Pest Identification

Deroceras reticulatum (Gastropoda: Agriolimacidae)
Roundback slugs Arion spp. (Gastropoda: Arionidae)

Identifying different life stages
Slugs have three life stages; egg, immature and mature adult.

Eggs of most slug species are typically small, 3-4mm in diameter, and spherical. They are often clear and appear gelatinous.

Immature slugs emerge from eggs and are morphologically similar to full grown adults, only smaller. The immature slugs develop and grow in size till they become sexually mature at which point they are classed as reproductive/mature adults.

All slugs, possess 2 pairs of forward pointing tentacles, one pair for sight the other for smell. Behind the head is a structure called the mantle. The mantle contains the respiratory apparatus and the breathing hole, pneumostome, almost always located on the right side of the body (Figure 1). Behind the mantle is the tail and along the underside is the foot upon which the slug moves through rhythmic waves of muscle contractions. Slugs commonly secrete mucus upon which they move, leaving characteristic trails behind them.

The grey field slug (Deroceras reticulatum) can be differentiated from most other slugs by its size and colouring. It is a small slug, up to 4cm long when fully mature and fully extended. Appearance can range from grey to brown in colour with darker brown flecks (Figure 2). The mucus is colourless or white

Life Cycle

Individual slugs are capable of breeding throughout the year but do so typically in times of favourable conditions, warm and wet weather.

Adults are hermaphrodites and once mated are capable of laying up to 500 eggs in batches of 20 -80. Eggs are laid in unexposed locations to avoid hot and dry environments. Eggs take between 2 to 3 weeks to mature and hatch. The emerging mobile immature stages immediately start feeding on susceptible plant material and take approximately one to two months to achieve sexual maturity. Due to dependence on wet, warm weather for survival, motility and hence breeding, sharp increases in the slug population numbers are often observed in spring and autumn where conditions are most favourable. Deroceras reticulatum typically live on the soil surface or in the top 2-3 cm of the soil profile but will move deeper during cold periods to avoid ground frost.

Sex

Adults are hermaphrodites, containing both male and female reproductive organs. Individuals rarely self fertilize preferring to mate with another con-specific individual.

Life stage that causes damage

Except for the eggs, all slug stages are capable of damaging susceptible plants.

When and where to identify

Slugs are particularly active in periods of high water availability, for example after rain fall. The grey field slug is nocturnal and can be easily found active on the soil surface, during wet periods, throughout spring, summer and autumn. During the day and dry periods, slugs are harder to locate as they typically will hide in sheltered areas, to avoid exposure to hot, dry environments.

Slug monitoring within a crop can be done by placing a small pile of bran underneath an upturned plant pot tray. The tray should have a stone on it to hold it in place. Monitoring stations should be checked during early morning when slugs will be on the surface before slugs move into the soil.

Geographical location

Slugs are found throughout temperate regions of the world. Human activities have helped with the spread of some slug species, for example the grey field slug D. reticulatum. Originally endemic to Europe, the grey field slug is now present in Asia, North and South America, Australia and New Zealand. Within these areas the grey field slug is almost exclusively restricted to cultivated areas.

Damage

Timings

Larvae feeding will occur from July to early November, when the larvae feeding on the plant root system will stop when temperatures fall and the larvae move deeper into the soil to avoid the cold winter temperatures. When this movement occurs will vary depending on geographical location. Typical high risk periods for seeing turf damage are September and October when the larger larvae consume more grass roots.  
Adult feeding will occur through late June and into July, however, It is rarely noticeable.

What and where

The first larval stage feeds on dead organic matter and small grass roots in the soil at a depth of 5 to 10 cm while, the second stage larvae feed on progressively bigger roots. The most damage is caused by the third stage larvae as they feed in preparation for hibernation when they are capable of eating the main grass roots, which severs the water supply and kills the plant. It is at this time that the major discolouration of turf grass is seen.

Symptoms

In potato crops, tubers are also susceptible to infestation with subterranean slug feeding, resulting innotches and tunnels in the surface of the tuber (Figure 5). Tubers are especially susceptible after burn down when the foliar cover is removed and the tubers are swelling just before harvest. Similar tunnelling damage can also occur to developing strawberry fruits.

In asparagus, slugs feed on spikes before they emerge from the soil leading to deformed growth. In cauliflower, slug activity on the plant head results in browning of the white cauliflower. Young seedlings are particularly vulnerable to slug infestations and can suffer high levels of mortality. Only a very low level of feeding on a seedling is needed to cause deformed growth or plant death.

As well as crop damage, a sign of slug activity is the presence of mucus trails which have a shiny silver appearance once they have dried.

Susceptible plant species

Most pest slug species are polyphagous and are capable of feeding on a wide range of different plants and crops. Slugs can also survive feeding on organic matter within the soil ift fresh plant material is not available.

The grey field slug, the most damaging slug pest, is known to infest cereals, sugar beet, oil seed rape and legumes. It is a common pest of vegetables such as potato, cauliflower, cabbage, brussel sprouts, asparagus and salads such as lettuce and celery. It will also damage fruit crops such as strawberries, and hardy ornamental stock, including Hostas.

Crops Affected by Slugs

• Lettuce
• Celery
• Cauliflower
• Cabbage
• Brussels sprouts
• Potato
• Asparagus
• Strawberry
• Hardy ornamental plants (e.g. Hostas)

Nemaslug^®

Nemaslug^® is based on the mollusc pathogenic nematode Phasmarhabditis hermaphrodita and provides control of a wide range of slug species. Nemaslug^® is mollusc specific and will kill a wide number of slug species, including the most important field slug Derocereas reticulatum. Nemaslug^® will also control other Derocereas species, many of the important Arion species and soil dwelling snails like Oxyloma species.

Nemaslug^® contains nematodes in their vigorously infective juvenile stage. These aggressive organisms actively seek out slugs and enter them through their natural openings. Once the slug is infected it stops feeding within a few days and then will move away from the crop and die underground approximately one week later. The nematodes then reproduce inside the slug and release a new generation of infective juveniles which disperse in search of further slugs.

Nemaslug^® is an unique product that provides superior performance over other control methods by killing slugs in the soil.

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