Forestry Commission ARCHIVE Bulletin 85
etonus micans in Britain
CJ King and NJ Fielding
FORESTR Y COMMISSION B ULLETIN 85
Dendroctonus micans in Britain - its Biology and Control
C. J. King and N. J. Fielding Entomologists, Forestry Commission
LONDON: HER MAJESTY’S STATIONERY OFFICE © Crown copyright 1989 First published 1989
ISBN 0 11 710276 8 ODC 145.7Dendroctonus micans : 453 : 411: (410)
KEYWORDS: Entomology, Integrated control, Forestry
Enquiries relating to this publication should be addressed to the Technical Publications Officer, Forestry Commission, Forest Research Station, Alice Holt Lodge, Wrecclesham, Famham, Surrey GU10 4LH.
Front Cover: Sitka spruce heavily attacked by Dendroctonus micans, showing typical resin bleeding and massed and single resin tubes. Inset. Adult beetle of D. micans.(36064) Contents
Page Summary/Sommaire/Zusammenfassung/PecjDepaT v-viii Introduction 1 The pest status of Dendroctonus micans 1 Initial control measures 2 Recognition and symptoms of D. micans attack 2 D. micans life cycle and breeding behaviour in Britain 2 Dispersal and host selection 4 Host susceptibility and breeding success 5 Natural enemies of D. micans 6 Biological control of D. micans by Rhizophagus grandis 6 Conclusions and implications for integrated control 7 Acknowledgements 9 References 11
Dendroctonus micans in Britain - its Biology and Control
Summary
The great European spruce bark beetle (Dendroctonus micans) was first identified as a breeding species in Britain in 1982. This insect’s distribution, pest status, biology, life cycle and control methods are described. Although its present population is limited to Wales, the English West Midlands and Lancashire, it is well established. Infestations are characterised by scattered groups of attacked trees which display symptoms of resin bleeding and resin tubes upon their trunks. Initial control measures of felling infested trees, debarking, and insecticide application, effectively reduced this insect’s population but could not hope to eradicate it. As a long-term means of control the predatory beetle Rhizophagus grandis — an effective and specific enemy of D. micans — has been successfully introduced into Britain. This biological control method has so far shown very encouraging results but it is still too early to consider R. grandis as a totally effective and permanent solution to D. micans. Surveys over the past 5 years show that D. micans is gradually increasing its range. Dendroctonus micans dans Le Royaume-Uni: Biologie et Lutte
Sommaire
On a constate l’hylesine geant de l’epicea (Dendroctonus micans) comme habitant reproductif dans Le Royaume-Uni pour la premiere fois en 1982. On decrit sa distribution, sa position comme insecte nuisible, sa biologie, le cycle de sa vie, et les methodes de lutte. Bien que sa population aujourd’hui se limite au Pays de Galles, au centre-ouest de l’Angleterre et a Lancashire, il se trouve bien etabli. Les infestations se caracterisent par des groupes repandues des arbres attaques, qui montrent sur ses tiges les symptomes de l’ecoulement de resine et des poches de resine. Les mesures premieres de lutte, c’est-a-dire l’extraction des arbres infestes, ecorgage, et l’application des insecticides, ont reduit effectivement la population de cet insecte, mais ils sont incapables de detruire D. micans completement. Comme mesure de lutte a long terme, on a introduit le predateur Rhizophagus grandis, ennemi effectif et specifique de D. micans, avec succes dans Le Royaume- Uni. Cette methode de lutte biologique a montree jusqu’ici des resultats tres encourageants, mais il est encore trop tot pour considerer R. grandis comme solution totalement effective et permanente du probleme D. micans. Des inspections pendant les cinq annees demieres montrent queD. micans graduellement agrandit sa distribution.
vi Dendroctonus micans in Grossbritannien: Biologie und Bekampfung
Zusamm enfassung
Der Riesenfichtenbastkafer (Dendroctonus micans) wurde zuerst als Brutart in Grossbritannien in 1982 nachgewiesen. Die Verbreitung, das Schadbild, die Biologie, der Lebenslauf und die Bekampfung dieses Schadlings werden beschrieben. Obgleich seine heutige Population in Wales, im westlichen Teil des englischen Mittellandes und in Lancashire beschrankt ist, ist er dort gut eingesessen. Der Befall ist durch zerstreute Gruppen befallener Baume gekennzeichnet, die Harzfluss und Harzgallen als Symptomen auf ihren Stammen zeigen. Die ersten Bekampfungsmassnahmen, namlich Aushieb befallener Baume, Entrindung und Insektizidverwendung, haben zwar die Population vermindert, aber es war nicht zu hoffen, den Schadling dadurch vollig zu vemichten. Wie langfristige Bekampfungsmassnahme hat man den Raubkafer Rhizophagus grandis, ein wirksamer und artspezifischer Feind von D. micans, erfolgreich in Grossbritannien eingefurht. Diese biologische Bekampfungsmassnahme hat bisher sehr ermutigende Ergebnisse geliefert, aber es ist noch zu fruh, R. grandis als vollig wirksame und dauerhafte Losung des D. micans — Problems zu betrachten. Aufhahmen wahrend der letzten fiinf Jahre zeigen, dass D. micans sein Verbreitungsareal allmahlich erweitert. BonbLuoM EnoBbiM Jly6oefl (Dendroctonus micans) b BennKo6pnTaHMM - Enonorn^1/1 Eopb6a c Hum Pecpepar
BonbiuoM enoBbiM ny6oefl 6bin o6Hapyx Hana/ibHbie Mepbi 6opb6bi (py6Ka 3apa>xeHHbix flepeBbeB, OKopKa, npMMeHeHMe MHceKMqMflOB) acjDcjDeKTMBHO noHMwanM nonyjinqHKD 3Toro BpeflHTerm,hono/iHoe yHHHTO>KeHMe HeB03M0>KH0.KaK flonrocpoHHaB Mepa 6opb6bi, 6bin ycnewHO nHTpoflyqnpoBaHb BenMKo6pnTaHMH xmqHbm >xyKRhizophagus grandis (scfDcjDeKTMBHbiii m cneqMcfiMHecKMM BparD. micans). 3Ta Mepa 6uo/ion/iHecKOM 6opb6bi y x e noKa3ana o6oApnTenbHbie pe3y/ibTaTbi,hoeiqe npexgqeBpeMeHHO CHMTaTbR. grandis nonHO acjjcfreKTMBHbiM m nocTOBHHbiM peweHMeM npo6neMbiD. m icans. Hafl3opbib TeneHi/ie nocneflHMX5 neT noKa3armhto D. m icans nocTeneHHO yBenMHMBaeTcbom apean. Dendroctonus micans in Britain - its Biology and Control C. J. King and N. J. Fielding, Entomologists, Forestry Commission Introduction 1972 (Bevan and King, 1983). In 1983 an isolated outbreak was found in the Trough of The great European spruce bark beetle, Den Bowland, Lancashire (Figure 1). droctonus micans Kug. (Coleoptera; Scolytidae), The accidental introduction and establish originally of a northern Eurasian distribution, ment of D. micans in Britain represents a very has spread steadily over the last 100 years or so, real threat to our most widely planted and probably as a result of increasing commerce in economically important tree species, as well as timber. Today it is distributed throughout the presenting us with the need to develop manage spruce growing areas of the European land mass ment strategies for a new forest pest. demonstrating a remarkable ability to adapt to a wide variety of climatic and forest conditions. In Europe the only limitation to its spread would appear to be the distribution of the spruces. The pest status o f Dendroctonus Serious damage to commercial forests by D. micans has been reported regularly over the past micans 90 years from western Europe, notably in Den Although a member of the bark beetle family, mark and Holland (Carle, 1975; Evans, 1984). It the Scolytidae, D. micans has a number of has also caused serious damage in both spruce atypical characteristics that distinguish it from and pine forests in Siberia and some Baltic other European scolytids. In particular: states. The most significant current outbreaks 1. It breeds almost exclusively in living and are in natural forests of Oriental spruce (Picea apparently healthy trees, which it can suc orientalis (L.) Link) in Turkey and the Georgian cessfully infest without mass attack to over SSR, and in plantation forests of Sitka spruce come tree resistance. Therefore some of the (Picea sitchensis (Bong.) Carr.) and Norway pest management strategies that apply to the spruce (Picea abies (L.) Karst.) in parts of majority of bark beetle species (such as the France. The potential threat posed by D. micans removal of dead or dying trees, in which these to Britain’s forests was recognised in 1964 when insects breed) are not appropriate to D. a study was made of a serious outbreak in micans. Denmark (Brown and Bevan, 1966). In 1982, 2. The larvae of D. micans aggregate and feed following an enquiry into tree deaths in a collectively within the bark. plantation of Sitka spruce in south Shropshire, D. micans was identified as the cause. Thorough 3. The life cycle is protracted and is not as and widespread surveys showed that the insect seasonal or predictable as that of most other was well established throughout much of east bark beetle species. ern, northern and central Wales and the west 4. Female D. micans are normally fertilised English Midlands from north Shropshire to the before leaving their original brood systems Forest of Dean. Further studies confirmed that and therefore have no need to locate a mate the beetle was breeding in Shropshire as early as after they emerge from the tree. Initial control measures of pure resin, varying in colour from white to pale pink or brown, often denote an abortive The first steps taken to deal with the outbreak in attempt at entry by the beetle. Others consisting Britain were to identify the distribution and of a mixture of resin and bark particles, coloured intensity of D. micans attacks by thorough forest purple-brown to dark brown, indicate successful surveys. Measures were then taken to reduce D. entry to the cambium but not invariably success micans populations by felling infested trees, ful brood establishment. Resin tubes may occur followed by debarking and applying insecticide singly or in large amalgamations of many tubes, to the resulting timber and bark residues. This sometimes reaching the size of a man’s fist (Plate exercise continued until 1984 and served to 1). Those which are crusty and hard are norm reduce the insect’s breeding population but ally old; fresh attacks have soft, malleable resin failed to eradicate it as even the most thorough tubes. However, breeding beetles sometimes surveys could not identify all infested trees. successfully re-enter old tubes. In such cases Broods also survived in the roots and stumps of fresh bark particles can be seen at the entry felled trees. Intensive surveys and piecemeal hole. If attack occurs below ground level the fellings of immature trees have proved costly, resin particles are pushed to the surface in while spot applications of powerful and persis granular form (Plate 2). tent insecticides to standing trees (practised in Removal of bark from infested trees reveals the USSR, Turkey and Western Europe on the brood system of larvae, (white legless grubs occasion) were avoided as being environmentally with a brown head capsule) feeding in large unacceptable. Legislation was also introduced to groups on a common feeding front (Plate 3). limit the accidental spread of D. micans through Destroyed areas of cambium are characterised movement of infested timber (Evans and King, by a concentration of resinous frass interspersed 1988; Anon., 1988). The need for an effective with tunnels (Plate 4). Pupation takes place long-term control method for D. micans as an among the frass in shallow oval excavations alternative to the ‘seek and destroy’ policy was (Plate 5). evident. Mature broods contain groups of adult beetles, Recognition and symptoms o f which by bark beetle standards are exception ally large, ranging from 5 mm to 7 mm in length, D. micans attack cylindrical, half as broad as long and ranging The denser spruce plantations where D. micans from pale brown when immature to pitchy black is well established are usually characterised by when fully developed (Front cover and Plate 6). scattered groups of affected trees representing D. micans broods, regardless of stage are fre various foci of infestation. Such groups usually quently attacked by woodpeckers and such activ consist of a few dead or partly dead trees ity can also be useful in recognising the presence surrounded with others with varying degrees of of D. micans in the crop. successful colonisation. In widely spaced heavily thinned crops, attack may be more general and D. micans life cycle and breeding widespread with scattered dead and dying trees. Closer inspection of infested plantations will behaviour in Britain show copious resin bleeding on the stems of The life cycle of D. micans has not been studied heavily attacked trees with the presence of many closely in Britain. There is no doubt that a resin tubes which are the main diagnostic complete generation takes at least 10 months to feature of D. micans (Front cover). Attacks may develop but, as breeding takes place at any time occur anywhere on the tree from the root system from March to October, much longer cycles to the upper crown. Lightly attacked stems may undoubtedly occur; perhaps as long as 18 months have only one or two resin tubes with the in certain circumstances. The cycle begins when attendant resin bleeding down the trunk. The the female beetle emerges from the brood system consistency of the tube varies considerably, those in which she developed. Females outnumber 2 Figure 1. Known distribution (brown areas) of Dendroctonus micans in Britain, 1988. Plate 1. Resin tubes of D. micans a. smooth consistency formed of pure resin, b. granular consistency formed of resin and bark particles (37201). Plate 2. D. micans attack at and below ground level results in piles of granular resin shown here. PI a te 3. Typ ical well developed D. micans larval brood. Note aggregated larvae. Plate 4. D. micans larvae pack their faecal pellets into hard resinous concretions which are later interspersed with tunnels — a highly diagnostic feature of the species. Plate 5. Pupae of D. micans in shallow excavations among their larval frass. Plate 6 Adults of D. micans remain massed under bark for protracted periods before emerging. Plate 7. Brood chambers and egg clutches of D. oiicans. Plate 8. D, micans larvae feeding collectively in living spruce bark. Plate 9. D. micans prepares emergence holes well in advance of leaving the tree. Copious amounts of bore dust are produced at this stage. Plate 10. Successful brood establishment in the roots and lower trunk of Sitka spruce frequently results in the death of the tree. males by at least 7:1 with a disparity as high as their heads into tightly packed concretions of 45:1 being observed. Copulation takes place resin and frass. As the larval system extends and before emergence, often between young, pale develops these areas of packed frass are inter adults and usually within the brood; mating is spersed with tunnels thus forming a series of therefore incestuous. ‘islands’ which form the ‘quilted’ appearance of After emergence females may re-enter fresh the system which is a strong diagnostic feature bark elsewhere on the same tree or may disperse of the species (see Plate 4). During the extension to nearby trees. In some cases, however, there is of a brood system several different feeding no emergence and new broods are established in groups of larvae may develop and feed in the live bark at the margins of the original brood different directions leading to haphazard and system. There is no aggregation behaviour in the irregular patterns. If the death of the tree occurs adult insect as there is with many other scolytid while broods are still actively feeding then the species. It is important to emphasise that pre larvae tend to disperse into small groups to emergence mating means that a single female continue to feed and develop in the dead cam has the potential to pioneer new areas and give bium. In such circumstances they also tend to rise ultimately to new foci of infestation. feed in the outer phloem thus engraving the Typically D. micans mines the cambium for wood. They may also feed within the bark where 1—2 cm and then constructs a roughly oval egg- this is very thick, generally feeding in an chamber; in this she will deposit a clutch of eggs upward direction. mixed with loose bark particles and a sticky Larvae feed through five moults or instars and secretion produced by the beetle. Egg numbers when fully fed measure up to 7 mm in length. As may vary considerably but normally average 80- they develop any sickly, dead or diseased larvae 100 per chamber, with each female constructing are walled up in the tightly packed frass, a charac up to three egg chambers joined by short teristic which may limit the spread of any diseases galleries thus depositing up to 300 eggs (Plate 7). between their healthy contemporaries. The re The oviposition period is protracted which sulting destruction of the living cambium may means that the first eggs will be hatched before be extensive, brood areas of 30-50 cm long and the last are laid. This ultimately leads to a some 10-15 cm wide are not uncommon. When mixture of larval stages within the family of a several females oviposit in close proximity to one single parent. There is no evidence that subse another the larval systems frequently coalesce quent pairing with male beetles is necessary for leading to very extensive wounding of the tree. refertilisation of egg-laying females. Little is When larval feeding is completed pupation known of the behaviour of male D. micans once takes place in small and usually closely spaced they have completed mating but laboratory excavations in the packed larval frass (Plate 5). evidence suggests that they are unwilling or The young adult beetles are a pale creamy even unable to mine into bark; they are known yellow colour when they first develop and gra to emerge and fly (Gregoire, personal communi dually darken through various shades of brown cation) or may die in their galleries after mating to their typical pitchy black coloration. The (Vasechko, 1988). massed adults (Plate 6) remain under bark for As the eggs of D. micans hatch into larvae, lengthy periods and show a variety of behaviour they begin to feed in the living bark at the edges al patterns. Frequently they will mine in large of the brood chamber. At this stage aggregation groups among their original excavation, chew takes place stimulated by chemical signals emit ing among the larval frass, but it is not known if ted by the larvae. Larval feeding is well organ they actually feed upon it. They also form long ised with larvae closely packed side by side at ‘nose to tail’ columns within existing tunnels the feeding front but occasionally leaving to among the frass or will sometimes feed singly or defaecate in the destroyed cambium in their collectively in the fresh bark at the edges of the wake (Plate 8). Individual larvae manipulate the original brood system. Extended feeding in the resinous faecal pellets by pushing them with fresh bark may lead to new broods being formed 3 adjacent to the original. Emergence holes are cut within a plantation during the early years of through the papery outer bark covering the colonisation; especially so when the trees are brood system well in advance of emergence closely spaced shutting out most of the available itself. During this process large amounts of loose, fight and thus reducing temperatures within powdery frass are ejected (Plate 9). Eventually and below canopy. The lower temperatures may emergence takes place over a protracted period discourage the beetles from flying. In crops that with many beetles using one emergence hole. allow greater light penetration and consequently The lengthy period spent by adults under bark higher temperatures increased beetle flight and their piecemeal emergence adds consider activity can lead to more wide ranging dispersal ably to the length of the complete life cycle. and attack. Timings of the life cycle vary greatly and Although flight is said to be rare it undoubt depend on rates of larval development and edly plays a very significant role in the dispersal conditions prevailing when adults mass under and territorial expansion of the insect. The flight bark prior to emergence. The shortest complete threshold temperature is stated by European cycle observed (10 months) resulted from eggs workers to be 23°C but in Britain flight has been laid in early summer but development was then observed at 20°C with secondary take-off at 18°C delayed by the winter months with the adults leading to sustained flight in air temperatures of emerging in the following spring. Therefore 14°C. The roles of other factors such as wind adult activity early in the spring would lead to speed and humidity, which dictate flight in other the shortest cycle. Development is slowest when scolytids, have yet to be determined for D. young larvae from late summer broods overwin micans. ter, the lowered temperatures reducing feeding Evidence provided by infested, isolated spruce rates. Larval diapause is not known in Britain plantations, where no forest operations have and sporadic feeding occurs during the average taken place since establishment, demonstrates winter. Some feeding by larvae has been that D. micans could only have reached such observed at temperatures as low as 2°C. A areas by flying; flight ranges of up to 4 km can be combination of slowly developing overwintering attributed to the beetle. Once aloft a flying beetle larvae leading to a prolonged adult presence is subject to a variety of meteorological condi under bark leads to the most protracted complete tions; thermal currents and winds may deposit cycle. Some adults emerge in late autumn and them at considerable distances from source. usually fail to oviposit, and then may overwinter Observations made on the initial flight of D. under bark in small gallery systems either micans show that after take-off, movement is a singly or in small groups, both above and below rapid upwards spiralling flight to above tree-top ground level, before breeding in the following height implying a positive orientation to fight. spring. The generally unsynchronised breeding There is abundant evidence that isolated behaviour of D. micans means that virtually any single spruces well beyond the range of crawling stage of the insect may be found throughout the insects are located and colonised. Scattered year, though it is uncommon to find eggs and spruces in mixed crops can become infested. pupae between December and March. In general Therefore it is probable that the beetle orien terms the population of D. micans in any one tates to host odours produced by the tree year may be predominantly larval or predomi although it is possible that the beetles may nantly adult. alight on many trees and select their preferred host species by ‘tasting’ or phagostimulation. The female beetle does not appear to have the Dispersal and host selection ability to select, from a distance, individual trees Adults normally disperse by crawling which of lowered resistance. Having located a poten clearly limits the extent of new infestations. This tially suitable host the beetle seeks a favourable tends to create small groups of infested trees site to attempt entry to the cambium in order to 4 breed. These entry sites are frequently beneath be tolerated before part or all of the tree dies. branch nodes, in the apices of forked stems, close Tree age seems to be of little significance, with to drought cracks, calloused wounds or in irregu successful colonisation of trees of 13 years to larities in the stem or root buttresses, although more than 100 years of age being observed. some entries also occur in open bark. Entry is also frequently made in roots at and below ground level. Evidence of attempted beetle col Host susceptibility and breeding onisation in the field shows quite clearly that D. micans does not pre-select individual trees success favourable to its breeding requirements. Howev The predisposition of trees to successful infesta er, when attacks are initiated upon individual tion by D. micans is by no means well under trees there is clear selection of favourable entry stood. Serious damage in Europe has been points upon that tree. closely associated with several years of drought Having selected an entry point the female conditions leading to a general weakening of beetle commences to bore into the bark of the crops. Danish research suggests that winter cold living tree. Once the outer bark is breached resin may be a more significant factor in susceptibility flows from the point of wounding, the quantity to D. micans damage (Bejer, 1988). This is not produced varying considerably between trees. likely to occur in the wetter, milder parts of The volume of resin flow is the limiting factor Britain where spruces are grown. The overall that is important in determining the breeding health and vigour of infested crops is the most success or failure of the invading beetle. How important factor in influencing population de ever, adult D. micans have an extraordinarily velopment of the insect. Badly sited, neglected, high tolerance of resin and, unlike most of the or older crops will have the lowest resistance to scolytids, are able to survive total immersion. colonisation and it is in such woodlands that the Attacking beetles, as they bore into the bark, most serious damage has been observed. Within will use their bodies in a piston-like motion to most spruce crops other factors may influence expel the liquid resin on to the outer bark. the development of D. micans. Trees of bad form, A clear preference by the beetle for breeding in and those with coarse growth and forked stems, Norway spruce has been observed in Britain and widespread extraction wounds, green brashing, in Europe; in mixed stands of Norway and Sitka or crowns broken or otherwise damaged by wind, spruce, or in adjacent pure stands of the two are all prone to successful infestation by the species, it is the Norway spruce that attracts the beetle. insect. None the less Sitka spruce, especially in Generally D. micans does not enjoy high pure stands, is susceptible to attack and the breeding success in Britain. Of all attacks population of D. micans in such crops develops initiated by the beetle only a relatively small normally. Virtually any other spruce species, proportion result in the successful establishment and occasionally Scots pine (Pinus sylvestris L.), of brood (Evans et at, 1984). Once a successful may be successfully used though successful brood is established within a tree it is very often breeding in the latter species is rare in Britain. followed by colonisation in the same tree by The most easily colonised species appears to be successive generations of the insect over several Serbian spruce (Picea omorika (Pancic) Pur- years. This leads to the bulk of the population kyne), which usually produces little resin and is being concentrated in a few stems with nearby readily killed by D. micans. In Sitka spruce, trees showing symptoms of attack but often with death may occur from relatively light colonisa few successful broods. In this way typical expec tion, the most serious form of attack being at the tations of damage, even in well colonised areas, root which is almost always fatal to the tree may be limited to the death of a few trees each (Plate 10). In Norway spruce, unless complete year. Unless crop health is generally poor or girdling takes place below the green crown or in declines rapidly due to other factors, widespread the roots, many years of successive attacks may tree death should not occur. 5 Natural enemies o f D. micans Table 1. Rhizophagus grandis release programme, Britain 1984—88. Excluding the role of the introduced predator, Rhizophagus grandis, mortality of D. micans is Numbers t Total influenced by a variety of other natural enemies Year released locations and environmental conditions. The most potent 1984 31168 942 of these is undoubtedly the greater spotted 1985 39392 655 woodpecker, Dendrocopos major L., particularly 1986 17604 815 during the winter months. The aggregated lar '1987 7100 71 vae and adults of D. micans and the flimsy *1988 7300 73 covering of dead outer bark over the brood Totals 102564 2556 system provides the bird with an easily obtained 'New locations found in 1986 and 1987 peripheral surveys. meal. A parasitoid, Dolichomitus terebrans t56% private woodlands, 44% state-owned woodlands. Ratz., normally associated in Britain with pine weevils of the genus Pissodes has adapted well to D. micans. A parasite of the pupal stage of its host, this insect is commonly found among most Belgium naturally occurring populations of R. well-established populations of D. micans. grandis are credited with maintaining D. micans Another limiting factor is the entomopathogenic populations at a low endemic level, drastically fungus Beauvaria bassiana (Bals.) Vuill. which reducing economic damage to the forests of the infects and kills larval, pupal and adult stages of Ardennes (Gregoire, 1984). D. micans. Other diseases of the larvae have also The potential usefulness of R. grandis in been noted but not as yet identified. Drowning of Britain was recognised shortly after the discov broods has also been seen, particularly in root ery of D. micans. In mid-1983R. grandis adults broods where rising water tables occur during were imported from Belgium and artificially winter, also in forked stems where the presence bred on a trial basis. Later in the same year a of active D. micans broods causes lesions in the policy of using the predator against D. micans bark allowing the accumulation of rainwater. was adopted. Facilities to breed this insect were Egg and first instar stages of D. micans may also established at Ludlow, Shropshire - convenient be engulfed by secondary resin flow and des to the main areas of the D. micans populations troyed. (King and Evans, 1984). Populations of R. grandis were reared under artificial conditions Biological control o f D. micans by and released among the spruce beetle population from 1984 to 1986 when all infested areas of any Rhizophagus grandis significance had been located (Table 1) (King, The predatory beetle, Rhizophagus grandis Gyll. 1987; King and Fielding, 1987). (Coleoptera; Rhizophagidae), specific to D. The first experimental releases, made in 1983, micans, is its single most important natural resulted in successful establishment of the pre enemy. A notable artificial introduction of this dator in one of the three trial sites. Observations insect to populations of D. micans began in 1963 in 1985 of the progress in the main release sites in Soviet Georgia. This programme of artificial showed that R. grandis had established and was breeding and release of the predator is still going breeding at 47 per cent of those sampled. In 1986 on and is influential in controlling serious and 1987 three long-term study plots were outbreaks of spruce beetle in extensive forests of randomly sampled. This showed that, in 1986, 34 Oriental spruce (Khobakidze et al., 1968, 1970). per cent of available D. micans larval broods A second such programme began in 1983 in were attacked by R. grandis. In 1987 this figure Belgium in collaboration with French forest had doubled to 68 per cent (Table 2). authorities to control various outbreaks of D. Other field observations and experiments micans in France (Gregoire et al., 1984). In show that R. grandis has a highly developed 6 Table 2.Sample surveys of three sites following introbiological control by R. grandis. Although the duction ofRhizophagus grandis, 50 pairs per site in breeding and release programme was initiated 1984 without the benefits of prior research and appraisal in Great Britain, it has enjoyed consid Percentage of D. micansbroods with erable success. Widespread surveys show that Random sample R. grandispresent the low density releases, about which there were Site (%) of total number some early misgivings, have proved effective in number o f infested trees 1986 1987 establishing predator populations among the 1 20% 35% 80 known British D. micans population. The most 2 10% 32% 66 recent appraisals of the R. grandis population 3 10% 38% 56 confirm that it is increasing rapidly and is well x34% x68% able to disperse naturally to occupy areas where introductions were less successful or D. micans infestations were overlooked during surveys. The level of D. micans brood systems attacked by R. grandis, currently 68 per cent, 4 years from host-finding ability and disperses over signi the first predator introductions compares favour ficant distances - up to 4 km. It is now well ably with European experience and falls only a established as a breeding species in Britain and little short of the theoretical 80 per cent infesta widely distributed throughout the forest areas tion level considered necessary for effective where D. micans is endemic. suppression of D. micans to minimum damaging levels, observed in the Ardennes forests in Belgium (Gregoire, 1984). Conclusions and implications Although these results are encouraging it is for integrated control still too soon to consider R. grandis as a totally effective and permanent control agent for D. Within its limited British distribution, D. micans is micans. Danish experience during several catas now firmly established as an introduced forest trophic D. micans epidemics between 1940 and pest. Legislation on the movement of potentially the mid 1960s suggests that R. grandis exerts infested timber to prevent the further accidental little control in these situations (Bejer, personal spread of the pest has so far been apparently communication). The most important overriding successful and is a vital part of future strategy. A factor is the health of our spruce forests; good steady, natural territorial expansion of the popu health and vigour in these crops means high lation may be expected particularly westwards resistance to D. micans as well as other pests and and southwards in Wales. Further spread to the diseases. D. micans control still requires a east, north and south of England should be human input; forest managers should always limited through the absence of spruce planta keep a watchful eye on forest areas where D. tions. The population of D. micans in the Trough micans is known to occur. If damage levels are of Bowland represents the greatest potential for seen to rise in such plantations then some the insects spread to the major spruce growing remedial action can be taken. Selective removal areas of north and west Britain. Annual surveys of the worst affected trees during routine thin at the edges of the known D. micans population nings can go a long way to reducing beetle monitor any expansion of the insect’s distribu populations and should be compatible with the tion. Any newly discovered outbreaks are sani role of the predator. The extremely protracted tised and are treated by the introduction of R. life-cycle of D. micans is advantageous to the grandis. forester as sudden and traumatic damage is not The systematic reduction of populations of D. to be expected. However, areas in which silvicul micans by survey and sanitation felling effec tural treatments and regular health appraisal tively prepared the way for the current project of are neglected may ultimately suffer serious 7 damage. In the long term, attention to sound from stocks of R. grandis maintained in culture choice of species and silvicultural treatment can by the Forestry Commission. do much to limit the future expansion and Future research on R. grandis and D. micans impact of D. micans populations. will aim at refining both timing and density of It would be unwise to consider D. micans predator introductions. There is already evi populations as static. Annual surveys on the dence that local high density releases at the edges of the known infested area show a steady right time of year can accelerate the impact of outward spread where spruce forest is available. the predator upon D. micans. A closer knowledge Forest managers so far untroubled by D. micans of the life cycle, biology and behaviour of both cannot be complacent and should look out for insect species is currently being sought. Studies suspicious symptoms in their forests. Early of dispersal and host-seeking by both predator detection and reporting can lead to rapid reme and prey are also subjects of current investiga dial measures including predator introductions tion. ACKNOWLEDGEMENTS The authors wish to thank David Wainhouse for editing the text; Andrew Martin, Terry O’Keefe, Margaret Kay, Elizabeth Dudley, Edward Hill, Joanne Boone, Alan Palmer and Veronica Taylor for their valuable contribution to this work. Also thanks to the many Forestry Com mission and private sector staff who carried out the predator introductions so thoroughly and effectively. The photographs were taken by Colin King and Martin Jukes. REFERENCES GREGOIRE, J-C., MERLIN, J , PASTEELS, J. ANON. (1988).Plant Health Information Sheet M., JAFFUEL, R., VOULAND, G. and No. 12. Forestry Commission, Edinburgh. SCHVESTER, D. (1984). Mass rearings and release of Rhizophagus grandis in Lozere. In, BEJER, B. (1988). Sitkagran og ‘Micans’.Saer- Biological control o f bark beetles, 122—123. tryk a f Dansk Skovforenings Tidsskrift 73, Proceedings, EEC Seminar, Brussels 1984. 34-42. KHOBAKIDZE, D. N„ TVARADZE, M-S. and KRAVEISHVILI, I. K. (1970). Preliminary BEVAN, D. and KING, C. J. (1983).Dendrocto results of introduction, study of bioecology, nus micans Kug., — a new pest of spruce in the development of methods of artificial rearing UK. Commonwealth Forestry Review 62 (1), and naturalisation of the effective entomo- 41-51. phage, Rhizophagus grandis Gyll. against the European spruce beetle, Dendroctonus micans BROWN, J. M. and BEVAN, D. (1966). The Kug., in spruce plantations in Georgia. Bulle great spruce bark beetle Dendroctonus micans tin of the Academy of Sciences o f the Georgian in north west Europe. Forestry Commission S.S.R. 60, 205—208 (in Russian). Bulletin 38. HMSO, London. KHOBAKIDZE, D. N., TVARADZE, M-S., CARLE, P. (1975). Dendroctonus micans Kug. YASHVILI, G. V. and KRAVEISHVILI, I. K. (Coleoptera, Scolytidae). The giant bark beetle (1968). Artificial rearing ofRhizophagus gran or European spruce beetle (Bibliographical dis Gyll. for the control of Dendroctonus Note). Revue Forestiere Frangaise 27 (2), 115— micans in Georgia. Soobschenie Akademii 128. Nauk Gruzinskoi S.S.R. 51 (2), 435—440 (in Russian). EVANS, H. F. (1984). Vigilance is vital in the KING, C. J. (1987). Rhizophagus grandis as a spruce beetle war. Forestry and British Timber means of biological control of Dendroctonus 13 (6), 21-23. micans in Britain. Research Information Note 124. Forestry Commission, Edinburgh. EVANS, H. F. and KING, C. J. (1988).Den droctonus micans—guidelines for forest mana KING, C. J. and EVANS, H. F. (1984). The gers. Research Information Note 128. Forestry rearing of Rhizophagus grandis and its release Commission, Edinburgh. against Dendroctonus micans in the United Kingdom. In, Biological control of bark beetles, EVANS, H. F., KING, C. J. and WAINHOUSE, 87-97. Proceedings, EEC Seminar, Brussels D. (1984).Dendroctonus micans in the United 1984. Kingdom: the results of two years’ experience KING, C. J. and FIELDING, N. J. (1987). Spruce in survey and control. In, Biological control of beetle four years on: Dendroctonus micans bark beetles, 20-32. Proceedings, EEC Semi update. Forestry and British Timber 16 (4), nar, Brussels 1984. 21-24. GREGOIRE, J-C. (1984).Dendroctonus micans VASECHKO, G. I. (1988). The problems that in Belgium: the situation today. In, Biological need to be solved for improvement of bark control of bark beetles, 48—61. Proceedings, beetle control. Journal of Applied Entomology EEC Seminar, Brussels 1984. 106, 1-2. 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