Field slug, grey field slug

Scientific name: Deroceras reticulatum

Grey field slug

Arion slug

Grey field slugs

Slug damage to clover
  • Key characteristics
  • Biology
  • Impacts
  • Control

  • Common throughout New Zealand
  • Can be a significant pest in pasture, agricultural crops, horticultural crops and home gardens
  • Particularly damaging to seedlings
  • Cultivation generally kills slugs but this should not be assummed to always be so
  • Direct drilled crops are at greater risk from slug damage – slugs are not killed by paddock preparation, food source is removed, drill slits provide an attractive refuge for slugs, vulnerable seeds and seedlings are exposed to slug feeding
  • Present all year round but damage is greatest in spring and autumn
  • Favoured by moist conditions

Slug species

  • New Zealand has a large number of native slugs but only the introduced European slugs are pests. Of these, only the grey field slug (Deroceras reticulatum) can be considered a significant agricultural pest. Other species are occasionally encountered in agricultural systems, but are never numerous.

Life cycle

  • Grey field slugs mate and lay eggs all year round, but most eggs are laid in early spring and autumn.  Groups of up to 50 clear 2 mm gelatinous eggs are laid in sheltered moist places.  Young slugs begin feeding immediately after hatching. Slug development is related to temperature, and sexual maturity is reached between three and nine months of age.  Slugs function as males when first sexually mature and as females later.  Cross fertilisation is usual, although self fertilisation may occur.  Grey field slugs can live for up to 13 months, and produce as many as 300 eggs.

Feeding

  • The grey field slug feeds mostly on plant leaves but will also feed on other plant organs and occasionally dead invertebrates and other slugs. Slugs feed by rasping away plant tissue. Characteristically, major veins and the lower epidermis of plant leaves are left intact giving the leaf a “windowed” appearance.

Activity

  • Slugs are active throughout the year, but during very dry or frosty weather they cease feeding and move down into the soil or shelter under debris. Not all slugs within a population will be active at the same time. Slugs are most active on still humid nights when soil is wet
  • Moisture is critical and forms 80% of slugs’ body weight when fully hydrated. Large quantities of moisture are used to produce mucous which is used in locomotion, to prevent desiccation and as a defence mechanism when attacked by predators. They can absorb atmospheric and soil moisture through their skin but are vulnerable to desiccation. They avoid climatic conditions and substrates which would dry them out.

  • Slugs damage a wide range of agricultural and horticultural crops. They have their greatest impact during seedling establishment as they feed on seedlings of all agricultural crops. Conventional cultivation kills slugs, and crops established following cultivation seldom suffer from slug damage
  • However slugs are a danger to direct drilled crops. The drill slits formed during direct drilling favour slug survival, and focus their feeding on seeds and emerging seedlings. When high numbers of slugs are present, establishing crops can be devastated overnight. Numbers as low as 15/m2 can cause significant damage. Slugs are generally more of a problem on heavy soils and in wet areas but damage is not restricted to these situations.

Detection

  • It is very difficult to accurately estimate the number of slugs present in paddocks. The mottled appearance of grey field slugs camouflages them amongst rotting plant foliage and makes them difficult to see. Slugs can not actively burrow, but their soft flexible bodies enable them to utilise cracks and the burrows of other animals in the soil, and they can be found 20 cm, or deeper, in the soil. The bases of large or clump forming plants are also favoured sheltering spots
  • Feeding damage in established plants is often the best indication of slug presence and potential damage to new crops.  Wet sacks placed in paddocks can provide a rough estimate of whether high numbers of slugs are present, but this and similar methods are often affected by environmental and climatic factors and should not be relied on.  Details of more accurate (but time consuming) sampling methods such as defined area trapping and turf immersion techniques can be obtained from AgResearch
  • After direct drilling, slugs can be counted easily by opening the drill slits. More than two slugs/m of drill row are likely to cause damage.

If the right farming practices are carried out it is not necessary to know how many slugs are present.

Stock management

  • Excellent slug control can be achieved using stock prior to direct drilling. Grazing the old vegetation hard kills slugs, and provides actively growing leafy material which will allow better herbicide activity. Grazing with 500 ewes/ha for three consecutive nights, 300 ewes/ha for five nights or 120 cows/ha for three nights, will give up to 90% control. Stock grazing closes cracks and crevices in the soil, trapping slugs either below the soil surface, from where they can not burrow out, or on the soil surface where they are exposed to adverse climatic factors (sun and wind) and vertebrate predation. Stock trampling also directly kills large numbers of slugs, especially if most of the population is active. Farmers practising rotational grazing, or similar stock management, routinely reduce slug numbers in their pastures. Lush pastures, those allowed to grow rank, and those with thick turf mats, such as found in browntop swards, often harbour potentially damaging numbers of slugs.

Drill type and coulter selection

  • The severity of slug damage will be influenced by the type of drill used, coulter design and soil conditions. Slug survival and access to seeds and seedlings is enhanced by open drill slits and when seeds are not covered by soil. Till seeders and rotodrills offer more protection to the seeds and less shelter to slugs than triple disc type drills but will not totally prevent slug damage. Winged coulters provide more favourable habitats for slugs than coulters that only create a vertical slot. Soils which crumble to cover the seed and close the drill slits favour slugs least. Don’t select your coulter with only slug control in mind. Remember, it takes more than coulter selection to control slugs, and more than slug control to successfully establish a paddock.

Cultural practices

  • Trash and crop residues left on the soil surface provide refuges and egg laying sites for slugs.  Eliminate them before drilling. Cultivation kills slugs and as a general rule the finer the tilth and the deeper the cultivation the greater the slug mortality achieved.

Molluscicides

  • Molluscicide baits are dry and must absorb moisture to make them palatable and attractive to slugs.  Usually sufficient moisture is absorbed from the soil. Baits eventually become unattractive, and therefore ineffective, as the grain or bran base used breaks down.For maximum effectiveness, baits should be broadcast evenly over paddocks prior to sowing. Because the entire slug population may not be active and feeding when baits are applied, and because, even though the baits contain attractants, some slugs will not come in contact with them, repeat applications of molluscicide baits provide higher levels of control than a single application. Drilling baits with the seed is also an option but baits tend to break down more quickly in drill slits than on the soil surface, and results are not as good as when baits are broadcast
  • If damage appears after sowing, application of molluscicide baits is the only control option. Discuss your bait needs and application techniques with your local chemical or seed rep.