Slugs

Milax gagates (Black Keeled Slug)

Deroceras reticulatum (Reticulated Slug)

Family: Helicidae

Order: Eupulmonata

Class: - Gastropoda

Description: Black Keeled Slug

Adult -

Colour - Black to brown.

Body - Elongated soft and slimy, 40-60 mm long with a ridge down its back.

Wings - None

Mouthparts -

Antennae -

Legs - None

Head -

Thorax -

Abdomen -

Egg -

Habits - Leaves a silver trail. They burrow 200 mm or more into the soil to escape the heat of summer and emerge after rain or when surface soil temperatures decrease.

Description: Reticulated Slug

Adult -

Colour - Light grey to fawn with dark brown mottling.

Body - Elongated soft and slimy, up to 50 mm long.

Wings - None

Mouthparts -

Antennae -

Legs - None

Head -

Thorax -

Abdomen -

Egg -

Habits - Leaves a silver trail. There is usually a generation in autumn and another in spring but they can breed anytime conditions are favourable. They are often found on the surface at night and shelter under rocks, debris or in soil cracks during the day and over the summer.

Biology:

The body remains moist making them susceptible to dehydration. They tend to remain in the soil or under debris to avoid drying out. Often they are only seen at night or after rain when the humidity is high.

They produce a mucous slime when they move leaving a typical silver snail trail.

They are mainly active during damp weather when temperatures are 15-25 degrees C. They are less active during heavy rain and in high winds.

They are hermaphrodites, all individuals may lay eggs. Mating usually takes place in mid autumn to mid winter. The eggs are laid into moist soil and cannot survive dry periods. Reticulated slugs can breed whenever moist conditions occur.

Life Cycle:

In spring, they seek shelter in the soil or under debris and aestivate to avoid the hot ground temperatures over summer. They become active again after the autumn rains. 1-2 mm of rain triggers feeding. Mating occurs 2-3 weeks after good autumn rains and lower temperatures and egg laying commences soon after mating. Egg clusters are laid in the top soil from autumn to spring. In moist conditions Reticulated Slugs will continue breeding. Eggs hatch about 2 weeks after laying. The juveniles feed in winter and spring and aestivate over summer to become sexually mature at one year old.

Habitats:

Underground or in protected areas.

Prefer loamy to heavy soils. They can't survive in light sandy soils or in compacted soils.

Origin and History:

Distribution:

Significance:

Major pest of Canola, Pulses and cereals on some soils.

Major pest of vegetables including potatoes.

Slugs and especially Black Keeled Slugs can feed on legume seeds in the planting furrow. They don't feed on Canola or cereal seed but will attack all plants soon after germination. The major economic damage is usually from feeding on young seedlings.

Damage is usually irregular pieces missing from the leaf edges or the removal of cotyledons in broad leaved crops resulting in plant death. Damage can be difficult to see if seedlings are being chewed down to ground level as they emerge. Cereals often recover from early damage whereas broad leaved crops often don't recover even after treatment.

Management and Control:

Baits are often used. They usually reduce the population by about 50%.

Look for them in the middle of the night or put out carpet squares or baits and look for them the next morning.

Cultivation provides some control.

A combination of cultural, chemical and biological control are usually required to provide control.

Graze or burn stubble to remove refuges.

Kill summer and autumn weeds and plants along fence lines to reduce food supplies and refuge areas.

Burning in autumn doesn't kill slugs but tends to reduce the population by removing food and shelter.

Monitor paddocks for snails in autumn before planting and lay baits early before egg laying commences in autumn. Look for them on moist, warm still nights or search for silver slime (mucous) tracks in the morning. Or lay a line of slug pellets with a rabbit baiter as the freshly disturbed soil will attract slugs to the pellets where they die and can be easily observed.

Use fortnightly applications of baits at lower rates (e.g. 5 kg/ha) rather than a single high rate of bait.

Bait refuge areas such as fence lines.

Use control options that minimise damage to biological control agents such as Ground Beetles.

Avoid liming paddocks as this aids survival of the snails.

Spring baiting is often ineffective because many populations are relatively immobile juveniles and there is ample alternative feed.

Cultivation reduces Reticulated Slug numbers but is not effective on Black Keeled Slugs.

Replant areas of broad leaved crops that have been damaged at emergence.

Thresholds:

Place slug pellets under wet carpet squares or Hessian bags that are 32 x 32 cm (1/10 square metre) on the soil surface and leave for a day or two then count the number of dead slugs and multiply by 10 to give an estimate of the slugs per square metre. 1-2 per square metre are worth treating in oilseeds and pulses and 5 /m2 are usually worth treating in pasture. In cereals the threshold is about 1-2/m2 for Black Keeled Slug and 5/m2 for the Reticulated Slug.

Biological Control:

Native Ground Beetles from the Carabidae family are general predators and help keep slug populations down. Baits containing mesurol can kill these beetles by secondary poisoning.

Related Species:


Similar Species:

References:

CSIRO. The Insects of Australia. Melbourne University Press. (1991)

WADA. Insects and Allied Pests of Extensive Farming. Department of Agriculture - Western Australia Bulletin No. 4185.

Micic, Svetlana, Henry, Ken, and Horne, Paul. (2007) Identification and control of pest slugs and snails for broadacre crops in Western Australia. Bulletin 4713. Department of Agriculture and Food Western Australia. Perth, 2007.

Acknowledgments:

Collated by HerbiGuide. Phone 08 98444064 or www.herbiguide.com.au for more information.