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Slugwatch 

Crop vulnerability  

Crop vulnerability


[Slugwatch]  [Slug foraging]  [Soil sampling for slugs]  [Surface slug activity] 

How & when slugs damage crops
Most arable crops are at their greatest vulnerability to slug damage at emergence. However, potatoes and certain field vegetables are most vulnerable to damage close to harvest.

Winter wheat
Winter wheat is highly vulnerable to slug damage. The seeds become attractive to slugs within hours after sowing, as soon as they have imbibed water. Slugs always eat the embryo, killing the seed, and they often eat part or all of the endosperm, resulting in characteristic seed hollowing.

Juvenile slugs are important in killing wheat seed, because they are often abundant in winter wheat seedbeds and can move through small spaces in the soil to reach the seed; weight-for-weight, they also kill more wheat seeds than larger individuals.

When wheat seeds germinate, slugs attack the growing shoots and roots and, if they are able to reach the seeds after germination, they will kill the seedlings by destroying the growing points, which remain at the base of the shoots, close to the seeds. If slugs are unable to reach the seeds, shoots growing up through the soil will be severely attacked but not necessarily killed. Wheat seedlings are grazed by slugs feeding on the surface from the time that they emerge, resulting in characteristic shredding of the leaves. Seedlings are normally vulnerable until the start of tillering. From then on, damage is less likely to affect the crop.

Other cereals
Winter barley and winter oat seeds have an extra seed coat and are less vulnerable than wheat seeds. However, as soon as they germinate, shoots and roots are damaged by slugs in the same way as winter wheat. Spring-sown cereals are at less risk of slug damage than autumn sowings, probably because slugs are generally less abundant and active in spring and because the crops grow quickly through the early vulnerable stages.

Oilseed rape
Seedlings of modern varieties of oilseed rape are highly susceptible to slug damage because of their low concentrations of glucosinolates and are at their most vulnerable soon after they begin to germinate. Seedlings remain at risk until they reach the four-true-leaf stage.


Potatoes
Slugs do not attack young tubers to any significant extent, but start to feed on them as they mature. They enter the tubers through small holes in the skin and hollow out feeding cavities in the tissue beneath. From the outside, the entrance hole made by slugs is very similar to wireworm damage but the internal damage is very different from the narrow tunnel made by a wireworm. Slug damage can be confused with cutworm damage, but slug damage is most prevalent on heavier soils in wet seasons, whereas cutworm damage is most prevalent on lighter soils in dry seasons. Some potato varieties show partial resistance to slug damage, whilst others are highly susceptible. Prompt harvesting reduces the period that the tubers are exposed to slug attack.

Sugar beet
Slug attacks on sugar beet are sporadic and tend to occur where agronomic and environmental conditions have favoured slug over-winter survival, e.g. where cover crops have been grown through the winter. Because sugar beet seeds are precision sown, seedling losses are even more serious than for cereals or oilseed rape.

Field vegetables
Many field vegetables are susceptible to slug damage at emergence (in the same way as oilseed rape or sugar beet) or just after seedlings have been transplanted into the field. However, the most difficult problems arise in crops close to harvest.

Brussels sprouts
Slugs feed on the outer leaves of maturing sprout buttons and, although the damage is usually quite small and does not affect yield, it greatly reduces the value of the crop because it will not meet the high quality standards demanded by consumers.

Broccoli, cabbage, cauliflower and kale
Slugs damage the maturing heads, resulting in loss of yield and unacceptable contamination with slugs and their faeces.

Carrots
Slugs excavate holes in roots, which can lead to bacterial rotting. Slugs do not survive well in sandy soils where carrots are grown, so attack is usually most severe at the field edges, due to slugs moving in from the field margins.

Peas for canning or freezing
These are particularly at risk from contamination by slugs at harvest. The harvesting operation is frequently carried out at night to meet the precise requirements of processors but this also coincides with maximum slug activity up in the crop canopy. As a consequence, pea-sized slugs may be harvested along with the crop.

Asparagus
Slugs make small bites in the sides of emerging shoots, causing distorted growth and downgrading.