Feeding by slugs and snails is so economically damaging to farmers, growers and gardeners that millions of pounds have been devoted to finding new ways to combat these molluscan pests.
Slugs and snails have a mouthpart, or radula, which is covered with minute teeth that rasp away at plant tissue. Although it is the feeding on living tissue that causes the damage, some species will also survive on decaying vegetation.
The slime trails they leave are a secretion of lubricating mucus that enables slugs and snails to glide over most types of surfaces. Slugs feed throughout the year, stopping only in dry periods when they look for moist conditions lower down in the soil. Snails hibernate during the winter, clustered in sheltered nooks and crannies. Both are most active in damp conditions and at night.
Aside from pellets, plenty of inventive non-chemical ways have been developed or tried for the control of slugs and snails, such as barriers, mulches and traps. While most are either too expensive or just not effective enough for growers to contemplate, copper-impregnated ground cover is a valuable option for container nurseries.
How to recognise them
Spanish Slugs Arion vulgaris
In 2012 a new species of slug, Arion vulgaris, commonly known as the Spanish slug (pictured), was identified in the UK. These are not just feeding on plants but other things too such as dog excrement and dead animals. It was first thought to be a species of slug called Arion flagellus, commonly known as the Spanish stealth, slug first identified in the UK in 1945-46. It was later discovered to be invasive and it is thought that the slugs entered the UK on imported salad leaves, bare-root trees or potted plants.
The Spanish slug varies in colour from between bright-orange and reddish brown, and they can grow to a size between 8cm and 15cm once they have reached maturity.
The Spanish slugs are known to produce twice as many eggs as native slugs and tolerate hotter and dryer environments. These slugs also have an extensive omnivorous diet that includes excrements, dead animals and crops that aren’t normally susceptible to slug feeding.
Due to their aggressive behaviour, large size and high population density, they push out other slug and snail species to dominate an area. Cannibalism is also common among these slugs. There is a risk of them building up resistance against commonly used molluscicides in the UK.
Field slugs Deroceras reticulatum and Deroceras panormitanum
Deroceras reticulatum is the most common of all slugs in the UK. Deroceras panormitanum is the most widespread in nurseries. Deroceras reticulatum varies in colour from light-grey to fawn or buff with darker flecking. Mucus can be milky in colour.
Garden slug Arion hortensis
Black in colour with yellow sole, producing yellow-tinged mucus. Feeds above and below ground.
Black slug Arion ater
Largest of the main pest species at more than 10cm long. Black or dark-brown textured skin with an orange fringe and grey sole. White mucus. Not as damaging as other slug species, despite its size.
Keeled slug Milax species
Black, brown or grey in colour with a prominent ridge, or keel, along the back. Colourless mucus. Mainly feeds underground.
Common garden snail Helix aspersa
Large, widespread snail species with grey-brown patterned shell as much as 3cm in diameter.
A semi-aquatic snail, found to be the most common snail species on container nurseries and most active during the day. Brown to black shell with three whorls and a pointed shape.
Slugs and snails are hermaphrodite, producing both eggs and sperm, and can mate with any adult of the same species.
Slugs lay clusters of up to 150 eggs in soil or organic matter. These can hatch in as little as three weeks or, if laid in the autumn, may not hatch before spring. There are generally two generations each year, but sometimes more in wet summers.
Snails lay up to 100 eggs in the soil. These hatch in two-to-four weeks. Adults can live for several years. Defra/AHDB studies on Deroceras panormitanum as a nursery-stock pest found egg-laying starts in March and can continue all through summer, with generations overlapping in protected structures.
Plant damage by slugs can occur above or below ground, so bulbs are also susceptible. Crops grown in heavy soil are particularly vulnerable because the conditions favour the survival of slugs. Trails of slime confirm the presence of slugs and snails, which will devalue ornamental crops.
Treatment: biological control
The parasitic nematode Phasmarhabditis hermaphrodita (also known as Nemaslug) can be applied via irrigation. Nematodes seek out slugs, penetrating them through their breathing pore (pneumostome). The mantle (the well-defined area behind the slug’s head) swells up and the slug stops feeding, dying between four and 21 days after infection. The nematodes need adequate soil moisture and a soil temperature of between 5°C and 25°C.
Treatment: cultural control
• Copper-impregnated ground cover Tex-R (Spin Out) will reduce slug and snail activity.
• Maintain a dryer irrigation regime during the autumn and winter where possible.
• Clear slug hiding places, such as weeds, debris and old plant material.
• Keep algae levels down in tunnels and under glass.
• Sand and some hard mulch materials may prevent slugs from moving over a surface.
• Natural predators include hedgehogs, slow worms, ground beetles, toads and birds, so these may be worth encouraging. Beetles feed on slugs and their eggs.
Treatment: chemical control
Active ingredient Ferric phosphate
Formulation Ferramol Max, Sluxx (Certis)
Action(s) Molluscicide A formulated as a pellet approved for use against slugs and snails in ornamental plant production, outdoors and under protection. After ingesting, the pest stops feeding and dies within three-to-six days. Mucus production is unaffected so pests are still able to move away — often no dead slugs or snails will be seen in the treated area.
Active ingredient Metaldehyde
Action(s) Approved for use against slugs and snails on both edible and non-edible crops, both outdoor and protected. Pests dehydrate by overproducing mucus. Best applied in mild and damp weather when pest is most active. Harmful to birds and mammals, some products contain a repellent. Do not apply when rain is imminent and avoid irrigating protected crops for four days after treatment.
Further to finding traces of metaldehyde in analyses from water abstraction catchments, the Get Pelletwise campaign was set up by the Metaldehyde Stewardship Group (MSG) to encourage operators to adhere to its best practice guidelines and, for qualified operators, to go for update training to achieve PA4S. Go to www.getpelletwise.co.uk for more information.
Fully updated by Dove Associates
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Dove Associates shall in no event be liable for the loss or damage to any crops or biological control agents caused by the use of products mentioned.