Causal agent(s) and transmission
Slugs belong to the phylum Mollusc (Gastropoda family) and their composition – over 85% water – makes them highly vulnerable to dessication in their environment. The behaviour of slugs is mainly nocturnal and is closely linked to the climatic conditions, mainly temperature and relative humidity.
Two main species of slugs are responsible for damage to potato tubers: the black slug and the grey slug.
The black slug, Arion hortensis (photo 1) measures 3 to 4 cm; it is blue- black in colour with an orange yellow ventral face, and the mucus is colourless. The body sides are dark and the tentacles, reddish. It preferably lives deep down in the soil and moves at a rate of 2 to 3 metres during a night. It can lay up to 150 to 300 eggs, mainly between spring and summer. It lives for between 12 and 18 months.
The grey slug, Deroceras reticulatum (photo 2), whose size ranges from 4 to 5 cm, is beige-grey in colour with reticulations in the form of elongated brown spots. Its mucus is milky white in colour. It preferably lives on the soil surface, and so covers greater distances (up to 7 metres during a night). The grey slug lays 300 to 400 eggs and very often produces two generations a year, in spring and in autumn.
Many cultivated crops may be attacked by slugs: rape, straw cereals, maize, beetroot and also the potato.
Symptoms on tubers
Attacks by slugs occur essentially at the end of the growth cycle of the potato, from the enlargement of the tubers until harvesting.
Damaged tubers show perforations in the skin (photos 3 to 5), of 4 to 5 mm in diameter, or galleries dug due to the slugs feeding on the flesh.
In potato crops, slugs, which can eat a third of their own weight in one night, attack mainly the tubers but can also defoliate the plants, leaving characteristic trails of slime (photos 6 and 7).
The qualitative or quantitative losses can represent up to 30% yield loss, and, in some cases, in the downgrading or the refusal of the affected potato batches.
Even if traps catch only a fraction of the actual slug populations, they are an efficient means to detect and limit the presence of slugs.
The most commonly used trap measures 50 cm by 50 cm and consists of a plastic layer lined with aluminium foil where the slug is attracted by specific pellets. Placing 4 of these traps per field will indicate the degree of infestation of the plots.
For potato crops, the chemical control threshold level is 5 slugs/m2 at the beginning of tuber enlargement. However, chemical control by the application of slug pellets reaches only 40 to 70% efficacy, thus preventive measures for the whole rotation should be a priority for the control of slugs.
The choice of the potato cultivar is another alternative: field experience demonstrates that not all potato cultivars attract slugs with the same intensity. Trials on the tuber sapidity of different cultivars have shown that, with a highly infested plot, all the tested cultivars are attacked to a similar high degree. However, in less infected fields, some cultivars do not attract slugs at all (lack of sapidity), whereas tubers of susceptible cultivars like Monalisa can be completely destroyed.
Slugs need to be controlled throughout the rotation, mainly by intercropping management but also with a series of preventive measures required once the potato crop is established. These methods are summarised as follows:
Intercropping soil management
- destroy the straw after the harvest of the grain crops; an initial and superficial soil operation as soon as possible after the grain harvest, ideally prior to a dry period;
- incorporate harvest residues in order to prevent “hollow” soils;
- limit the green manure period to a minimum and facilitate its decomposition by a soil incorporation prior to ploughing. In general terms, limit all food sources for the slugs during winter (groundkeepers, inadequately incorporated green manure, etc.).
Planting and monitoring the potato crop
- avoid planting potatoes in heavy soils, these are favourable to slugs; avoid the proximity of compost heaps or piles of organic waste, as all of these provide a base from which the slugs will carry out their midnight raids;
- carefully prepare the soil before planting, in order to bring eggs to the surface so they will be eaten by birds and later to limit the slug movement. In a high risk situation, select potato cultivars showing resistance or low attraction for the slugs and avoid late maturing cultivars;
- adapt chemical control to the presence of the slugs by monitoring “slug activity” with the use of traps especially at the high risk period of tuber enlargement stage;
- keep irrigation to a minimum;
- reduce the delay between haulm destruction and harvest.