AHDB Information Sheet 04 Integrated slug control
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AHDB Information Sheet 04 Winter 2018 Importance Oilseed rape Slugs are most damaging to seedlings because the growing point of a germinating oilseed rape shoot is, unlike cereals, above ground. Serious damage occurs up to the four true leaf stage. Cereals Slugs are most damaging to cereal crops when they cause seed hollowing. Each slug can kill up to 50 seeds in the first week after sowing, indicating the need for immediate control. Weight-for-weight, smaller slugs destroy more seeds than larger slugs. Feeding on shoots and leaf shredding is also important and cereals are most vulnerable up to GS14 (four leaves unfolded) but remain vulnerable up to G21 (one main shoot and one tiller). Potatoes Slugs are most damaging at the early stages of tuber bulking. They enter through small holes in the skin, causing irregular-shaped holes on the tuber surface extending into large cavities in the tuber (below). Horticulture Slugs can attack and cause damage to vegetable crops at varying stages of their development. Attack in the early stages can result in plant loss by direct feeding on seedlings. At later stages, feeding can lead to cosmetic problems. Slug damage can cause contorted growth of the growing tip, eg in asparagus, also causing rejection. The tolerance to feeding damage varies from zero in salad crops to small permitted tolerances in other crops like Brussels sprouts. Faecal and slime contamination to mature crops make them unmarketable and the presence of live or dead slugs in harvested produce will lead to rejections. Celery, carrots, asparagus, long season Brassicas (such as Brussels sprouts, winter and spring cabbage), lettuce and spinach tend to be the most susceptible vegetable crops. Early planted crops like lettuce and salad crops are known to be very vulnerable. Horticultural crops covered by nets and covers may be more susceptible to slugs, as the humidity within the crop is increased. Always read product labels, consider your local conditions and consult a professional BASIS-qualified agronomist, if necessary. Latest information – The approval for methiocarb slug pellets expired on 19 September 2015. – Metaldehyde can be detected in raw (untreated) water above the drinking water standard and was widely detected above the standard in autumn 2012. – A ban on the outdoor use of metaldehyde slug pellets is to be introduced across Great Britain from spring 2020. For details, see the statement published at ahdb.org.uk/knowledge-library/ integrated-slug-control Action – An integrated approach to slug control using several different techniques is more successful than relying solely on molluscicide pellets. – Put slug traps out before cultivation to assess slug risk. – Monitor slug activity throughout the susceptible crop growth stages. – Find out if you are in a Drinking Water Protected Area or Safeguard Zone using the new tool at www.wiyby.co.uk – Refer to the metaldehyde stewardship guidelines at www.getpelletwise.co.uk Integrated slug control Slug-damaged potato Slug-damaged oilseed rape Slug-damaged Brussels sprouts Courtesy of The Food and Environment Research Agency (Fera), ©Crown Copyright ©ADAS ©Allium and Brassica Centre
Transcript of AHDB Information Sheet 04 Integrated slug control
AHDB Information Sheet 04 Winter 2018
ImportanceOilseed rapeSlugs are most damaging toseedlings because the growing pointof a germinating oilseed rape shootis, unlike cereals, above ground.Serious damage occurs up to thefour true leaf stage.
Cereals Slugs are most damaging to cerealcrops when they cause seedhollowing. Each slug can kill up to 50seeds in the first week after sowing,indicating the need for immediatecontrol. Weight-for-weight, smallerslugs destroy more seeds than largerslugs.
Feeding on shoots and leaf shreddingis also important and cereals are mostvulnerable up to GS14 (four leavesunfolded) but remain vulnerable up toG21 (one main shoot and one tiller).
PotatoesSlugs are most damaging at the earlystages of tuber bulking. They enterthrough small holes in the skin,causing irregular-shaped holes on thetuber surface extending into largecavities in the tuber (below).
HorticultureSlugs can attack and cause damageto vegetable crops at varying stagesof their development. Attack in theearly stages can result in plant lossby direct feeding on seedlings. Atlater stages, feeding can lead tocosmetic problems. Slug damagecan cause contorted growth of thegrowing tip, eg in asparagus, alsocausing rejection.
The tolerance to feeding damagevaries from zero in salad crops tosmall permitted tolerances in othercrops like Brussels sprouts. Faecaland slime contamination to maturecrops make them unmarketable andthe presence of live or dead slugs inharvested produce will lead torejections.
Celery, carrots, asparagus, longseason Brassicas (such as Brusselssprouts, winter and spring cabbage),lettuce and spinach tend to be themost susceptible vegetable crops.Early planted crops like lettuce andsalad crops are known to be veryvulnerable. Horticultural cropscovered by nets and covers may bemore susceptible to slugs, as thehumidity within the crop isincreased.
Always read product labels, consider yourlocal conditions and consult a professionalBASIS-qualified agronomist, if necessary.
Latest information– The approval for methiocarb slug
pellets expired on 19 September2015.
– Metaldehyde can be detected inraw (untreated) water above thedrinking water standard and waswidely detected above thestandard in autumn 2012.
– A ban on the outdoor use ofmetaldehyde slug pellets is to beintroduced across Great Britainfrom spring 2020. For details, seethe statement published atahdb.org.uk/knowledge-library/integrated-slug-control
Action– An integrated approach to slug
control using several differenttechniques is more successful thanrelying solely on molluscicidepellets.
– Put slug traps out beforecultivation to assess slug risk.
– Monitor slug activity throughoutthe susceptible crop growthstages.
– Find out if you are in a DrinkingWater Protected Area orSafeguard Zone using the newtool at www.wiyby.co.uk
– Refer to the metaldehydestewardship guidelines atwww.getpelletwise.co.uk
Integrated slug control
Slug-damaged potato
Slug-damaged oilseed rape
Slug-damaged Brussels sprouts
Courtesy of The Food and Environment ResearchAgency (Fera), ©Crown Copyright
©ADAS
©Allium andBrassica Centre
Key slug species
The grey field slug (Derocerasreticulatum) and other Derocerasspp.The grey field slug is the mostwidespread and troublesomespecies. It is usually light grey orbrown, grows to 5 cm in lengthand produces milky white mucus.
Populations tend to have a mixedage structure, so damage occurswhenever conditions arefavourable for activity. It continuesto be active in damp weather andeven when temperatures are closeto freezing.
Breeding is generally at a peak inApril and May and then again fromSeptember to October. However,in favourable conditions, it willbreed throughout the year. Inoptimum conditions it can start tolay eggs within 16 weeks ofhatching.
The garden slug (Arion hortensisand Arion distinctus)The garden slug is usually smallerthan the grey field slug, growing to 3 cm in length. The body is dark andthe foot (underside) ranges fromyellow to orange. It produces orangeor yellow mucus. Egg hatchingreaches its peak in late spring/earlysummer. Young slugs can developrapidly to produce a further generationwithin the year. Arion species areonly active at temperatures above5°C and are less active on the soilsurface than the grey field slug.
The keeled slug (Milax, Tandoniaand Boettgerilla spp.)Keeled slugs are more localised inarable crops than field or gardenslugs but they can be important.They vary in size and generallyproduce a colourless mucus.Keeled slugs have annual lifecycles, with eggs hatching fromautumn to spring. All keeledspecies are generally subterraneanbut can be seen on the surface,especially during the breedingseason.
The Spanish slug (Arion vulgaris)Spanish slugs can be brown, black,fawn or mustard coloured and cangrow up to 15 cm long. Unlikeother slug species, the Spanishslug is omnivorous, eating deadanimals, excrement and plantmaterial. They produce twice asmany eggs as native slug species.
©Dr Gordon Port,Newcastle University
Slug movement occurs mostfrequently at night but they willreturn to their resting site by dawn ifweather conditions are unfavourable.They do not travel far from wherethey were hatched, often taking onlya circular route of a few metres insearch of food.
Life cycleAll slug species are hermaphrodite(each individual is both male andfemale). While some species areself-fertile, most mate before layingeggs in batches of 10 to 50 in soilcavities, between clods, understones or at the base of plants.
Up to 500 eggs per slug may be laidover several weeks. Eggs developslowly in the winter but will hatchwithin a few weeks when thetemperature starts to rise.
The number of active slugs found atany one time and place is dictated byboth the slug population density andthe suitability of the weather foractivity (activity-density). Rapidreproduction and growth is enhancedby mild, moist weather conditions,sufficient food supply and ampleshelter. Such conditions prevail in thespring and early autumn, makingcrops like lettuce and Brassicas morevulnerable at these timesof year. Adults
Juveniles
EggsTail
Foot fringe Foot Pneumostome(respiratory opening)
Mantle Opticaltentacles
Sensorytentacles
©Dr Ian Bedford
Risk factorsMoisture and temperatureActivity, survival and reproductionare dependent on temperature,moisture, light and soil structure.The optimum temperature for slugsis 17°C but they are active between5°C and 20°C. The grey field slug isactive even when temperatures areclose to freezing.
Crops grown under fleece orpolythene covers provide warm,damp and occasionally weedyconditions favourable for slugs.Irrigating crops after planting resultsin a conducive environment for slugactivity.
Soil typeSlugs are more abundant in heavysoils with high clay or silt content.
Previous croppingSlug damage is much greater afterleafy crops which create moist soilconditions. New plantings thatfollow crops that have a longgrowing period are moresusceptible.
Crop residues, organic matter andweedsCrop residues or applications ofmanure, especially in the autumn, aswell as weeds and volunteers,provide slugs with a source of foodand shelter.
Slugs prefer soft, nutritious tissueprovided by young ground plantsrather than more fibrous material. Adamp environment combined withsucculent food is essential for theirsurvival, so leafier crops that offerground cover are often preferred.
CultivationDirect drilling, as well as delayeddrilling, increases the risk of slugdamage.
Open, damp and cloddy seedbedsallow slugs easy movement andprovide more shelter than friable,frequently cultivated soils.
Crop typeAutumn crops are slow growing andso are more at risk than spring-sowncrops. Barley and oat seeds have anextra seed coat, so are lessvulnerable to attack than wheat.There is variation in susceptibility toslug damage between potatovarieties but there are noindependent variety resistanceratings available.
Small fields surrounded by ditches,wasteland, hedgerows or greenfallowIn the field, slugs tend to be evenlydistributed depending on soilmoisture. The risk of damage can beincreased on the perimeters offields. Headlands are close to fieldboundary vegetation and are oftencompacted, with poor drainage. Thiscan create a moist refuge for slugs.
Other agronomic conditionsLack of nutrients, poor drainage andweed competition can all result inslow growth, prolonging thevulnerable period of establishment.
Cultural controlCultivationSeedbed preparation and quality arepotentially more important thanchemical control of slugs, particularlyin combinable crops. Ploughing is agood way of reducing slugpopulations but even minimum tillagegives a considerable reduction in slugdamage compared to direct drilling.
Seedbed cultivations will increasemortality depending on machineaction, soil type, timing of cultivation,depth and intensity. Firm seedbedsreduce slug activity by making itharder for them to move around andreducing the availability of saferesting places.
A fine, consolidated seedbed isimportant and will protect seeds andprevent slugs accessing seedlingsbefore emergence. A consolidatedseedbed is important in providinggood seed to soil contact, enablingseeds to germinate quickly and growrapidly through the vulnerableestablishment stage.
Increase sowing depth of wheat to4–5 cm if the seedbed is cloddy.
Minimise weedsMinimising weed growth inpreceding crops and seedbeds willreduce sources of food and shelter.
Beetle banksThe establishment and managementof beetle banks in field margins ashabitats for carabid beetles has beenshown to reduce slug numbers bypredation, mainly from June toSeptember.
Carabid beetles
©Roger Umpelby
Monitoring for slugsTo assess the risk of crop damage, itis important to estimate the size ofslug populations present. Samplingin the field is best done using refugetraps. Put slug traps out beforecultivation, when the soil surface isvisibly moist and the weather is mild(5–25°C). When soil conditions aredry and slugs are not activelyseeking food, trapping will have littlevalue in determining the threat to thecrop.
Traps consist of a cover about 25 cmacross, such as a plant pot saucer,with a small heap of bait underneath.A suitable bait would be two heapedspoonfuls of chicken layers' mash ora cereal grain-based food (NOT slugpellets). Leave a small gap betweenthe trap and the soil to allow slugs toenter. It may be necessary to put aweight on the trap in windyconditions.
In each field, nine traps (13 in fieldslarger than 20 ha) should be set outin a ‘W’ pattern spread over theentire area of the field. Also,concentrate on areas known to sufferdamage. In standing crops, place the
traps just to the side of tramlines andmark with canes to allow them to belocated.
Leave traps overnight and examineearly the following morning while thesoil surface is still moist, counting thenumber of slugs present and notingany slime trails. On warm days, it isimportant to check the traps earlywhile the temperature is still cool, asslugs will leave the trap as it getswarmer.
If no slugs are found, continue to trapuntil crops have passed theirvulnerable stage.
The following thresholds indicate apossible risk when soil and weatherconditions favour slug activity.
Monitoring slug damageOilseed rape crops should bemonitored regularly for slug damagefrom sowing to the four true leafstage when the crop is vulnerable.
Winter cereal crops should beregularly monitored from sowing tofirst tillering (GS21). Damage afterthis stage is less likely to result infurther plant loss but monitoringshould continue through winter.
Potatoes have two critical controlperiods (below), at 50% to 75%canopy closure and at the earlystages of tuber bulking.
Closely monitor slug activity duringthese periods to determine if controlis necessary. Continue to monitorthe crop until burn down.
Lettuce crops should be monitoredin January/February and Brusselssprout crops in March/April.
Criticalcontrolperiod 1
Criticalcontrolperiod 2
Typical interval4 to 6 weeks
Potato illustration courtesy of Bayer CropScience
Refuge trap using chicken layers'mash bait
Crop Threshold (average number of slugs/trap)
Winter cereal 4
Oilseed rape (standing cereals) 4
Oilseed rape (cereal stubble) 1
Potatoes 1
Field vegetables 1
Biological controlBiological control of slugs isparticularly suited to organicsystems. Phasmarhabditishermaphrodita is a nematodeparasite of slugs that is commerciallyavailable as a biological molluscicide.Unlike pellets, nematodes can targetsoil-dwelling slugs, as well assurface-active slugs, although slugsmay avoid feeding or resting onsubstrates treated with nematodes.
The nematodes enter the slug'sshell sack (mantle), where bacteriafrom the nematode's gut arereleased and start to multiply. Thenematodes feed on the bacteria andthe bacteria cause the slug's mantleto swell and burst. It usually takesbetween 7 and 21 days for aninfected slug to die but its feedingstops soon after the nematodeenters the body, protecting plantsfrom damage. When the slug dies,the nematodes will continue to feedon it until the food source isdepleted, before leaving in search ofmore slugs.
P. hermaphrodita will kill most slugs.The grey field slug is highlyvulnerable but larger species areonly susceptible when they areyoung and small. Therefore, when
larger species are likely to causedamage, it is important to applynematodes in spring when juvenileslugs are present.
Nematodes are best applied in dullweather, in the evening and beforerain. They can be applied in advanceof expected damage, at sowing orany time during the crop’s lifetime;however, success is dependent onwet conditions after application.In ideal conditions, nematodes will
provide a reduction in damage forabout six weeks after application. Toget the best out of the product, itmay need to be used soon afterpurchase: always follow theinstructions carefully.
The use of nematodes is most viablefor high-value crops. They are alsoextremely useful in situations whereit can be difficult to target the slugseffectively with molluscicide pellets,such as mature lettuce crops.
Chemical control
Ferric phosphateFerric phosphate is suitable fororganic systems. It causes slugs toquickly stop feeding; they thenbecome less mobile and die within3–6 days. As slugs often dieunderground, effectiveness of thetreatment should be measured bythe decrease of feeding damage inthe crop.
MetaldehydeMetaldehyde is a selectivemolluscicide and principally acts onslugs by inducing excessive
secretion of mucus, leading to subsequent dehydration and death. At high temperatures of around 20°C, the activity of metaldehyde is optimised; at low temperatures its toxic effect may be diminished. Metaldehyde does not harm predatory ground beetles, which can help to restrict slug populations.
A ban on the outdoor use ofmetaldehyde slug pellets is to beintroduced across Great Britainfrom spring 2020. For details, see the statement published at:ahdb.org.uk/knowledge-library/integrated-slug-control
Applications of slug pellets should be integrated with cultural control measures. The effectiveness of pellets isdependent on their chemical content, constituents which affect their attractiveness to slugs and their durability underfield conditions. Large slugs need to ingest more of the active substance than smaller ones to cause death so it iscritical that the pellet is palatable to slugs to ensure enough bait is eaten. If there is too much active substance in apellet, slugs may detect it and stop feeding before a lethal dose is consumed.
www.getpelletwise.co.uk
Slug infected with Phasmarhabditis hermaphrodita
©ADAS
Application method and timingApplication in cereals and oilseed rape: Broadcasting pellets is the best method of application. Broadcasting kills slugs more quickly than pellets drilled with seeds and can facilitate stewardship. It gives more consistent slug control, particularly in combination with fine, firm seedbeds that help protect seeds and seedlings.Broadcast slug pellets as soon aspossible after drilling. Application ismost effective up until the four trueleaf stage in oilseed rape and GS14in wheat.
Admixed slug pellets (pellets mixedwith seed at drilling) are ineffective infine seedbeds because both seedsand pellets are unavailable to slugs,which survive to attack emergingseedlings.
Pellet admixtures with wheat seedswhen direct drilling or in open cloddyseedbeds can be effective.
The best time to apply pellets is justbefore the susceptible crop stage,when optimum weather conditionsprevail. It can be beneficial to repeattreatment when new feeding damageis observed, when traps indicate anincrease in activity or if pelletsdisintegrate or go mouldy. It isimportant to note that pellets can berendered ineffective after prolongedheavy rain.
Critical control periods for potatoes:1. 50% to 75% canopy closure,usually in late June to early July.The canopy is sufficiently open toallow pellet penetration.
2. Early stages of tuber bulkingbefore slugs go underground tofind developing tubers. August isthe pivotal month for follow-upapplications and when damageusually begins to appear.
Sensitive phases for fieldvegetablesThe sensitive phase for lettuce andcelery lasts for the whole of thegrowing period. Pellet applicationbefore a crop is planted may bejustified.
Brussels sprouts are most sensitiveat the seedling stage and whenharvestable buttons start to develop.Application of slug pellets should belimited to the sensitive phases only.
Metaldehyde slug pellet stewardshipKILL SLUGS NOT METALDEHYDE
Metaldehyde is often detected in rawwater above the drinking waterstandard, with peaks followingrainfall. While levels detected poseno danger to health or theenvironment, the UK’s environmentagencies and Defra are responsiblefor the implementation of the WaterFramework Directive (WFD). Unlessaction is taken, regulatoryrestrictions, or even withdrawal, maybe enforced.
What’s in Your Backyard?Find out if you are in a DrinkingWater Protected Area (DrWPA) orSafeguard Zone by entering yourpostcode into the EnvironmentAgency’s online tool ‘What’s in YourBackyard’ www.wiyby.co.uk
Use WIYBY tocreate a targetedslug controlprogramme that isup to date andtailored to yourindividualsituation.
Application guidelines– Use minimum active per hectare toavoid drainage and run-off losses.
– Maximum application rate:210g metaldehyde a.s./ha*. Foradditional protection of water,BASIS-qualified suppliers or advisorsmay recommend rates reduced to160g a.s./ha or less*.
– Maximum total dose from 1 Augustto 31 December: 210g metaldehydea.s./ha*. For additional protection ofwater, BASIS-qualified suppliers oradvisors may recommend ratesreduced to 160g a.s./ha or less*.
– Maximum total dose rate: 700gmetaldehyde a.s./ha/calendar year*.
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No pellets to be allowed to fall within a minimum of 10 metres of any field boundary or watercourse.
Do not apply when heavy rain is forecast.
– If drains are flowing, do not applymetaldehyde-based slug pellets.
*from any combination of metaldehydeproducts. 700g is also the statutory limit.a.s. = active substance (or activeingredient)
Further information
www.ahdb.org.uk/integrated-slug-control
The Metaldehyde StewardshipGroup www.getpelletwise.com
What's in Your Backyard?www.wiyby.co.uk
Project Report 393: Integratedslug control in arable crops: Riskassessment, trapping, agronomyand chemical control (AHDB,2006)
Research Review 79: Implicationsof not controlling slugs in oilseedrape and wheat in the UK (AHDB,2014)
FV 379: Leafy salads & Brassicas:slugs – a review (AHDB, 2010)
Publication orders0845 245 [email protected] [email protected]@ahdb.org.uk
AHDB Information Sheet 04Summer 2017Produced by AHDB Cereals & Oilseeds, AHDB Horticulture and AHDB Potatoes.
An electronic version can be downloaded at cereals.ahdb.org.ukTo join the mailing list, contact: [email protected]/[email protected]
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