil level some species attack root crops - the small Black Keeled Slug being particularly destructive, especially to potatoes and the bases of plant stems. Many species spend most or all of their life underground and those seen are only a small proportion of those present.

Slugs begin to move, hatch, feed, and lay eggs when the temperature is above 5 °C. During dry and cold conditions they remain deep in the soil. The wetter weather and milder winters of late has been very benificial, and most gardeners have noticed an increase in their numbers. They are usually nocturnal, but will venture out on dull, damp days.

There are three stages in the life cycle: eggs, immature stage and adults - they can overwinter in any stage. They are hermaphrodite having both male and female organs, so every individual can lay eggs - up to 300 each in batches of 10 to 50 in moist, but not waterlogged, crevices; sometimes down the sides of pots. One individual has the potential to produce about 40,000 offspring. Eggs are gelatinous, watery, about 3 - 4mm across and usually spherical like tapioca. The period of development of the eggs varies depending on the temperature, during warmer weather they hatch after 10 days, but this can be up to 100 days in cooler conditions. After hatching the slug matures in less than a year and can live for two or more years.

Treatments

There are many suggestions to kill or deter this pest, but even the most diligent collection regime has been shown to have little effect on the damage caused. The best thing is to try and cope with them by following some general hygiene methods.

Remove their hiding places and spots where they lay eggs, eg. rotting boards, logs, stones dead leaves and other debris. Place compost heaps away from vulnerable plants. But don't be too tidy as Ground Beetles which feed on the eggs, like to hide during the day as well, large stones and pebbles will do for them. Another creature which appears to eat the eggs is the New Zealand Flatworm - 'snail caviar' perhaps.

Some other means of control:-

• Use traps, eg. stones, boards, upturned pots empty grapefruit skins or wet sacking, the slugs take cover underneath and can be collected. They will also lay their eggs here so they can be destroyed as well.
  
  
• Go slug hunting at dusk and chop them up or put them in hot water, this is the most effective method of reducing their numbers, but the most destructive species do not emerge often. The slimey bodies are not very pleasant to handle so use tweezers or chopsticks.
  
  
• Ordinary table salt is lethal to slugs, but overuse is detrimental to plants and other creatures. It is probably best to drop them into a container of this, rather than sprinkling it about.
  
  
• A tablespoonful of oats acts as an attractant when placed beside more favoured plants. The slugs can be collected and destroyed.
  
  
• Attract hedgehogs into the garden with dog food (not bread it clogs their digestive system and milk is bad for young hedgehogs). Keep them there with a concealed, dry home with straw bedding and don't light bonfires without checking for sleeping 'hogs.
  
  
• Beer traps should have the entrance about 3cm above soil level to prevent Ground Beetles falling in - put a few twigs inside to help any which do fall in, to climb out. Beer cans can be quite effective, I discovered this when tidying urban gardens where late revellers use the front garden as a bin. They seldom finish the beer so the cans are already baited, the slugs crawl in and drown in a stupor, I suppose. An alternative to beer is a mixture of sugary water and yeast or milk, but the best bait is said to be stout - a bit of the black stuff. Slugs are also partial to cat and dog food, so this too makes a suitable bait.
  
  
• A home-made trap can be constructed from an empty margarine or ice-cream tub with a hole cut in the side near to the top - the lid makes escape more difficult and stops the trap filling up with rain water.
  
  
• Researchers in Hawaii have found that a 1 to 2% solution of caffeine will kill slugs and a 0.1% solution will deter them if is sprayed on leaves. However, this latter strength solution also scorched the young leaves of cabbages. They were using purified caffeine and a 2% solution would be lethal to most things including beneficial insects. It appears to act on the nervous system. Instant coffee contains 0.05% caffeine so it might be possible to use it as a deterrent. It would probably have to be continually applied especially in wet weather and it is not known if it is harmful to other beneficial creatures. Some people already use spent coffee grounds to deter, but it is not very effective. Further work is being carried out so a suitable formulation which might be safe to use on food crops as well, may be available in the future. The spent coffee grounds can be spread around vulnerable plants and they don't like to crawl over it - any remaining caffeine could also be affecting them.
  
  
• Frogs, toads, hedgehogs, some beetles and their larvae (ground & rove), centipedes, parasitic flies, birds, chickens, and ducks are natural enemies. Attract these creatures by providing shelter like a thick hedge.
  
  
• A dry mulch will reduce slug activity, this is achieved using a dutch hoe in dry weather. Alternatives are grit, sawdust, cocco shells, weathered cinders, wood ash or crushed egg shells. However their protective slime can allow them to crawl over a razor blade or sharp glass, so these methods are of limited success at best and are usually wishful thinking!
  
  
• Fito Slug Stoppa granules form a rough barrier and absorb slime so make it difficult for slugs to move over them, but are quite expensive.
  
  
• Diatomaceous Earth is a dry granular substance made up of fossilised algae. It is applied as a barrier to exclude the slugs. It loses its effectiveness when wet, but regains it after drying out. Also can be used to eliminate other insects the dry powder sticks to their exoskeleton, abrading it and leading to dehydration.
  
  
• A copper wire or strip placed in a circle around the area is said to deter by creating a micro-current as the slug touches it. Check for the presence of slugs or their eggs first and destroy - there is no point in trapping them inside! Strips around pots could also be decorative. A self-adhesive copper tape is available in some garden centres or online. There is also a woven mesh which can be erected as a small fence, but it must be in contact with the soil along its full length otherwise they will crawl underneath.
  
  
• A mulch of hair clippings are said to deter them as it sticks to their slimy underside, making movement difficult.
  
  
• Cut the bottom off a plastic drinks bottle and remove the cap, to form a protective cylinder around vulnerable young plants.
  
  
• A biological control is the nematode (eel worm) Phasmarhabditis hermaphrodita which occurs naturally and is used in Slugsure. They invade the slug and bacteria which they carry cause the slug to stop feeding, go underground and die. As they decompose the eel worms reproduce. I suppose it's like giving them the 'flu'. This natural control is less effective on surface dwelling species as the eel worms live in the moist conditions below ground, but they are the best treatment for the underground species which attack root vegetables. Nematodes are unaffected by wet weather. They remain active for about 6 weeks, and need a minimum temperarure of 5°C. Another brand is Nemaslug (Phasmarhabditis hermaphrodita), which is available by mail order or on-line. They can be ordered between March and October, and keep in the fridge for a few weeks after delivery.
  
  
• A slick of petroleum jelly around the top of containers prevents them crossing to the contents, but makes handling the pots difficult and it becomes covered with dust and leaves.
  
  
• For use when on slug patrol, make a half-and-half mixture of vinegar and water in a handsprayer. One squirt should kill them. Also a 1 : 3 mixture of household ammonia in water should work.
  
  
• Slug pellets, said to be safe by their manufacturers if sprinkled thinly around. They are dangerous to pets if they find the packet, 12 dogs died in 1998 after eating spilled or badly stored packets. Birds are not likely to eat them directly and they do not eat the corpses, but slightly affected slugs may be eaten and some studies show that the bird's fertility may be reduced. Also the active ingredient will be washed into the soil and will affect other organisms. The same chemicals, usually metaldehyde, can be applied as a drench.
  
  
• Another slug bait is one containing ferric phosphate which works best in moist conditions. It is considered to be organic and although it is less effective than metaldehyde, it can be used where crops are grown and is not hazardous to pets. It degrades over time in the soil to iron and phosphate.
  
  
• Some species or varieties of vulnerable plants with thicker leaves are said to be resistant.
  
  
• Sacrificial plants such as lettuce planted near to vulnerable ones, may draw the slugs away and keep them interested for long enough to be picked up and removed.
  
  
• Keeping vulnerable plants such as hostas in drier conditions makes the leaves tougher and a bit less attractive to tha slugs.

Most of the above measures apply to snails as well.