Science & Opinion

invertebrate species from undecayed What do we really wood where the host is the tree species, through early decay where the decay fungus is the host, to late decay where the fauna has become independent of tree know about species or fungus species. The suitability of particular dead woody tissues for different invertebrates depends jewel and very much also on whether the tree itself is in control of the death of these woody tissues. A large proportion of such wood acute oak decline? is heartwood which has died under the control of the tree, with accessible nutrients having been withdrawn and Dr Keith Alexander, Ancient Tree Forum Trustee and waste products deposited. A similar Honorary Adviser to the IUCN on European saproxylic process takes place when lower branches become shaded out by the crown. A different process occurs when sapwood dies by accident, because of disease or This article is provided by the Very few beetles become familiar other damage, where the tree has not Ancient Tree Forum, which names to arborists and the ones that been able to withdraw nutrients first. The champions the biological, cultural do tend to achieve that status because freshly dead woody tissues are much and heritage value of Britain’s they have somehow become perceived more nutritious as a result, and it is the ancient and veteran trees, and gives as ‘problems’. But in the case of cambial zone that is most nutritious as advice on their management at oak jewel beetle biguttatus this is where the living tissues had been www.ancienttreeforum.co.uk. perceptions have been manipulated most active, in generating new xylem and by experts who should know better; phloem. The first wave of ‘deadwood’ the experts have produced no directly on undecayed wood as they have invertebrates to be attracted to this fresh evidence in support of their campaign micro-organisms in their gut which are habitat – all – target the cambial of blackening its name within the capable of digesting wood. These species zone. They respond to airborne chemicals industry. In this article I aim to outline also tend to be specific to particular tree to orientate themselves towards freshly what is actually known about the or shrub species. They feed on freshly available habitat; antennae are biology of this beetle – and some of dead or even dying woody tissues – very acutely sensitive to volatile hydrocarbons what is not known – and how this has few can overcome the natural defence given off by plants. A living healthy tree been misrepresented in the press. mechanisms of living tissues. As decay smells very different to a dying tree. proceeds, then the woody tissues are Most of these insects have no interest in There are in excess of 2,000 different abandoned by these early arrivals and living healthy trees, but a dying or freshly species of invertebrate known in Britain begin to attract different species which are dead tree is very attractive. As ‘fresh’ which are dependent on the process of more associated with the broad type of dead sapwood is a short-lived resource wood decay. They are often referred to fungal decay – brown rot or white rot. As for the insects – one or two years at as ‘deadwood’ species, but this is more the decomposition progresses further, the most – they have to be highly mobile a name of convenience than an accurate species composition of the invertebrate so that the habitat can be found and description of their habits. The majority fauna changes again, when the level of exploited quickly, before it becomes ‘old’ actually require the activity of fungi to moisture and volumes of accumulating deadwood. begin breaking down the otherwise debris (wood mould) become more indigestible cellulose and lignin content of important. Eventually the decayed material the woody tissues – they are wood-decay becomes indistinguishable from soil and The biology and conservation species. A small number of the species it is typical soil invertebrates that are status of oak jewel beetle (less than 5%) are, however, able to feed present. There is a clear succession of Oak jewel beetle is amongst those early-successional species whose host is the oak tree, rather than a decay fungus. Its larvae feed directly on the undecayed dead or dying cambial zone and outermost sapwood of physiologically stressed, dying or recently dead trunks or larger thick-barked boughs of mature and older oak trees. The oak jewel beetle is not alone in this habitat: other typical associates include the longhorn beetle Phymatodes testaceus and the bark beetle Dryocoetes villosus. Similarly, freshly dead smaller branch wood with thinner bark is the habitat for another jewel beetle, Agrilus laticornis, and another bark beetle, Scolytus intricatus. Many other insect species are also attracted, including The oak jewel beetle Agrilus biguttatus. (Roger Key) predators and parasites.

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course the only beetles active in freshly A national recording scheme was set dead oak wood and so great care needs up in 1984 to collate records of all jewel to be taken with identification using the beetles and soldier beetles, and it quickly shape and size of emergence holes; they became apparent that the British range are an indication of species only and not of oak jewel in particular appeared to 100% reliable. It is always better to find be expanding (2003 Provisional atlas supporting evidence for the identification, of the Cantharoidea and Buprestoidea by examining the type of larval galleries for (Coleoptera) of Britain and Ireland. example. Biological Records Centre). I commented that an increase in its abundance across The adults of the oak jewel beetle are parts of the south-east in the early 1980s rarely seen – unlike those of many other had been boosted by the great storms beetle species, they are not attracted of the late 1980s and the (then) recent to blossom for feeding; they appear appearance of chronic oak decline. An adult emergence hole of oak jewel beetle to feed on leaf material high in the Climate change may have been a key from bark on a parkland oak in Tatton Park, crowns of trees, and so the species is factor in the original increase. Our Cheshire. readily overlooked unless observers are have subsequently shown themselves familiar with the larval galleries and adult to be particularly susceptible to various The adults of the above beetle species emergence holes in oak bark. In areas diseases or decline syndromes in recent are the first to arrive on the scene of which suffer from tidiness – where dead decades, and oak jewel has continued recent death; the females insert their eggs trees are felled and removed, and where to prosper as a result. The most recent into the bark and the larvae feed on the fallen branches are cleared away – the Species Status Review (2014) by Natural nutrient-rich tissues of the dead cambial species could remain undetected for England has concluded that it is no longer zone. These species are different shapes decades. The species becomes confined scarce or threatened in England. This and sizes, and so the emergence holes of to aerial deadwood, where limbs are should be good news, that a species the adult beetles are very distinctive. The damaged but do not fall, and to patches once confined to a few refugia in our most emerging adult beetle appears to be aware of trunk bark which die beneath torn-out ancient forests has been able to expand of its cross-sectional shape and does or sawn branches. There is no evidence back out into the wider countryside, but not waste time and energy by excavating at all that the presence of small colonies unfortunately this expansion has been any other shape in order to emerge from in patches of dead bark leads to the death the result of continued and unsustainable the bark and fly off. Jewel beetles have of the living healthy tree. losses of our mature oaks. flattened upper surfaces and round undersides and so excavate D-shaped The adult beetles are sun-loving insects holes for emergence. The longhorn has and tend to favour oaks growing in open Scientific research on the round upper and undersides, and so situations such as parkland. It has been beetle in relation to acute oak excavates an oval-shaped emergence widely noted that egg-laying tends to decline hole. The distinction between D-shaped take place on the warmer south-facing and oval can be easily seen in many cases side of host trees. As with so many It was natural, then, for suspicion to fall but the two general shapes can also other wood-decay insects, the beetle’s on the oak jewel beetle as perhaps more grade into each other making identification European range is mainly central, where directly involved in the death of the oaks problematic. The bark beetles are different relatively warm continental summers are than had previously been supposed. But sizes, with the Scolytus larger than the typical. The English population is near the despite extensive study of the various Dryocoetes, and so broadly speaking northern edge of the range and may be named oak diseases and syndromes – the emergence holes are round, but of expected to fluctuate according to local including the current concerns over acute differing diameters. These are not of weather patterns, with poor performance oak decline (AOD) – no cause-and-effect in relatively cool and damp summers, but has proved demonstrable for oak jewel. strengthening in relatively warm and dry This is really not surprising given that the sunny summers. beetle is only attracted to a tree which is already ailing or dead in part at least, Oak jewel beetle has a very interesting and so it would have no interest in a living history in England. It was until recently healthy tree other than any patches of sufficiently rare for an assessment of its damage. Could the adults be carrying conservation status by the national authority the agents of disease with them? A on the group, Brian Levey, to conclude that reasonable question, but no supporting it was ‘Vulnerable’ (1987 British Red Data evidence is available. Can the suspected Books: 2. Insects). The history of the beetle causal agents spread through tissues of does date back a long way in England and the tree via the larval galleries? Again, this it seems reasonable to conclude that it is is not proven – the larval galleries do not a true native. Oak jewels were recorded enter the living healthy cambial zone of from Sherwood Forest, the New Forest the tree. and Windsor Forest, but few other sites. The beetle was assessed as being a relict Of recent scientific papers that I have seen, old forest indicator species on the basis of only one has examined the biology of oak records collated through the late 1970s and jewel objectively – a review in Forestry early 1980s (1986 Pasture-woodlands in (Brown et al., 2014) which notes that: Lowland Britain. A review of their importance for wildlife conservation. ITE), although it • Despite the recent emphasis on it as a Keith Alexander searching for tracks, trails and signs of was also noted that it was occasionally insect, it has been relatively little beetles. imported in timber. studied.

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Any proposed management programme selected trees over 16 sites) – a rather should consider that Agrilus biguttatus is biased statement which implies that most a native British beetle and, until recently, AOD has co-occurrence with the beetle. considered rare. But turn that same figure around and 10% of trees showing AOD have no co- Unfortunately for people who only read occurrence with the beetle. So if the beetle abstracts, the key points only appear in is involved in AOD, how did those 10% the main text; this is a common problem become infected? Given that the beetle is in the pest management industry where attracted to diseased trees it is surprising the language itself is tainted with value- that the figure of co-occurrence is so low! judgements – notably ‘attack’ when But, so far as AOD is concerned – and they really mean ‘attracted to’. Once one according to evidence currently available understands the biology of oak jewel, the – the presence or absence of oak jewel is frequent association of AOD with the larval irrelevant. Denman et al. (2014) comment galleries and distinctive adult exit holes is that a key issue is to determine whether the not at all surprising but does not indicate high co-occurrence of oak jewel and AOD cause and effect. Brown et al. (2014) is merely a coincidental relationship or an correctly refer to this lack of evidence for essential part of the disease. The authors cause and effect in the main text but not in admit that they have found no evidence, the abstract. but only after they have argued that the presence of the beetle is a sign of AOD. In contrast to the objectivity of this review is a parallel paper (Denman et al., 2014) The oval-shaped exit holes of a longhorn which claims to describe the symptoms of beetle – presumably Phymatodes AOD and yet includes the larval galleries testaceus – are also noted by Denman et of oak jewel. There are many dictionary al. (2014) in their sample trees, but this fact A dying mature oak in Moccas Park. There are many medium- definitions of ‘symptom’ but the one that is not commented on further. What was aged oaks dying or dead in this nationally important old covers most people’s understanding of the the level of co-occurrence between this parkland which is amongst the richest English sites for term is: evidence of disease or physical beetle species and AOD? Not surprisingly, saproxylic beetles. The oak trees had been dying prematurely long before oak jewel beetle recently colonised the site. disturbance. But in the case of oak jewel populations of this longhorn beetle have there is no evidence for cause and effect also been increasing over the same period – as clearly stated in the paper by Brown as has oak jewel beetle – why is it being et al.; our current understanding of oak ignored by the researchers? Why have they jewel biology is that the beetle follows the chosen to pick on oak jewel rather than • Only a few Agrilus species globally disease, not that it is involved causally. this longhorn – or any other insect species are considered pests; typically these So, while oak jewel might conceivably be that happens to show obvious signs of its are exotic introductions, e.g. emerald considered a symptom of a dead tree, it is presence? ash borer, an east Asian species illogical and unhelpful to describe it as a introduced to North America and symptom of AOD. Is control premature? western Eurasia via wood packing material. Denman et al. (2014) note that oak jewel Suggestions for removal of oak-jewel- has been reported from 90% of trees colonised trees are clearly seriously • Where host and pest species have with AOD (19 out of the 21 randomly premature. What evidence is there that co-evolved, healthy trees are generally able to resist attack, so trees tend to be physiologically stressed or altered in biochemical status before they become susceptible.

• Female beetles are thought to select oviposition sites on host trees that are physiologically stressed by one or a combination of factors.

At present the cause-and-effect order remains unclear; oak jewel may be attracted to trees affected by AOD or may simply be selecting weakened hosts predisposed to decline; host condition may be the critical factor in determining which trees suffer from AOD.

To date, no Agrilus species has been demonstrated to actively vector a plant pathogen.

The old oak pollards at Cwm Byddog Nature Reserve in Powys. One of these live and healthy At this time, there is no evidence to oaks had a small patch of dead trunk bark below where a large limb had recently ripped out contradict its status as a secondary stem- and which had been colonised by oak jewel beetle, and where a few of the distinctive adult boring beetle attacking weakened oak. emergence holes were noted.

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this might impact on AOD, given that researchers have yet to work out how the causal agents of AOD get around? Standing deadwood is a rare and threatened habitat throughout modern tidy-minded and risk-averse Britain. It would seem unlikely that oak jewel is the only native wildlife present. We know that mature trees are increasingly scarce in the landscape and so recommending removal of standing dead trees only adds to the sterilisation of our countryside. Brown et al. (2014) comment that trees standing dead for more than a year will be unlikely to yield any further individual beetles.

Brown et al. (2014) mention two known parasitic wasps of larval oak jewel – the ichneumonid Deuteroxorides elevator and the braconid Spathius curvicaudis – but provide little additional information. Only the latter is known from England. The two Kedleston Park, Derbyshire. Mature oaks had been dying here prematurely for many years key national experts on the Spathius (now before the site was recently colonised by oak jewel beetle. regarded as the same species as Spathius erythrocephalus) say that this wasp is to oak jewel, so any attempts to increase AFTER trees have already been affected almost certainly a widespread species its impact would be misguided. by AOD. The research papers have clearly and so will already be impacting on oak demonstrated that some trees with AOD jewel populations. They do not believe Conclusions symptoms have no signs of oak jewel that any artificial manipulation of the wasp beetle – this is evidence that the beetle is population would have any further impact. There is currently no evidence that oak not an essential causal factor. There are It is a gregarious species that parasitises jewel beetle has a causal role in AOD, still questions which merit investigation, medium-sized beetles to be found in the and indeed, the known biology of oak but it is very premature to be describing subcortical area of fairly recently dead or jewel strongly suggests that it is merely oak jewel beetle as a pest of oak dying trunks or branches. It is not specific attracted to dying and dead tissues trees and advocating its control.

and advisers. The handbook is now Have you booked a place on News available to freely download from the the ATF’s Summer Forum? ATF’s site. For those wanting a hard copy The 2015 Ancient Tree Forum update it can also still be purchased for £30. conference is to be held in East Anglia on Thursday 18 and Friday 19 June, Valuing and managing and it will bring together a wide range veteran trees: a ‘training of experts and enthusiasts interested New website for the Ancient the trainer’ course in the ecology and management Tree Forum The Ancient Tree Forum is of ancient and veteran trees. The The Ancient Tree Forum (ATF) has running ‘Valuing and managing veteran two-day event will include field developed a new website, which trees: A three-day advanced course for visits to wood pastures, orchards is designed to be fully accessible for trainers’ from 29 September to 1 October and other sites, presentations from those on the go as it adapts for viewing in Cumbria and from 8 to 10 December expert speakers, and networking on tablets and mobiles. The site, www. at Gilwell Park in Essex. The object of opportunities. Booking is essential. ancienttreeforum.co.uk, already includes the course is to train people who will For more information and details of a wealth of information on ancient and in turn be able to deliver a one-day how to book a place, see the ATF’s other veteran trees and their management, course on valuing and managing veteran website or email [email protected]. and more content will be added over the trees. The course is a mix of indoor coming weeks and months. and outdoor activities and is intended ATF autumn field meeting to be interactive and participatory. The ATF’s handbook now available content includes topics such as the The ATF’s autumn field meeting will take online aging process, the value of veteran place in the Lake District on Thursday Many ARB Magazine readers are likely trees for biodiversity and heritage, roots, 8 October. As always, the event is open to be familiar with the ATF’s book management of veteran trees and site to everyone, but places will be limited Ancient and other veteran trees: further management. Participants will be given and booking is essential. Details will be guidance on management, edited by all the material needed to deliver the available on the ATF’s website. David Lonsdale. The 212-page handbook one-day course including presentations on the management of ancient and and handouts. Early booking is Sign up for our newsletter veteran trees is widely recognised as advisable, and full details and booking If you would like to receive the ATF’s an invaluable source of information and information can be found at www. free digital newsletter, email enquiries@ advice for arboriculturists, landowners ancienttreeforum.co.uk. ancienttreeforum.co.uk.

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