How slugs live

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The Biology of slugs

Slugs are nocturnal animals. Their activity varies greatly depending on the period of the year, the temperature and humidity.

The slug life cycle, their population density, their speed of reproduction and their growth are all dependent on climatic conditions, light and available food. Mild winters favour the survival and development of eggs and young slugs.

On the other hand a very dry cold winter can reduce the slug population of a piece of land.

Activity and way of life

Slug activity varies very much from one species to another, and within one species from one individual to another, and for the same individual from one day to the next.

In general slugs come out at night once the temperature exceeds a certain minimum and does not exceed a certain ceiling.

Garden slugs no longer move if the temperature falls below 5°C. The optimum temperature for grey slug activity is in the region of 18°C and they are inactive at 0°C. Slugs die if the temperature falls to –3°C but in general they are buried deep in the soil when the periods of frost occur.

Without humidity the slug cannot produce mucus and as a consequence cannot move about. It takes refuge in the soil to await the return of rain.

Feeding

Slug feeding habits are very variable. They feed most often on vegetable matter but they can also consume animal waste.

Large slugs and the small grey slug prefer plants at the soil surface. They are often attracted to plants already damaged. In periods of drought small slugs live mainly in the soil and nibble the underground parts of plants.

A slug can eat up to half of its weight in a single night. The grey slug takes in between 30 and 50 mg per day, but a large slug can eat 5 to 10 g per day. Certain slugs feed in one go, others eat several times in a single night.

Reproduction

Slugs are hermaphrodites, i.e. a single individual is both male and female, but not at the same time. The male organs become active first followed by the female organs. The activation of the two phases of sexual activity is regulated by a hormonal system.

The female lays the eggs between several days and several weeks after mating, depending on the species. A slug can lay between 100 and 500 eggs in groups of 10 to 50. It puts them in a hole dug in the ground or in a shelter. The eggs are spherical, creamy white or transparent.

The incubation period of the eggs is directly linked to the climatic conditions, particularly the temperature. At 5°C incubation takes up to three months while at 20°C two or three weeks is sufficient. Soil humidity must be between 40% and 80%.

Slugs can produce a generation per year, one every

two years or two per year. The best periods for reproduction are autumn and spring.

When the eggs hatch the immature slugs measure a few millimetres and are transparent. Slugs live between nine and eighteen months depending on the species and the region. As an example the grey slug produces one generation per year in regions with hard winters (e.g. the north-east of France) and two generations per year in regions with mild winters (e.g. the south-west of France)..

On the other hand the black slug produces a single generation per year what ever the region.

Movement

Slugs do not move about very much. In optimum conditions the grey slug travels between 4 and 7 m a day and the black slug between 2 and 3 m.

Mucus is essential for slug movement. When they move about a gland situated at the front extremity of the foot secretes mucus which is squeezed below the crawling sole and allows them to slide along.

When they are in an area with abundant vegetation the slugs do not move more than 50 cm per day..

They find their food using their olfactory organs and their taste buds.

Natural enemies of the slugs

There are numerous slug predators:

• Most birds, 
• Mammals such as moles, hedgehogs, shrews, rats and badgers,
• Toads, frogs, slow worms, and lizards,

But their effect in controlling population is rather limited.

Factors affecting the development of slugs

Factors	Favourable conditions
Humidity	Air humidity Soil humidity: in the 5 first centimetres from the surface
Temperature	The quantity of food ingested depends on the temperature: maximum between 15° and 20°C
Type of soil	Generally : well aerated soils, with clods and stones Above all: clayey and clay/chalk soils Occasionally: silty soils, Rarely: sandy soils Presence of organic matter, vegetable detritus, harvesting debris Proximity of scrub, woodland, fallow land
Soil preparation	Reduction of cultivation Sowing too superficial
Preceding use	Oil-seed rape, straw cereals, fallow grassland Harvest debris Intermediate crop planting
Other factors	Irrigation Direct sowing Presence of straw

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