Fighting Slugs in Market Gardening

Technical articles - Insects and pests

Market gardening Market gardening Soil cover Soil regeneration Slugs

Slugs limaces and snails cause significant losses every year in productions of market gardening. And while lettuces suffer the most damage, all species are potentially affected.

Which slugs?

Two species of slug are distinguished:

Grey slug

• The grey slug (Deroceras reticulatum) which moves on the soil surface.
  • Grayish to yellowish-brown color, with elongated spots.
  • White mucus, respiratory opening at the rear of the shield.
  • Size : up to 70 mm extended.
  • In humid weather, it consumes all green vegetation.
  • In wet conditions, it attacks roots.
  • It can consume 1/3 of its weight in one night.
  • This slug is especially harmful during drought periods, as it is less sensitive to drought than other slugs and resists cold down to frost.
  • Polyphagous, it attacks various crops : protein peas, legumes, carrot, endive, strawberry, tomato and lettuce.

Black slug

• The black slug (Arion hortensis), less mobile, usually found in the soil and less often on the surface. These slugs are quite difficult to destroy.
  • Slate black color.
  • Foot (underside) yellow-orange.
  • Yellow mucus.
  • 40 mm extended.
  • Omnivorous, it can consume 40% of its body mass in 2 hours.
  • This species is mainly harmful to autumn crops in rainy and persistent weather. It causes significant damage during crop establishment by eating roots underground.
    

These two species have mainly nocturnal activity. In overcast and humid weather during the day, they are also active. In France, 80% of attacked plots are infested by grey slugs.

Development factors

Période de présence :

J

F

M

A

M

J

J

A

S

O

N

D

Slugs show a development peak in spring.

The development factors of slugs are multiple :

• Weather conditions : mild winters and rainy summers favor slug multiplication, while frost and drought reduce their activity.
• Availability of refuges : the amount of refuges depends on the vegetation cover and soil structure. Cloddy and aerated soils and burying crop residues provide a perfect habitat for slugs.
• Presence of natural enemies : slugs have many enemies, predators and parasites among invertebrates and vertebrates. Their presence helps regulate slug density.
• Agronomic conditions : soil type and preparation influence slug populations. These mollusks are especially common in clay and clay-loam soils, but sometimes show high concentrations in loams. They prefer well-aerated soils with cavities allowing movement. They are mainly found in cloddy, stony areas, in roughly prepared soils. In tunnels, they find refuge on edges and benefit from humidity. Solarization is an effective way to reduce populations.

Consequences of slug presence

Periods of high risk

On vegetables, the risk of damage from slugs can exist from emergence to harvest :

• The periods during which vegetables are most sensitive to this pest extend from April to mid-June and from September to October.
• At the end of winter, slugs attack young shoots of crops because they lack other food sources.
• In summer, slugs have difficulty moving on dry, hot soil, so they remain dormant in the soil. At each rain, they wake up and come to the soil surface. Since there are few young plants in summer, slugs attack better-formed leafy vegetables.

Damage caused

Slugs cause two types of damage to crops :

• Direct damage : by consuming aerial organs, or even underground organs (roots, tubers…), slugs and snails can cause :
  • mechanical losses (plants disappear),
  • commercial losses (vegetables soiled or holed are no longer marketable).
• Indirect damage : by being physically present in the harvest, slugs and snails cause outright rejection of the batch by commercial operators.

Risk assessment

Slug population density and activity can be estimated by observing presence indicators : mucus trails, torn leaves, and trapping.

For this, it is possible to place, before sowing, 4 traps (tiles, moistened cardboard covered with a tarp…) per plot. Counting the number of mollusks trapped 3 days later allows estimating the number of individuals per m². This count allows comparing slug pressure against damage thresholds :

• For sensitive crops (lettuces, cabbages, radish, turnips, strawberries, spinach) : from the threshold of 1 slug/m².
• For endives: 2 slugs/m².
• For alliums (leek, garlic, onion, shallot…) and bean: occasional problem.

Mat traps : work without bait, moisture alone attracts slugs. Check early in the morning.

• Importance of anticipating from crop rotation or after harvest, not just at emergence.
• The issue is managed on a multi-year scale.
• Solarization is very effective, soil preparation is beneficial, and tunnel edges can be refuge zones.

To facilitate slug counting, it is possible to use LIMACAPT, an autonomous and connected sensor for slug detection and counting in the field.

Prophylactic control methods

Slug management relies on understanding their lifestyle and reasons for presence.

It is therefore possible to implement effective prophylactic measures to manage slugs and limit damage.

Crop rotation choice

The choice of cultivated species and the succession of species directly influence slug presence according to :

• Their palatability,
• The refuge possibilities they offer,
• The winter cover they provide.

Crops covering the soil in winter such as rapeseed or cereals are favorable to good slug feeding and facilitate their development in autumn and spring. Thus, winter crops and fallows are the riskiest predecessors for following crops.

Short intercrop rotations favor gastropods the most.

Clover paspalum is very relevant because, in the short term, it acts as a refuge, localizing slugs. However, in the long term, it can "fatten" them and lead to population buildup and egg-laying. Risk of "double penalty" if palatable and irrigated crops are nearby.

Managing the intercrop period

The intercrop period is a time of greater sensitivity for these pests because their main resource disappears, whether for feeding or shelter.

• It is possible to intervene during this period to increase slug vulnerability :
  • In summer, stubble cultivation operations are particularly effective to dry out eggs.
  • Catch crops for nitrate trapping (CIPAN) are often favorable to slugs: the choice of cover and destruction date are elements to consider to limit risk. Legumes and cereals are appreciated by slugs, while some mustards are less palatable.
  • Maintaining bare soil is unfavorable to slugs if it provides neither food nor shelter.

Soil work

Soil work can help reduce slug populations by disturbing their habitat and limiting their movement and shelter possibilities.

• Soil moisture at the time of these operations is decisive : the less saturated the soil is with water, the greater the benefits.
• Closing or compacting the soil after each tillage is also very beneficial… if there is no risk of compaction or crusting.
• Deep tillage reduces populations by burying and destruction.
• Plowing also has the advantage of eliminating food sources for slugs but its effect seems limited in time (one or two weeks).
• Superficial tillage, effective in dry weather, also destroys eggs, young individuals, and adults directly or indirectly by exposing them to natural predators and sun action (drying out eggs and adults).
• It is estimated that most slugs and their eggs are in the top 10 centimeters of soil.
• To limit slug movement, when possible, in addition to fine preparation, soil rolling is recommended.
• Stubble cultivation after harvest is particularly important in summer. Eggs are then exposed to solar radiation causing significant mortality by desiccation.
• Moreover, it is recognized that burying seeds deeply enough during sowing reduces attacks.

Managing slug shelters and food

There are two types of management for slug shelters and food.

1. Remove all available food resources for slugs, especially during intercrop periods
   • It is preferable to chop and bury crop residues to prevent them from serving as shelter or substitute food.
   • Controlling weed development in the intercrop reduces population increase in the plot.
   • Also, controlling vegetation on edges prevents the formation of mollusk hotspots that can migrate from weeds to crops.
2. Offer slugs more palatable food in spring than the crops in place to avoid attacks
   • They particularly like young crucifer shoots
   • Favor shady, cool spots to attract slugs away from the garden.

Encourage slug enemies

• Wooded areas near cultivated plots would provide good shelters for many beneficials that consume mollusks : hedgehogs, toads, birds, ground beetles, rove beetles, spiders, glow-worms...
• Creating hedges or grassy strips would restore some biodiversity that would limit mollusk development as well as other pests (aphids, thrips…).

• To encourage slug enemies, provide them with a food stock : insects. One way to attract insects is to place decomposing wood near market gardening plots. A pile of decomposing wood at the plot edge will produce fungi, which attract Springtails that are at the base of the food chain. In spring, all these springtails decrease due to predation by rove beetles, which gradually turn to slugs in crops and regulate their population. From there, rove beetles and ground beetles can be attracted to the garden as soon as they start to lack food at the end of winter. Slugs can also be attracted to wood piles by sowing crucifers that slugs like.

• It is also possible to directly introduce slug enemies into vegetable plots : Indian runner ducks, chickens...
  

Manual collection

A tedious solution is to manually collect slugs by hand (or with a slug tongs).

For this, it is recommended to place a tile, a piece of cardboard, or a wooden board in the plot to offer shelter to slugs and concentrate them in one place. At dawn, simply lift the shelter and collect the slugs present. To be effective, this technique must be repeated regularly.

Natural slug barriers

There are many natural slug barriers, whose effectiveness is sometimes unproven : eggshells, ashes, coffee grounds, sand, hair, chestnut burrs, needle branches, mulch, garlic extract ...

A comparative study of these barriers is available here.

Copper

Copper is a natural repellent for slugs, snails, and other gastropods.

There are many copper-based barriers : strips, nets, rings.

• Copper adhesive tape protects potted plants or raised beds against slugs.

• The copper ring is highly effective and easy to use. About 5 cm high, it keeps slugs, snails, and other gastropods away from young plants.

Direct control

Parasitic nematodes

There are possibilities for biological control against slugs.

• The nematode Phasmarhabditis hermaphrodita is a specific parasite of slugs.
• It is currently marketed under the name Phasmarhabditis-system and authorized on all vegetable crops.
• These microscopic worms develop inside the mollusk body, causing its death.
• The product is diluted in water, then sprayed on the soil. Heavy irrigation then allows nematodes to penetrate the soil. Under certain conditions, the product's effectiveness is very good, but results are still somewhat variable.
• The very high cost (about €35 for 100m²) currently limits use to organic gardeners or market gardeners cultivating small high-value areas.

Iron phosphate

Iron phosphate (Ironmax, Sluxx HP, Métarex Ino) is the main solution in market gardening, including in Organic Farming.

The use of molluscicides allows direct control of slugs.

• Granules based on iron orthophosphate are authorized in Organic Farming at a dose of 50 Kg/Ha.
• Treatments must be applied judiciously.
  • All studies show that early applications (at sowing or before emergence) give better results, while late applications during vegetation only limit damage.
  • Field results: up to 70% activity reduction, even under high pressure.
  • Indicative doses: Sluxx HP (4–5 kg/ha before sowing, 5 kg/ha after sowing), Métarex Ino (5 kg/ha after sowing, 1.5 kg/ha in sowing row).
  • Once slugs are detected, molluscicide spreading must be done at sowing or before crop emergence. After this first treatment, plots must be monitored and application renewed if slugs persist or granules deteriorate due to rain.
  • Applications during vegetation, when crops are well developed, are not very effective, only limiting damage from an established attack.
  • Several granule spreading methods are possible, depending on available equipment : the basic and most common solution is surface broadcasting in the open, which can also be localized on the sowing row; another option is mixing granules with the seed.
    

Advice

• Favor early control, it is always more effective.
• Avoid curative applications : whether for slugs or snails, intervening when the pest is present in or on the plant is less effective.

• Slug protection should also focus on plot edges to limit reinfestation risks; it can be renewed until harvest, especially in case of snail risk.
• Regularly observe and adjust doses to activity level.
• Locate granules in the sowing row if conditions are difficult (straw, cloddy soil).
• Anticipate upstream: rotation, soil work, traps 3 weeks before sowing.
• Prevention is better: managing risk before emergence is the key to success.
  

La technique permet de favoriser la présence des auxiliaires et bioagresseurs suivants

• Indian runner ducks

La technique limite la présence des auxiliaires et bioagresseurs suivants

• Slug
• Snail

La technique est complémentaire des techniques suivantes

• Chelated iron phosphate application in slug control
• Protect spring crops from slugs
• Plant hedges
• Plant grassy and floral strips at plot edges
• Practice biocontrol in market gardening

Cette technique fait référence aux outils d'aide à la décision suivants

• LIMACAPT