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P R A C T I C E N O T E

231 Corstorphine RoadEdinburghEH12 7AT

www.forestry.gov.uk

INTRODUCTION

Since their introduction into Britain in various locationsbetween 1876 and the 1920s, grey squirrels have spreadrapidly. They have displaced the red squirrel throughoutmost of England and Wales and in central and southeastScotland1. In addition to displacing this native species theyfrequently cause damage to woodlands by stripping barkfrom the main stem and branches of trees2.

Such damage acts as a major disincentive to the planting ofbroadleaved and coniferous trees for timber as it reducesthe value of the final crop. Increasingly wider impacts arebeing recognised as potentially of major significance towoodland conservation, biodiversity and sustainability. Intime damage may lead to a loss of particularly vulnerablespecies (e.g. beech) within the mature canopy of woodlandsand this may be accompanied by loss of associated fungaland invertebrate fauna and their predators. In addition,there may be indirect competition, e.g. for food, betweengrey squirrels and native fauna such as the red squirreland common dormouse; or predation by grey squirrels,e.g. on woodland bird populations. Grey squirrels also carrysquirrelpox virus, an infection fatal to red squirrels3,4.

Many people enjoy the grey squirrel as a regular andapproachable resident of our woodlands, parks and gardens.It is not practical to exterminate grey squirrels from areaswhere they are already established. However, targetedcontrol is often necessary to reduce or prevent damage.This Practice Note focuses on the control of bark-strippingdamage by grey squirrels in woodlands. Factors influencingdamage and its occurrence are described, followed byguidance on damage risk assessment and best practicemethods to control populations and reduce damage.

Grey squirrel control in urban areas is described in anArboriculture Advisory and Information Service Note5.The control of grey squirrels for red squirrel conservationis covered in Forestry Commission Practice Note 5: Redsquirrel conservation6. As research to determine the impactof grey squirrels on woodland birds has only recentlystarted, guidance on this subject is not yet available.

FCPN4 (REVISED)

A U G U S T 2 0 0 7

BARK STRIPPING DAMAGE

Bark stripping damage (Figure 1) usually starts at the endof April and continues until the end of July (early Septemberin high-risk years). Grey squirrels do not strip bark at anyother time of year7. Trees may be stripped anywhere onthe main stem and branches, with vigorously growing anddominant trees generally being most affected. Damage levelsvary between years and across sites within the same year.

Planted or naturally regenerated trees aged between 10 and40 years, especially sycamore, beech (Figure 2), oak, sweetchestnut, pine, larch and Norway spruce, are mostvulnerable to damage. However, other species (especiallybroadleaves) and age classes may be damaged, particularlyin high-risk years (Table 1). Trees younger than 10 yearsare not normally damaged because their stem andbranches are too small (<50 mm diameter) to support agrey squirrel. Bark on the main stem of trees older than 40years is normally too thick to strip, but grey squirrels willstrip the thinner bark on the larger branches in the crown.The planting of ‘sacrificial’ species in mixtures has notbeen shown to reduce damage to ‘high risk’ species8.

Damage is recognised to be associated with high numbersof squirrels, especially juveniles entering the populationduring early summer9, and tends to occur at densities of 5or more squirrels per hectare (ha). This occurs when therehas been successful spring breeding (from January toApril) following a good seed/mast crop the previous

B Y B R E N D A M A Y L E , M A R K F E R R YM A N A N D H A R R Y P E P P E R O F F O R E S T R E S E A R C H

Controlling Grey SquirrelDamage to Woodlands

AlastairBaxter

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Figure 2

Fresh basal stripping damage to young beech tree; note the barkfragments on the ground. This type of damage is often confusedwith similar bark stripping carried out by rabbits during winter.

Figure 3

Top blown out of40-year-old NorwaySpruce followingsquirrel damage(Forest of Dean).

Damage risk will be highest where stands ofvulnerable trees are adjacent or in close proximity tomature mixed woodland areas producing good seedcrops, as these will support high squirrel densities.

Trees from which the bark is most easily stripped bysquirrels are generally the fastest growing and those withthe most sap. They discard the outer bark (an importantfeature for identifying fresh damage – see Figure 2) andeat the unlignified tissue beneath, implying that it is afeeding activity10. However, squirrels strip bark when foodand water are plentiful, and damage is most likely to bethe consequence of aggressive interactions between youngsquirrels. In general the greater the number of juvenilespresent, the greater the number of interactions whichoccur. This in turn results in more damage9.

Tree seeds, particularly from large-seeded broadleavedtrees, are the principal food of grey squirrels; seedabundance controls population densities11. Grey squirrelsalso feed on tree buds, shoots and flowers and willscavenge bird tables and litter bins. In poor mast years,wheat provided by game-keepers is a significant additionto the squirrel’s diet. Roots, bulbs, invertebrates, birds’eggs and nestlings may also be taken.

Effects of damage

Up to 5% of damaged trees may die and many more willhave degraded timber value through stem deformation, rotand broken tops. Oak, poplar, Scots pine and Norwayspruce are particularly vulnerable to stem breakage(Figure 3). Fungal invasion at the damage site causesstaining and rotting, reducing the value of the timber.Callusing is common (Figure 4) and disguises damage orstaining present in the timber at felling age. There may be areduction in annual increment of up to 2 yield classes, anddamage to branches in the canopy may cause dieback, withtimber yield being affected if 30% of the canopy is lost.

Figure 1

Severe grey squirrel bark-stripping damage (pale patches), in afarm woodland plantation.

autumn. Note that average summer densities in mixedwoodland habitats are around 8, but may reach 16 per ha.

Control measures are most likely to be required aroundthese vulnerable stands, especially where timber productionis an objective and where the value of the trees, includingamenity benefits, is sufficient to justify the cost. Damagemay also occur to young trees in continuous cover stands.

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DAMAGE PREDICTION

Predicting years of high squirrel damage will enable bettertargeting of control, reducing both costs and the numberof squirrels killed. A new method (Index Trapping) iscurrently being developed. This explores the relationshipbetween grey squirrel populations, winter foodavailability, spring breeding success and damage thefollowing summer12. Grey squirrel traps are set during aweek in early January in habitats likely to hold high-density squirrel populations (i.e. mature broadleaved ormixed woodland) adjacent to damage vulnerable habitat.

Figure 4

Cross-section from beech stem at felling showing callus arounddamage.

If no squirrels are trapped this indicates that squirrel numbersare very low or that natural food availability is high andthat early breeding, and a high risk of damage betweenApril and July, is likely (Table 1). Confirmation of the factorexplaining the low trapping results can be obtained byevaluating natural food availability during the same week.

Any animals captured are killed and assessed for breedingcondition (it is an offence to release a captive greysquirrel). Males with dark, prominent scrotal testes withstaining are likely to be reproductively active13. Femalesare checked for pregnancy (number of embryos in theuterus) and lactation (nipples raised and hairless, andexpressible milk)14. This provides further information onwhich to base damage-risk decisions.

Results from the studies currently under way will be usedto provide guidance on trapping protocol and interpretationof the information to identify high, medium or lowdamage-risk years and on the intensity and extent ofcontrol required to minimise damage. This guidance willbe published as soon as it becomes available.

DAMAGE CONTROL STRATEGY

Control for tree damage prevention should aim to reduceresident squirrel densities to below damaging levels (<5per ha) just prior to and during the main damage period(April–July). It should be targeted in good squirrel

Scenario 1 Scenario 2 Scenario 3

Autumn tree seed availability in highdensity habitats

Poor Moderate High

Winter breeding None Some High

Overwinter survival Poor Moderate Good

Winter trapability Good Moderate Poor

Population size during the followingApril–July

Low Moderate High

Damage level in damage-vulnerablehabitats near high density habitats

Very low Moderate High

Control effort None

ModerateTargeted to high density

habitats adjacent to damage-vulnerable habitats

HighWidespread. Control in all highdensity habitats within 1 km of

vulnerable areas

Table 1 The rationale behind the Index Trapping method based on three different scenarios leading to three different controlrequirements (after Gurnell and Pepper, unpublished).

In poor seed years, overwinter survival and breeding will be poor, leading to relatively low populations the following spring and summer and hence reduced risk of bark-stripping damage. In ‘mast’ years with high numbers of broadleaved tree seeds available, survival and breeding overwinter will be good, leading to high spring/summerpopulations and high damage risk. Control will need to be intensive in high density habitats near to vulnerable habitats.

Dark banded areas show link between winter trapability and expected damage level in damage-vulnerable habitats.

the bulk purchase of materials and bait, and the collectivedisposal of unused bait, empty containers, contaminatedpersonal protective clothing and other equipment.

If it is not possible to target control within holding areasthen it should be located around the perimeter ofvulnerable plantations, particularly at likely points ofsquirrel incursion from the holding areas. Control shouldnot be located within the centre of vulnerable plantationsas this is likely to draw squirrels into the area, andincrease the risk of damage occurring.

Conflicts with pheasant rearing

Pheasant feeding stations provide an alternative foodsource for grey squirrels, particularly when there is littlenatural food available. This is likely to influence greysquirrel breeding and survival and should be taken intoaccount in damage prediction and control strategies. Ifsquirrels are drawn to pheasant feeder locations thesesame locations can be used to site traps and hoppersduring the control period. However, it is generallyinsufficient to rely solely on the pheasant feed sites andadditional trap/hopper sites will be required15.

Cost effectiveness

Control efficacy should be monitored through the seasonusing maps of the control areas and locations of thewoodlands vulnerable to damage. The numbers of squirrelskilled per ha should be recorded or estimated from theamount of warfarin bait used (200–250 g required to killone squirrel depending on bodysize16). It may be necessaryto move traps or hoppers that are clearly not being visitedby squirrels to new locations. Damage assessment invulnerable plantations at the end of the summer will showthe efficacy of the control operation. This can be doneusing either a quick visual assessment or by the ‘NearestNeighbour’ assessment method17. Comparison with areaswithout control will be helpful. If unacceptable damagehas occurred this will suggest that control was eitherincorrectly targeted in time or place, or insufficientlyintensive or widespread. This information should be usedto help in the planning of future control programmes.

Chemical reduction strategies

UK Approved Codes of Practice for pesticide userequire that pesticides are only applied where and whenthey may reasonably be expected to prevent unacceptablelevels of damage. Users are also expected to take steps toreduce pesticide usage through ‘integrated crop management’

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‘holding’ habitat adjacent to damage-vulnerableplantations. However in areas managed under ‘continuouscover’ holding and vulnerable areas will be intimatelymixed. Studies have shown that all resident squirrels in apreviously marked population can be trapped or poisonedwithin 5 weeks15. However, grey squirrels are extremelymobile and can recolonise isolated woodland within 3months and a non-isolated area within 1 month. Thuskilling squirrels at any other time of year will not reducesubsequent levels of squirrel damage.

The timing of control is important. Starting too early willallow grey squirrels in neighbouring areas to move inbefore the beginning of the damage period, resulting in agreater control effort and unnecessary killing of animals.Tree seed remaining on the woodland floor will also reducethe efficacy of the control measures9. Starting too late maynot allow time to reduce the population sufficiently toprevent damage. Control, using live traps or warfarinpoison (see pages 6–13), should continue throughout thedamage period to remove recolonising animals.

Resource availability and squirrel density will also influencethe timing of control. It may be necessary to start earlierrather than to use more intensive control where the areato be controlled is extensive or there are very high squirreldensities. If many areas are to be controlled with limitedresources (more likely when live trapping is used) thencontrol will need to be carried out in a pulsed manner,with each area targeted at different times starting at themost central site (see Trapping programmes andmanagement, page 10). This may necessitate an earlierthan optimum start to control in the first area. However,it is illegal to use warfarin poison before 15 March.

In amenity areas where timber production is not anobjective the decision not to control may be taken. However,consideration should be given to the fact that trees may beaffected in different ways: some may lose their naturalform, others may have a reduced life expectancy as aresult of secondary fungal infection, some may not evenreach mature canopy stage, and mature trees may dropdamaged branches, presenting a public safety hazard.

Collaborative squirrel control

Effective squirrel control may be difficult where theholding and adjacent damage-vulnerable woodlands are indifferent ownership. Collaborative control, through eitherinformal or formal ‘Squirrel Control Groups’, enablesbetter planning of control over wide geographic areas. Italso provides the potential for joint contracts for control,

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or other strategies. The voluntary forest certificationscheme offered by the UK Woodland Assurance Standard18

imposes stringent requirements for pesticide reduction. Inrelation to grey squirrel control, targeting control tomoderate and high-risk years, and in high-density habitatsin close proximity to vulnerable habitats, provides ameans to reduce chemical (warfarin) use in the environment.

CONTROL METHODS

Warfarin poisoning and live trapping using baited cagesare the most effective methods of reducing grey squirrelpopulations. The live-trapping method was developed toprovide a more effective alternative to shooting or tunneltrapping, and is easier for relatively inexperienced operatorsto use. The use of warfarin poison has been favoured inmost commercial woodland situations because it is lesstime consuming and hence a less expensive operation whilebeing at least as, if not more, effective than live trapping.In 2006 The Pesticides Safety Directorate confirmed thatwarfarin active ingredient had been included on Annex 1of Council Directive 91/414/EC concerning the placementof plant protection products on the market. One conditionof this inclusion, imposed by the Commission, stipulatesthat only ready-to-use warfarin baits may be used.

Shooting, either on its own or in conjunction with pokingout dreys with poles, is generally ineffective at reducinggrey squirrel numbers to the level required to preventsummer bark-stripping. Grey squirrels are more visible inwinter when there are no leaves on the trees, but animalskilled at this time will usually be replaced before thesummer damage period. Drey poking and shooting in lateFebruary/early March will have most effect, but is only ofuse to prevent damage in extensive areas of woodland iffollowed up with trapping or poisoning. Shooting (withdrey poking) may also risk disturbance to nesting birdsand accidental injury to protected species. Shootingsquirrels that are seen at poison hoppers or live capturecage traps does not improve the efficiency of eithermethod. It may actually reduce the efficacy, as squirrels inthe tree canopy are encouraged to come down to visittraps and hoppers when they see others feeding there.Shooting may be effective for the removal of occasional‘nuisance’ squirrels, for example those found ravaging astrawberry bed, hazelnut crop or specimen trees in anamenity area. As the presence of steel shot in timber maycause degrade, non-steel shot is recommended.

Spring traps set in tunnels may be useful for short-termcontrol (see Tunnel trapping, page 13) but are less

selective than cage traps and are therefore not recommendedas a main control method.

In some sites a combination of methods may be mosteffective.

Control site selection

The success of a control operation depends onidentification of:

• woodlands that support a resident squirrel population;• sites in those woods which squirrels will visit to feed

at traps/hoppers.

Control is most productive in mature or semi-maturemixed broadleaved woodlands, so should be targeted inthese habitats. As squirrels can be difficult to draw totraps in pure conifer stands within mixed broadleaf/conifer woodlands, control should be avoided in theseareas. The best sites for traps or hoppers are generallyunder the largest trees in a wood, particularly those withbranches extending well down the trunk with tips near tothe ground. Yew and holly trees or a single conifer in apredominantly broadleaved wood will often make goodsites, as do tree stumps that are regularly used as feedingsites. The presence of many stripped cone-cores (Figure 5)and the peeled outer skins of large broadleaved seedsidentify these. The ground beneath the chosen tree andaround the stumps should be bare so that scattered bait isvisible to squirrels in the canopy. Squirrels avoid dense orwet vegetation, so it may be necessary to removevegetation to create sufficient clear areas. Using a longerpre-bait period, or placing traps up trees can improvecaptures in conifer woodlands.

Figure 5

Scots pine cone remains following squirrel feeding activity.

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SSiinnggllee--ccaappttuurree ttrraappssThese are designed to catch one squirrel at a time so thetrap is out of commission until the animal inside isremoved and the trap reset. The catching mechanismgenerally consists of a treadle connected to a wire (strandor rod) that holds the trap door open. The wire is releasedwhen an animal puts weight on the treadle and the doorcloses either under its own weight (Figure 7) or by aspring (Figure 8).

Accidental release of squirrels from this type of trap willoccur if the trap rolls over when occupied or duringremoval of the squirrel for dispatch. Another design(Figure 9) has a single lift door with baffle bars. Thedrawback of this particular trap is that another squirrellifting the door to gain entry may release a captivesquirrel.

LIVE TRAPPING

The bait

Live trapping involves attracting squirrels to a trap withfood (bait). Many types of bait have been compared,including wheat, rice, peanuts, acorns and hazelnuts butyellow whole maize has proved to be the best all roundbait. Not only is it a favoured food, but it is readilyavailable in quantity, is relatively cheap, stores well and isvery visible to squirrels when scattered on the ground.Uniquely, grey squirrels only eat the germ of the maizegrain and discard the remainder (Figure 6). This can beused to advantage to show whether squirrels have visited atrap site. The maize bait can be supplemented with a smallquantity of acorns, peanuts or hazelnuts to increase itsattractiveness at times when squirrels are difficult to trap.

Figure 8

Spring activated single-capture trap (mink type).

Figure 7

Single-capture trap. The door closes under its own weight oncethe animal stepping on the treadle releases the trip wire (Leggsingle type).

Grains intact Grains with germs removed by squirrels

Grains whichhave beenchewed by mice

Figure 6

Whole and damaged maize grains.

The trap

Live capture traps generally consist of a wire mesh cagewith sprung, drop or lift doors. The mesh size of a wirecage must be no greater than 25 mm x 25 mm and thewire diameter must not be less than 1.6 mm. Internalcondensation is a problem in solid metal box cages andwill cause considerable discomfort and suffering totrapped animals which may die if held in a dampenvironment. Wood and plastic are unsuitable materialsbecause a confined grey squirrel will gnaw its way out.

There are two types of trap: single-capture and multi-capture.

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MMuullttii--ccaappttuurree ttrraappssOnce set, these catch continuously and, depending ontheir size, can hold up to seven squirrels although one tothree is the norm. They have two lift doors in a tunnel(Figure 10). The second door is fitted to prevent anysquirrels in the trap being released by the next incomingsquirrel. Traps with mesh doors may be more effective asthe squirrels can see the bait behind the door. Behind eachdoor there is a set of vertical and horizontal baffle bars(Figures 10 and 11). These bars prevent captive squirrelsopening the doors and their shape and position in the trapis crucial. As manufacturers occasionally modify their trapdesign the dimensions of the doors, tunnel and baffle bars

Figure 9

Single-capture trap with single flap door entrance (midgettype), best suited for use in gardens.

177 mm

133 mm

108 mm 78 mm

12 mm

330 mm

7 mm

96 mm 110 mm

27 mm

56 mm

27 mm

12 mm

7 mm

Side view

Plan view

Bait tray

Figure 11

Multi-capture trap specifications showing flap door (red) andbaffle bar (blue) positions.

Figure 10 Multi-capture trap showing bait tray below the entrance tunnel. The baffle bars prevent captive squirrels from opening thedouble ‘lift’ doors. To be most effective, baffle bars should be curved as in Figure 11.

Baffle barSheet metal door

Squirrel holding area

Bait tray

Sheet steel screen

Entrancetunnel

Exitdoor

should be checked before accepting delivery to ensure theycomply with the specifications (Figure 11). These trapshave one or two sliding doors through which the capturedanimals are extracted.

Some traps are also termed ‘permanently baited’ becausethey have a metal tray that sits beneath the wire mesh trapfloor and is filled with maize. The maize in the tray isprotected from mice, voles and squirrels by a fine wiremesh above the tray. This mesh must not be greater than6 mm x 6 mm (Figure 10).

Single capture traps are recommended for use if there arealso red squirrels in the area. However red squirrels aremore likely to enter the ‘open door’ type of trap (Figures 7and 8) compared to traps with a flap door entrance.

The trapping session

A successful trapping session can remove at least 90% ofthe resident grey squirrels in the wood at the time. It ispossible to achieve this by simply setting out the traps andvisiting them daily to check the catch, but the aim shouldbe to trap the squirrels in the shortest possible time tominimise labour costs. To achieve this, a period of pre-baiting is essential to give the squirrels a chance to findthe traps and become used to feeding from within them(see section on Pre-baiting). The presence of animalsfeeding at traps will attract others passing through the treecanopy down to the ground. As traps on pre-bait are onlyvisited every other day, there is less human disturbancethan when traps are set immediately and visited daily.

Pre-baiting lasts for a minimum of five days after whichthe traps are set for a period of four to five days.Extending the pre-bait period to two or three weeks, usingold hoppers for the bait, prior to trapping the site may beparticularly beneficial in conifer areas. Using mainlywhole wheat with some maize and weekly visits willminimise costs. Without a pre-bait period it may take twoto three weeks to catch the squirrels with a daily(preferably twice-daily) visit necessary throughout.

The trapping method

The density of traps/sites required in a wood is dependenton whether single or multi-capture traps are used. Singlecapture traps are spaced 75–125 m apart, but may beplaced in pairs, and multi-capture traps 150–200 m,equating to approximately 1 trap per ha and 1 trap per 2ha respectively. Experience will determine the mostproductive sites and be far more effective than placingtraps to a regular grid.

The trap sites are normally laid out to form a trap roundwhich may use existing tracks, paths or rides forconvenience. However, to avoid or minimise vandalism,sites should be chosen to ensure the traps are not visiblefrom the track, particularly in woods with high publicaccess. Single capture traps may be more easilycamouflaged by placing them in a bank or tunnel, singlyor in pairs. Leaving both ends uncovered encouragessquirrels to investigate the ‘tunnel’.

In areas where it is difficult to draw squirrels to theground, for example, pine mixtures, or where traps aredisturbed by badgers, deer or wild boar, it may benecessary to site single capture traps on platforms on thetrees.

Trap deployment

A sound trap in good working order (as described onpages 6–7) is allocated to each chosen trap site. Whetherthe entrance is facing towards or away from the tree orstump makes no difference to the ability of the trap tocatch squirrels. However, it is sensible to ensure thatsquirrels can be removed from the exit door withouthaving to move the trap.

An area of ground, large enough to accommodate the trapand approximately 1 m in front, is levelled. Traps fittedwith a bait tray require a shallow depression to be dug.The bait tray is placed in the depression so that the rim ofthe tray is flush with the surface of the ground, half filledwith a layer of soil and then filled with a layer of yellowwhole maize. The trap is placed in position so that theflap doors in the entrance tunnel are above the tray andthe wire netting floor of the trap is flush with the ground.Some soil may be riddled through the wire netting roof ofthe holding area of the trap to fill in any hollows underthe trap and to form a false floor. This helps to preventmice and voles burrowing under the trap and stealing themaize. Once in position the traps are secured to theground using pegs cut from the surrounding wood, smalltent pegs or meat skewers.

Pre-baiting

During the pre-bait period trap entry doors should bepropped open and fixed securely so that there is nopossibility of accidental capture and any exit doorsremoved or pushed under the trap. At least two largehandfuls of yellow whole maize are placed in the mainbody of the trap (Figure 12). Black plastic (e.g. a bin liner)is used to cover the trap to make the inside weather proofand dark and therefore attractive to a squirrel. The plasticis held in place with branch wood, stones or any othersuitable material, which can also be used to camouflagethe trap. More maize is scattered liberally all around thetrap for up to 20 m.

After 2 days the traps are visited again and the maizeoutside and inside each trap inspected to see if the germhas been eaten out of any of the grains (Figure 6). Thisindicates whether squirrels have visited the site andentered the trap. However, mice, voles, birds, particularlypheasants and pigeons, and deer may also eat the maizeand there may be little or none left. The presence of a baittray is particularly useful where mice populations arehigh, as it ensures that bait is always present (see page 7).The maize inside and outside the trap should be

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replenished if necessary, but not broadcast as widely as atthe first visit. If there is little evidence of squirrel presenceat the trap sites this procedure is repeated after a furthertwo days.

Setting traps and catching squirrels

Two days after the last pre-bait visit the traps are set byreleasing the doors and activating the treadle mechanism,if fitted, after first checking that all moving parts areworking properly and removing bait or debris capable offouling the system. The sliding exit door, when fitted, isreplaced firmly and checked to see that the bottom of theexit door is below the floor of the trap in order to preventit being lifted up by a captured squirrel. The maize insidethe trap is replenished and a few grains (8–10, more ifmice have been removing bait) placed outside the entranceto the trap. Before leaving the trap any covering materialis replaced (Figure 13).

Traps that are set must be visited regularly to ensurecaptured animals do not suffer (Animal Welfare Act2006). Although daily inspection should be the minimum,twice a day is recommended especially where the squirrelpopulation is considered to be high or where singlecapture traps are used. At each trap the covering is firstremoved to identify the number of squirrels caught. It maythen be necessary to replace some of the covering to calmthe squirrels, enabling them to be removed from the trapin a controlled manner and dispatched humanely. Gloves,preferably waterproof, should be worn while handlingsquirrels to minimise the risk of bites, scratches andexposure to leptospirosis (Weil’s disease).

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Figure 12

Multi-capture trap prepared for pre-bait period, with lift doorswedged open and exit doors removed.

Figure 13

Multi-capture trap in situ with plastic sheet and camouflagecovering. Traps are best sited away from final crop trees.

It is illegal to release a grey squirrel into the wild19.

A medium weight hessian sack is used to extract thesquirrels from the trap. If the sack is too thin there is ahigh risk of being bitten or scratched through the fabric,and if too heavy it will be difficult to control themovement of a squirrel inside. Polythene sacks have anopen weave and are unsuitable. After first checking thatthere are no holes in it, the open end of the sack is placedaround the trap exit door and the width of the sack rolledup to form a tunnel. The exit door is opened and captivesquirrels are driven, one at a time, into the sack and theexit door closed. When squirrels are reluctant to leave thetrap, e.g. in poor light conditions, they can be encouragedto move into the sack by blowing on the base of their tail.The operator should position themselves so as not to deterthe squirrel from entering the sack. The squirrel is thenmoved into the corner of the sack and with the headpositioned carefully within the corner, killed rapidly andhumanely by a single blow to the back of the head with ablunt instrument.

Some people regard shooting at close range through thetrap with an air gun as an equally humane method ofdispatch. However, air rifles may be cumbersome to usefor this and air pistols are generally not sufficientlypowerful to achieve an instant kill. There is also a risk ofinjury to the operator as the wire mesh or the ground maydeflect the pellet. Blood in the trap may deter other animalsfrom entering and pose a risk of squirrelpox disease transferwhere both red and grey squirrels are present. Drowningis not a humane method of dispatch and is illegal.

Once empty the trap mechanism should be checked andreset. The exit door should be put firmly in position, themaize inside the trap and at the tunnel entrancereplenished, and the trap covering replaced.

If there are no squirrels present in a trap the maize shouldbe checked to see if the germ has been removed (Figure 6).This would indicate that a squirrel has entered the trap andhas either not been caught or escaped. The trap should bechecked to ensure that the mechanism is working effectively,the baffle bars have not been dislodged and the exit dooris in place, and that there are no escape holes in the wiremesh made by a captive squirrel gnawing at the wire.

Trapping normally lasts for just four or five days providedit was preceded by a pre-bait period. The highest numberof squirrels will usually be captured during the first threedays and fewer thereafter. However, prolonged heavy rainmay delay captures. A particularly high squirrel populationor too low a trap density may also result in good numbersbeing caught on days 4 and 5. The trapping period shouldtherefore be extended until no squirrels are caught.

Poor trapping success

There are some situations that prevent successful trapping.Squirrels are generally most trappable from mid March tothe end of July, although there may be a short periodwhen they may feed on flowers of broadleaved trees andcaptures will be low. Squirrels will not be attracted totraps when there is abundant natural food available, e.g.the autumn/winter tree seed crop.

Brand new traps should be left outside for a few weeks beforeuse to weather off the shine and manufacturing smells, other-wise squirrels will avoid them. Traps that have been visitedby a stoat or a weasel, both of which leave a strong muskysmell, will need to be lifted and left aside for a few days.

Very occasionally, and for no apparent reason, a trap sitewill not be visited. In these circumstances just moving thetrap to a new site 10–20 m away is often sufficient and thetrap should then catch.

Trap maintenance and storage

After trapping operations are complete, traps should bewashed, thoroughly checked and repaired as required.They should be stored with exit doors removed. Multi-capture traps should be stacked upside down and themechanism of other traps secured either open or closed toprevent animals and birds being inadvertently captured.

Trapping programmes and management

The trapping methodology described above should be usedirrespective of the number of traps deployed or the location(e.g. garden, park or woodland). A number of trappingsessions will be required when traps are to be used inquantity, for example in a forest or series of small woodlands.Traps should be moved sequentially from one trapping areato another and in these situations a rolling programmealong the following lines may be employed, starting in thisexample on a Wednesday, which avoids weekend visits:

• Day 1 (Wednesday) traps are put out and pre-baited.• Day 3 (Friday) traps are visited and rebaited.• Day 6 (Monday pm) traps are set.• Day 7 (Tuesday) set traps are visited twice.• Day 8 (Wednesday) set traps are visited twice and a

second batch of traps are put out and pre-baited in anew area.

• Day 9 (Thursday) set traps are visited twice.• Day 10 (Friday) set traps are visited once and lifted

and held ready for putting out in the next area to betrapped the following Wednesday. The second batchof traps are visited and pre-baited so that they areready for setting on the following Monday.

This sequence is repeated until all the target areas havebeen trapped. It is possible for one person to manage twobatches of around 30–40 traps provided there isreasonable access between the trap sites and the twotrapping areas are not too far apart. Extending the pre-bait period may increase trapping efficiency in coniferwoodlands.

WARFARIN POISONING

Warfarin poison may only be deployed out of doorsagainst grey squirrels for tree protection and only between15 March and 15 August. A successful poisoningoperation will be at least as effective as cage trapping,even though few, if any, dead squirrels will be found.

Legislation

The Grey Squirrels (Warfarin) Order 1973 permits thepoisoning of grey squirrels with the anticoagulantwarfarin for the purpose of tree protection. The Controlof Pesticides Regulations 1986 specifies, on the productlabel (ref. MAPP 13020_Grey Squirrel Bait) how, whereand when it may be deployed. These specifications includethe bait, and the design and dimensions of the hopper.

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11

operation should be covered by a written Risk Assessment.Disposal of contaminated personal protective and otherequipment should be through a registered waste contractor.

HoppersFigure 15 shows the required dimensions of the hopper,which are set to prevent animals larger than grey squirrelsgaining access to the poison. Selective access hoppersshould be used. These have a flap door in the tunnelentrance preventing access by smaller animals20 and reducebait usage by approximately 50%. There are two types ofdoor: the weighted door which is the most reliable; andthe magnetic door that is equally effective when operatingcorrectly, but the magnet may become dislodged or losepower with age or from a coating of debris.

Checking door operationThe correct operation of the flap door can be measuredwith a small, 85 mm wide, electronic pocket balance. Thebalance is modified in two ways by attaching a probe,metal or plastic rod, 6–8 mm diameter, to the weighingplatform (Figures 16 and 17). Two small adhesive plasticcable clips are adequate. The probe protrudes 70 mmbeyond the platform and is rounded off at the end. Themeasurement is made, by sliding the balance, probe first,along the hopper tunnel floor, until the door is liftedabove the floor. The reading on the digital scale should bebetween 100 and 115 g (Figure 17).

Handle Lid

1.5 kgcapacityor more

Bait container

45°

Weight

Tunnel

Front view

80 mm6 mm

Flap door

10 mmbaffle

60 mm

15 mmgap

20 mm screwNut and bolt

110 mm Doorweight

Side view

Flap door Plan view

240 mm110 mm

125 mm

90 mm100 mm

85 mm

Figure 15

Warfarin hopper, showing maximum tunnel dimensions andposition of the flap door which prevents access by non-targetanimals.

Warfarin must not be used outdoors where red squirrelsor pine martens occur. Operators must be trained beforeusing warfarin. The National Proficiency Tests Council(NPTC) provides a nationally recognised Certificate ofCompetence for Vertebrate Pest Control, which includesgrey squirrel control. The responsible use of warfarin isvital and every attempt must be made to prevent or reducerisk to other species. Hoppers should hold no more than 4kg bait and be clearly labelled with a warning that theycontain a poison, as shown in Figure 14(a) and (b). Label(a) is more informative and is preferred to (b).

Poisoning method

The baitOnly the 0.02% warfarin bait (Grey Squirrel Bait MAPPno. 13020) may be used. The approval for Grey SquirrelLiquid Concentrate MAFF no. 06455 has been revoked.Users of the Grey Squirrel Liquid Concentrate whopurchased product before 31 December 2006 have until31 December 2007 to use the product (effectively August2007 for tree protection use). Gloves (unlined or flock-lined synthetic rubber or PVC to European Standard EN374) should be worn when handling the bait and the control

Figure 14

Examples of labels for poison hoppers.

POISONThe bait in this hopper is

treated for the specific control of grey squirrels.

These animals cause severe damage to trees.

Disturbance of the hopper or bait can pose a danger to other wildlife.

For further information on squirrel control contact:

Tel: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

(b)

(a)

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Hopper deploymentThe choice of sites for hoppers is as described in controlsite selection (page 5). Sites should be spacedapproximately 200 m apart and distributed throughoutthe control area at a density of one hopper to 1–4 ha,depending on intensity of control (Table 1). At each site ahopper is placed at the base of a tree or stump with thetunnel tilted slightly down to prevent surface waterflowing down the tunnel into the bait. The entrance mayeither face into or away from the tree/stump. Placinghoppers facing the tree 20–25 cm away limits access bynon-target species, and bait spillage. The hopper is firmlysecured with branch- wood or with one or two stakes(Figure 18) which must not prevent the lid from closingfully. The hopper is held to the stake with either tyingwire or bands made from a car tyre inner tube.

Branchwood, stone or turf can then be used to camouflagethe hopper from the public. An alternative is to dig thehopper into a bank. Occasionally, badgers, muntjac deeror feral wild boar will persistently disturb hoppers despitebeing well staked and secured. Another site should first besought, but if this is not possible or it is also disturbed, thehopper must be placed above the ground either in the forkof a tree or on a platform (Figure 19). Hoppers sitedabove ground are difficult to hide.

Bark-stripping damage can occur on trees close to a well-used hopper due to subordinate animals ‘waiting’ for adominant squirrel to finish feeding. The addition of asecond hopper 3–5 m away will generally curtail furtherdamage. It is advisable to avoid placing hoppers near tofinal crop trees.

Figure 16

Warfarin hopper and modified balance with metal or plastic rodattached to the weighing platform to check effort required tolift the hopper door.

Probe

Plan view

115g

Balance platform

Front view Side view

85 mm85 mm 70 mm70 mm

Figure 17

Specifications for modification of balance to check the effortrequired for an animal to lift the hopper door. A probe, metalor plastic rod 6 to 8 mm diameter, is attached to the weighingplatform.

Figure 18

Hoppers should be firmly secured and carry a label explainingtheir use.

Figure 19

Hopper sited (a) on a tree branch and (b) on a platform about1 m above the ground, away from badger access.

(a) (b)

Poisoning operationEach hopper is filled with between 1.5 and 4 kg (legalmaximum21) of poisoned bait. Two or three handfuls ofyellow whole maize are initially scattered around thehopper to attract squirrels to the site. If pheasants removethe maize the tunnel entrance can be extendedapproximately 150 mm with rabbit netting (Figure 20).

It is essential that a continuous supply of bait is availableas each squirrel needs to consume 200–250 g bait(depending on bodysize) over a 10-day period to kill it16.The hoppers must not be allowed to become empty andshould be visited once every 2–3 days initially, particularlyif squirrel populations are very high, and thereafter atleast once every two weeks to check that they have notbeen damaged or disturbed and to top up the bait. (Forexample, where there are 10 squirrels per ha present, toremove 5 squirrels per ha at least 1.25 kg of warfarin baitper ha would need to be consumed over a 10-day period.)A plastic jug, or similar utensil, labelled ‘poison’, shouldbe used as a scoop to top up hoppers. Bait is too easily spiltwhen pouring it from a polythene bag into the relativelysmall opening of the bait container. All spillage caused orfound at any hopper should be cleared up at each visit.

Hoppers should be checked at each visit to ensure thatmoisture has not caused the wheat grains to stick togetherand prevent the bait running. A thin stick is used to checkfor and free any blockage, but should be used withrestraint to avoid pulling out too much bait into thetunnel. Any grain that has been soiled by moisture and isnot in a suitable condition to be replaced in the hoppershould be removed for safe disposal, i.e. burned orthrough a registered waste contractor. The flap doorshould also be checked at each visit to ensure that the

retaining screws are both in place (as well as the magnet,when fitted) and that the door is working properly, i.e. thecorrect weight is required to lift the door (see page 11).Squirrels generally die in their dreys but the area shouldbe searched at each visit, particularly under dreys, and anydead bodies removed.

Hoppers are generally left in place throughout the controlperiod to ensure removal of both resident and recolonisinganimals. Bait consumption is greatest during the first 4weeks or so, followed by a decline, indicating a reductionof the resident population. A period of about 3 weeks oflittle or no take of bait will follow before recolonisinganimals start feeding. Some maize should be scatteredaround the hopper to attract incoming squirrels andtopping up of the hoppers reduced to maintain thembetween half and quarter full. Control operations shouldstop during mid to late July and certainly before 15August. At the end of the control period all the baitremaining in hoppers should be removed and stored safelyif in a sound condition (dry and with no weevils) or sentfor safe disposal, along with any empty containers andcontaminated personal protective and other equipment,through a registered waste contractor.

Hopper maintenanceAt the end of the control period hoppers should be washedand dried thoroughly before being stored for next year. Looseor missing flap door retaining screws should be tightened orreplaced and the doors secured in the open position to avoidthe accidental trapping of small mammals during storage.Hoppers may remain in situ after the end of the controlperiod (15 August) but must be emptied completely anddoors secured to prevent small mammals being trapped.

TUNNEL TRAPPING

Tunnel trapping, also known as spring trapping, can beeffective when used by a skilled operator and can beuseful when a rapid kill is required; for instance, whenunexpected damage is occurring and the squirrels must beremoved immediately. Otherwise it is not recommendedas a principal method of control. Grey squirrels may becaught incidentally in woodlands on game estates wherespring traps are routinely used for predator control.

Spring traps have arms or jaws activated by strong springswhich, when triggered, close violently around thesquirrel’s neck. Death should be instantaneous. In practicethere are many variables that may influence this outcome,such as speed of movement over the trap, size of animal

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Figure 20

Hopper tunnel entrance extended approximately 150 mm withrabbit netting to prevent access by game birds.

and the way the traps are individually sited and set. If theanimal is caught by a leg, the tail or around the abdomendeath will not be instantaneous and the animal will sufferif a trap is not visited for some time after capture. Onlytraps that have been approved under the Spring Traps(Approval) Order 1995 may be used. The Pests Act 1954and the Wildlife and Countryside Act 1981 require thetraps to be set in tunnels. Traps must be inspected at leastdaily.

Tunnel traps should be sited in places and runs normallyused by grey squirrels. This requires considerable expertiseif the trapping effort is to be successful. Fenn and Springertraps are set in tunnels so that the treadle is level with thetunnel floor. Tunnels may be natural, such as the hollowbase of a tree, dry drains and holes in banks and walls, orartificial, constructed from turf, logs, pipes, bricks, flatstones or wood. The tunnel should be at least 600 mmlong and its internal dimensions sufficient (15 cm wide x13 cm high) to allow the arms of the trap to strike. Theentrance holes at either end of the tunnel should be nogreater than 60 mm to deter animals larger than a squirrelfrom entering. Any animal capable of travelling throughthe tunnel is at risk. Stoats and weasels are particularlyvulnerable as they may be lured by the scent on the trap ofa previously captured grey squirrel. Body grip traps (e.g.BMI Magnum) may also be used successfully to take greysquirrels by making a wire mesh tunnel on a branch thatsquirrels run along and setting the trap inside. Kania trapsare also sited up trees and come fitted with their owntunnel.

Red squirrels, when present, will also be at risk althoughthey are less likely to enter tunnels. It is stronglyrecommended that spring traps should not be used whenred squirrels are present. Although there is no case law, asthere are no reasonable precautions that can be taken toprotect red squirrels from spring traps, it is probable thattheir use is prohibited under the Wildlife and CountrysideAct 1981 in areas where red squirrels are known to bepresent. Similar concerns apply where other protectedspecies such as pine martens, polecats and hedgehogsoccur and measures to reduce the risks to these speciesshould be taken.

Baiting tunnel traps to attract squirrels increases the riskto birds if the bait is visible. A few grains of maize orwheat under the trap plate can attract squirrels into thetunnel without being noticed by birds. Advice on thesiting and use of spring traps is available from the GameConservancy Advisory Service (see page 15).

DISPOSAL OF CARCASSES

As carrion can play an important part in forest ecosystemsit may be acceptable to leave carcasses, believed to be freefrom disease, distributed in suitable locations in thewoodland. Alternatively carcasses may be buried providedthey are not diseased or poisoned. Carcasses showingsymptoms of squirrelpox virus (scabs around eyes, nose,mouth and feet) should be sent to the State VeterinaryService for investigation. Carcasses of animals that havehad access to warfarin poison should be disposed of inaccordance with the statutory conditions relating to theuse of warfarin; for further guidance contact theEnvironment Agency (08708 506506) or your localScottish Environment Protection Agency office. Carcassessuspected of being infected with diseases communicable tohumans or animals cannot be buried and must be sent forincineration or rendering. Advice should be sought fromthe local office of the State Veterinary Service.

IMMUNOCONTRACEPTION

Fertility control is being investigated as an innovativemethod to keep populations of grey squirrels below thedensity at which they damage trees. However, research intosquirrel reproductive physiology and the chemical mechan-isms controlling fertility is required to enable the developmentof a suitable vaccine. Once an agent has been identified,the minimum dose required to produce a long-term effect,a suitable carrier bait and delivery systems, and confirmationof species specificity will be required before legislation ispassed to enable its use. Such studies are both long term andcostly, and the method will not be available for many years.

REFERENCES

1. HUXLEY, L. (2003). The Grey Squirrel Review.Profile of an invasive alien species. Grey Squirrel(Sciurus carolinensis). ESI. Dorset.

2. GURNELL, J. and MAYLE, B. A. (2003). Ecologicalimpacts of the alien grey squirrel (Sciurus carolinensis)in Britain. In: Conference Proceedings: MammAliens –a one day conference on the problems caused by non-native British mammals. Peoples Trust for EndangeredSpecies/Mammals Trust UK, London, 40–45.

3. SAINSBURY, A.W., NETTLETON, P., GILRAY, J.and GURNELL, J. (2000). Grey squirrels have highseroprevalence to a parapoxvirus associated withdeaths in red squirrels. Animal Conservation 3,229–233.

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4 McINNES, C. J., WOOD, A. R., THOMAS, K.,SAINSBURY, A. W., GURNELL, J., DEIN, F. S. andNETTLETON, P. F. (2006). Genomiccharacterization of a novel poxvirus contributing tothe decline of the red squirrel (Sciurus vulgaris) in theUK. Journal of General Virology, 87, 2115–2125.

5 PEPPER, H. (2004). Grey squirrels in parks, urbanwoodlands and amenity plantings. ArboriculturalPractice Note 7. Arboriculture Advisory andInformation Service, Farnham.

6. PEPPER, H. and PATTERSON, G. (1998). Redsquirrel conservation. Forestry Commission PracticeNote 5. Forestry Commission, Edinburgh.

7. HODGE, S. and PEPPER, H. (1998). The preventionof mammal damage to trees in woodland. ForestryCommission Practice Note 3. Forestry Commission,Edinburgh.

8. KENWARD, R. E. (1996). Grey squirrel management[Letter to Editor]. Quarterly Journal of Forestry 90,218.

9. GURNELL, J. (1999). Grey squirrels in woodlands:managing grey squirrels to prevent woodland damage.Enact 7, 10–14.

10. KENWARD, R. E. (1983). The causes of damage byred and grey squirrels. Mammal Review 13, 159–166.

11. GURNELL, J. (1996). The effects of food availabilityand winter weather on the dynamics of a grey squirrelpopulation in southern England. Journal of AppliedBiology 33, 325–338.

12. GURNELL, J. (1989). Demographic implications forthe control of grey squirrels. In: Mammals as Pests,ed. R. Putman. Croom Helm, London, 131–143.

13. FERRYMAN, M., MAYLE, B. A. and MORGAN, G.W. (2006). Visual method for evaluating the state ofsexual development in male grey squirrels (Sciuruscarolinensis). Reproduction, Fertility andDevelopment 18, 383–393.

14. GURNELL, J., SAINSBURY, T. and VENNING, T.(1997). Conserving the red squirrel in Thetford Forest.Report for English Nature, Forestry Commission andthe People’s Trust for Endangered Species. EnglishNature, Peterborough.

15. KENWARD, R. E., DUTTON, J. C. F., PARISH, T.,DOYLE, F. I. B., WALLS, S. S. and ROBERTSON, P.A. (1996). Damage by grey squirrels. 1. Bark-strippingcorrelates and treatment. Quarterly Journal ofForestry 90, 135–142.

16. PEPPER, H. and STOCKER, D. (1993). Grey squirrelcontrol using modified hoppers. Research InformationNote 232. Forestry Commission, Edinburgh.

17. PEPPER, H. (1998). Nearest neighbour method forquantifying wildlife damage to trees in woodland.

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Forestry Commission Practice Note 1. ForestryCommission, Edinburgh.

18. UKWAS STEERING GROUP (2006). The UKWoodland Assurance Standard. Second edition.UKWAS, Edinburgh.

19. PEPPER H. W. (1990). Grey squirrels and the law.Research Information Note 191. Forestry CommissionResearch Division, Farnham.

20. PEPPER, H. W. (1997). Hopper design and targetedgrey squirrel control. In: The conservation of redsquirrels, Sciurus vulgaris L., eds J. Gurnell and P.Lurz. People’s Trust for Endangered Species, London.

21. KILLGERM CHEMICALS LTD. (2007). GreySquirrel Bait MAPP no. 1302 label. KillgermChemicals Ltd, Ossett, West Yorkshire.

STATUTORY ACTS ANDORDERS

• Abandonment of Animals Act 1960• Animal By-Products Regulations 2003• Control of Pesticides Regulations 1986• Grey Squirrels (Warfarin) Order 1973• Pests Act 1954• Animal Welfare Act 2006• Spring Traps (Approval) Order 1995• The Animal By-Products Amendment (Scotland)

Order 2001• Wildlife and Countryside Act 1981

FURTHER ADVICE

Details of areas where the use of warfarin is restricted orprohibited are available from National Offices of theForestry Commission, see: www.forestry.gov.uk. Details ofequipment manufacturers and suppliers can be found at:www.forestresearch.gov.uk/greysquirrels.

TRAINING

NPTC Stoneleigh ParkWarwickshire CV8 2LG T: 024 7685 7300F: 024 7669 6128E: information@nptc.org.ukwww.nptc.org.uk

Certificate of competence in vertebrate pest control (Level 2)

Forestry CommissionForestry Training ServicesDean Management Training Centre,Prosper Lane, Coalway, Coleford, GL16 7JYT: 01594 832096F: 01594 810514E: forestry.training.services@forestry.gsi.gov.ukwww.forestry.gov.uk

Rabbit and grey squirrel control (course no. 4.68)0.5 day each – rabbit/squirrel1.5 days for PA01 certificate

British Pest Control Association (BPCA)Ground Floor, Gleneagles HouseVernongate, Derby, DE1 1UPT: 01332 294288F: 01332 295904enquiry@bpca.org.ukwww.bpca.org.uk

Mammal management: grey squirrels and other mammals:1 day (covers trapping, shooting and poisoning)Safe use of pesticides course: 1 day

Killgerm Chemicals LtdPO Box 2, Ossett, West Yorkshire, WF5 9NAT: 01924 268400F: 01924 264757sales@killgerm.comwww.killgerm.co.ukModular training courses: Foundation module: Safety and legislation module.Vertebrate module (covers squirrels).Training information: 01924 268445

Other sources of training

Colleges: Countryside management, Forestry andwoodland management and Controlling vertebrate pestsEnvironmental consultanciesGrey squirrel control groups

For grey squirrel problems in urban areas advice may beobtained from:

The Mammal SocietyT: 02073 502200F: 02073 502211www.mammal.org.uk

© CROWN COPYRIGHTISSN 1460-3810ISBN 978-0-85538-735-8 16

Enquiries relating to this publication should be addressed to:

Mark FerrymanForest ResearchAlice Holt LodgeWreccleshamFarnhamSurreyGU10 4LH

T: 01420 22255F: 01420 23653E: mark.ferryman@forestry.gsi.gov.ukwww.forestresearch.gov.uk

Tree Help LineAlice Holt LodgeWreccleshamFarnham GU10 4LHT: 09065 161147 (calls charged at £1.50 per minute)

The Game Conservancy Advisory ServiceFordingbridgeHampshire SP6 1EF T: 01425 651013advisory@gct.org.uk

FCPN004(REVISED

)/FC-GB(EC

D)/WOP-2K/A

UG07