Tag Archives: Hylobius abietis

November 13, 2019 · 9:50 am

Weevily clever – on being behaviourally resistant

I am currently sharing my office with a Tupperware container of weevils, Hylobius abietis, the Large pine weevil to be exact.  The reason, just in case you were wondering, is that I have had an undergraduate doing her final year research project with me on ways in which this highly pestiferous weevil might be prevented from feeding on newly planted conifers.  The weevils in my office are those that were left over from her project and being the old softie that I am, and having worked on Hylobius since 1987 I couldn’t bear to throw them away :.)

My office pets – easy to maintain and quite cute

Adult and larvae of Hylobius abietis

You might think that having worked on an insect with the sole aim of trying to reduce its pestiferousness, that I might have succeeded by now.  Say that to the many scientists who have addressed this problem for more than a century and you will be rewarded with the sound of hollow laughter.  The laughter is even hollower if you point them to the statement made by the first UK Forestry Commission entomologist,  J W Munro, who a mere ten years after the formation of the Forestry Commission wrote “The pine weevil (Hylobius abietis) problem still occupies the attention of the Forestry Commissioners” Munro (1929).  Ninety years on I can make exactly the same statement and judging by the global number of papers written about Hylobius, I think I can confidently state that the same can be said for the forest industry as a whole.

Not a problem that is going away! Papers published on Hylobius abietis since 1910.  Data from Google Scholar and Web of Science.

So why is the large pine weevil, or Hylobius as those of us who work on it or attempt to control it, call it so hard to manage? The simple answer is that we have helped it become a pest in the first place and in the second place it has a couple of attributes that give it a bit of an edge. You might even go so far as to say that it is a clever little beast.

First a little bit of history is in order. Up until the beginning of the 20th century references to Hylobius are few and far between, especially in the UK, although there are some German references from the latter half of the 19th Century, a reflection of the fact that the German forest industry was well in advance of that in the UK. Prior to the establishment of conifer plantations, populations of Hylobius would have been small and scattered as the larvae need conifer stumps or large pieces of fallen branch in which to develop.  The adults, which can live for up to four years (Leather et al., 1999), would normally feed on the cambium of thin barked twigs in the upper canopy of conifer trees, and the larvae, depending on how shaded the host stump was, could take from a year to two years to reach adulthood.  The adults are extremely responsive to host volatiles (Nordenhem & Eidmann, 1991) and can locate host plants and egg-laying sites remarkably quickly*.  Plantation forestry with its cycles of clear-fell and subsequent restocking with two year old conifer saplings has been akin to setting up a deliberate breeding programme for Hylobius.  In some cases 100% of all new planting can be destroyed by the adults ring-barking the saplings and on average 30% would be lost if plants were not pre-treated with insecticide.

How to turn an innocuous forest insect into a major pest. Plantation forestry and how it created a forest pest. (Figure adapted from Leather et al, 1999).

Over the years there have been a number of attempts at controlling Hylobius without using insecticides, including cultural methods, physical barriers and biological control using entomopathogenic nematodes (Williams et al., 2013), none of which have been as effective as insecticidal treatment. The latter, although reasonably effective at preventing sapling damage, may not, however, be reducing Hylobius numbers.  This is because Hylobius is, as well as being good at detecting host volatiles, also great at detecting and avoiding insecticides.  A former PhD student of mine, Dan Rose, showed this is in a series of elegant experiments where he manipulated insecticide presence and absence at different scales (Rose et al., 2005).  First he tested if adult Hylobius could detect the presence of an insecticide at a whole plant level, by giving them a choice in semi-field conditions between treated and untreated saplings.  They could, they avoided feeding on treated plants.  Then he gave them a choice of plants where he had sprayed half the canopy with an insecticide, and, yes, you guessed it, they only fed on the untreated parts.

 Given a choice, adult Hylobius abietis will not feed on insecticide treated plants or on those parts of a tree that have been treated with an insecticide

Dan wondered just how good their discriminatory powers were, so using our standard choice boxes,

Standard Hylobius abietis host choice test box

he presented his weevils with pieces of pine twig that had had insecticide painted on to them alternating with equal width untreated stripes, and yes, you guessed, they only ate the untreated parts of the twig.

  Adult Hylobius abietis only fed on the untreated stripes.

Next he sprayed twigs all over, but some with large droplets and some with fine droplets and then gave them the choice between a coarse sprayed twig and a fine sprayed one and as you may have guessed,  they were able tell the difference, and fed on the twigs with the bigger spaces between the droplets of insecticide.

Given a choice between twigs treated with a large droplet spray and a fine droplet spray, adult Hylobius abietis will feed on the twigs with the large droplet size spray application.

 

So this is an indication that adult Hylobius are behaviourally resistant to insecticides, well at least the ones he tested them against. Hylobius are not alone in possessing this trait, other weevils (Haddi et al., 2015) and at least one aphid species (Fray et al., 2014) are also able to detect and avoid insecticide treated substrates.

Hylobius adults are also quite resistant to insecticide poisoning when you force them to eat treated plant material. Some individuals take almost three weeks to die and then if they are removed from the insecticide treated food they soon return to normal.

Figure borrowed from Rose et al.,( 2005)

Remarkable rate of recovery (Figure borrowed from Rose et al., (2006)

 

Hylobius abietis adults are able to recover from pesticides if given the chance, even after a week of exposure.

Given that they are able to recognise and avoid eating treated plant material and if they do, show remarkable powers of recovery, it is very likely that in the field, the reason that the insecticidal treatment works is more to do with repellence than toxicity, so it is unlikely that weevil popualtions are reduced.

To reduce populations rather than divert them elsewhere and given the pressure to remove pesticides from the forest environment, a biological control approach is the logical best option. Entompathogenic nematodes are probably the best option and have received  a lot of attention over the last thirty years or so (Williams et al., 2013), but again Hylobius has a tactic or two up its elytra to make it more difficult to control than other insect pests.  First, like its North American cousin, Hylobius pales (Cornell & Wilson, 1984; Moore, 2001), it can play dead, a phenomenon known as thanatosis or death feigning. In human terms, when they see/feel a nematode approaching, they hold their breath and collapse in a heap. In insect terms, they close their spiracles, the point of entry for the nematodes, and hope that the nematodes give up and go away before they have to breathe again.  If they do have to breathe when the nematodes are still in contact with them then clever old Hylobius is able to brush them away (Ennis et al., 2010). Biological control of adult Hylobius is thus unlikely to be successful, and the larvae and their stump habitats are now the main target of biological control methods (Williams et al., 2013).

Clever, cute and long-lived, what more can you ask for in a pet or should that be pest? 🙂

 

References

Cornell, J.A. & Wilson, L.F.  (1984) Dispersion and seasonal activity of the pales weevil, Hylobius pales (Coleoptera: Curculionidae), in Michigan Christmas tree plantations. Canadian Entomologist, 116, 711-717.

Ennis, D.E., Dillon, A.B. & Griffin, C.T. (2010) Pine weevils modulate defensive behaviour in response to parasites of differing virulence. Animal Behaviour, 80, 283-288.

Fray, L.M., Leather, S.R., Powell, G., Slater, R., McIndoe, E. & Lind, R.J. (2014) Behavioural avoidance and enhanced dispersal in neonicotinoid-resistance Myzus persicae (Sulzer). Pest Management Science, 70, 88-96.

Haddi, K., Mendonça, L.P., Dos Santos, M.F., Guedes, R.N.C & Oliveira, E.E. (2015) Metabolic and behavioral mechanisms of Indoxacarb resistance in Sitophilus zeamais (Coleoptera: Curculionidae). Journal of Economic Entomology, 108, 362-369.

Leather, S.R., Day, K.R. & Salisbury, A.N. (1999) The biology and ecology of the large pine weevil, Hylobius abietis (Coleoptera: Curculionidae): a problem of dispersal? Bulletin of Entomological Research, 89, 3-16.

Moore, R. (2001) Emergence trap developed to capture adult large pine weevil Hylobius abietis (Coleoptera: Curculionidae) and its parasite Bracon hylobii (Hymenoptera: Braconidae). Bulletin of Entomological Research, 91, 109-115.

Munro, J.W. (1929) The biology and control of Hylobius abietis L. Part 2. Forestry, 3, 61-65.

Nordenhem, H. & Eidmann, H.H. (1991) Response of the pine weevil Hylobius abietis L. (Col. Curculionidae) to host volatiles in different phases of its adult life cycle. Journal of Applied Entomology, 112, 353-358.

Nördlander, G., Hellqvist, C., Johansson, K. & Nordenhem, H. (2011) Regeneration of European boreal forests: effectiveness of measures against sedling mortality caused by the pine weevil Hylobius abietis. Forest Ecology and Management, 262, 2354-2363.

Rose, D., Leather, S.R. & Matthews, G.A. (2005) Recognition and avoidance of insecticide-treated Scots pine (Pinus sylvestris) by Hylobius abietis (Coleoptera: Curculionidae): implications for pest management strategies. Agricultural and Forest Entomology, 7, 187-191.

Rose, D.R., Matthews, G.A. & Leather, S.R. (2006) Sub-lethal responses of the large pine weevil, Hylobius abietis, to the pyrethroid insecticide lambda-cyhalothrin. Physiological Entomology, 31, 316-327.

Williams, C.D., Dillon, A.B., Harvey, C.D., Hennessy, R., McNamara, L. & Griffin, C.T. (2013) Control of a major pest of forestry, Hylobius abietis, with enomopathogenic nematodes and fungi using eradicant and prophylactic strategies. Forest Ecology & Management, 305, 212-222.

 

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February 17, 2014 · 9:08 pm

How ready is the UK to combat current and future threats to our forests and woodlands?

Almost exactly two years ago (February 2012) a consignment of ash trees sent from a nursery in the Netherlands to one in Buckinghamshire, were confirmed to be infected by the fungus causing ash dieback, Chalara fraxinea.  By October of that year, it had been confirmed by Food & Environment Research Agency (FERA) scientists to be present in a number of woodland sites within the natural environment.  The story was quickly picked up by the national press http://www.telegraph.co.uk/earth/earthnews/9660538/Ash-dieback-now-beyond-containment.html and other media http://www.bbc.co.uk/news/science-environment-20079657 and articles about the severity of the disease and our inability to control it spread proliferated at  a fantastic rate.  Partly as a result of this, the Tree Health and Plant Biosecurity Expert Taskforce was convened by the Government’s Chief Scientific Advisor in November 2012.  I was invited to be a member of the Taskforce which was an independent, multi-disciplinary group of members of the academic community, https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/200428/tree-taskforce-tor.pdf and very willingly, agreed to serve on it.   Our remit was to “provide advice on the current threats to tree health and plant biosecurity in the UK and make recommendations about how those threats could be mitigated”.   What surprised me and other members of the Task Force was the interest and emotional responses that ash dieback generated among the general public.  After all, a few years earlier another one of our iconic tree species, oak, was under threat by another fungal disease, Phytopthora ramorum, somewhat misleadingly known as Sudden Oak Death, which despite its potential threat to cause landscape level changes comparable with those caused by Dutch Elm Disease (Potter et al., 2011) failed to cause the same  level of media hysteria.  Our best guess for why there was such an outburst of press and media coverage and subsequent public concern about ash dieback, was that the Chalara outbreak was the straw that broke the camel’s back.  People, had perhaps become sensitised to forestry due to what seemed to be a constant stream of stories of threats, both man-made, such as the proposed sell-off of parts of the Forestry Commission’s estate by the UK government in 2010 http://www.telegraph.co.uk/earth/countryside/8082756/Ministers-plan-huge-sell-off-of-Britains-forests.html and natural, such as Sudden Oak Death and other pests and diseases.

For the record, although Chalara  fraxinea is now being treated as a quarantine pest under national emergency measures and is widespread across the  United Kingdom and Northern Ireland, it no longer makes the front pages of our national newspapers.

Ash dieback distribution

http://www.forestry.gov.uk/chalara

We in the Tree Health Taskforce did not just consider ash dieback; we reviewed the whole range of biotic threats, both current and future, and highlighted a number of reasons that we felt had contributed to the problems and made recommendations about how these could be rectified.  In essence, how could we stop yet another ash dieback scenario occurring. Our joint report was published in May 2013 https://www.gov.uk/government/publications/tree-health-and-plant-biosecurity-expert-taskforce-final-report.  One of our major findings was that the UK as a whole lacked, or would shortly lack, enough trained personnel able to recognise and respond to threats to our forests and woodlands from native and alien pests and diseases.  One of the more immediate outcomes of our report was the rapid commissioning of some research to determine just how serious the situation actually was.

The results of this report were published by Defra on February 5 of this year,  TH0115 Strategic Analysis of Capability and Capacity to undertake Tree Health Research and Evidence Activity in the UK.  The report highlighted research and evidence themes identified by key policy stakeholders and forest researchers from the university sector, research institutes and forest industry.

Ten themes were identified – Horizon scanning, Pathways and trade, Pest and pathogen biology and epidemiology, Detection and surveillance, Ecological patterns, Control and Management, Adaptation and resilience in forests and forestry, Governance and contingency planning, Economic evaluation and analysis and finally Public engagement, communication and citizen science.

Three of the themes – Pest and pathogen biology and epidemiology, Control and management and Adaptation and resilience in forests and forestry, were identified as areas where existing research providers lack current capability and/or capacity in one or more types of expertise.

The report also highlighted that there are serious skills shortages in the UK in mycology, plant pathology and entomology, especially in relation to forest health. Even in those disciplines where universities still run undergraduate degree courses, tree specific expertise such as silviculture, the care and cultivation of forest trees, was also noted as being in short supply.

So how did we get into this mess?  Why are we seeing what appears to be an unprecedented assault on the UK by invasive forest pests and diseases (Defra 2013).  Exotic and invasive insects are not a new phenomenon in the UK; the European spruce sawfly, Gilpinia hercyniae was first recorded in 1906, the Douglas fir woolly aphid Gilleteela (Adelges) cooleyi) in 1913, the web spinning larch sawfly Cephalcia lariciphila in 1953, Megastigmus spermotrophus, the Douglas fir seed wasp since at least the late 1940s,  Ips cembrae, the large larch bark beetle, since at least 1955

Ips cembrae

Ips cembrae  http://www.padil.gov.au/pests-and-diseases/Pest/Main/135614

and the great spruce bark beetle, Dendroctonus micans since at least 1973 (Crooke & Bevan, 1957; Bevan 1987).  Apart from Dendroctonus, none of these insects has however, had landscape level effects or for that matter, made the headlines to the same extent that ash dieback did.   Since the beginning of the current century the situation has changed dramatically, the influx of tree pathogens has continued to rise at an almost exponential rate and the number of potentially landscape changing insect pests has also seen an increase e.g. the horse chestnut leaf miner, Cameraria ohridella, first seen in London in 2002  (Straw & Williams,  2013) is now found as far north as Liverpool in the West and North Yorkshire in the East (personal observation); the pine tree lappet moth Dendrolimus pini, established in Scotland since 2004.  The oak processionary moth, Thaumetopoea processionea, has been firmly established in London since at least 2006 and looks set to spread further north and west (Townsend, 2013); it is probably only the bizarre weather we have had the last couple of years that has slowed it down slightly.  The Asian longhorn beetle, Anoplophora glabripennis, caused some concern when an outbreak was found in 2012 in Kent; the eradication of which resulted in the felling of several hundred healthy trees.

Anapolophora

Anopolophora glabripennis  (source USDA)

A related species, the Citrus longhorn beetle A. chinensis, is often intercepted but so far is not known to have established in the UK (Nigel Straw personal communication.)

Given the time that it takes for an exotic insect to reach noticeable population levels, all these insects may have actually established four or five years earlier and it could already be too late to eradicate these pests.  Attempts to eradicate the Oak processionary moth from London have, for example, now ended and been replaced by a policy of containment and eradication is only attempted in the case of new outbreaks outside London (Forestry Commission 2013).  Another species which has often been intercepted since the 1970s, is Ips typographus, a severe pest of spruce.  Other possible invaders include the pine processionary moth Thaumetopoea pityocampa, other Ips species attacking pine and spruce, and of great, and increasing concern, the emerald ash borer, Agrilus planipennis, a native of Asia which is now spreading rapidly outwards from Moscow (Straw et al., 2013).

Agrilus_planipennis_001

Agrilus planipennis  (source Pennsylvania Department of Conservation and Natural Resources – Forestry Archive)

So what may have caused this flood of new forestry pests in the UK?  The most obvious change to forestry practice in the UK which undoubtedly influenced the rise of the exotic conifer pests of the first half of the 20th Century was the large-scale afforestation programmes of many non-native tree species, brought about by the formation of the Forestry Commission in 1919.  This rapid afforestation of sites, many of which had not had trees on them for centuries,  provided new hosts for native pests and pathogens and inadvertently allowed the introduction of non-native insects.  The other major change over the last 50 years or so is in global trade patterns; the world is a much smaller place, goods travel extremely quickly, come from much further afield and in greater volumes.  The ability to transport living plant material has also much improved.  In pre-container and pre-bulk air transport days, goods that were packed with unprocessed or poorly processed timber (pathways exploited by many bark beetles) took many weeks to make the long sea voyages and the insect pests often did not survive to make it to land and a new host plant.  Long sea-voyages also meant that the transport of living plant material and their accidental insect passengers also had less chance of surviving to reach the UK.  Another major change to our trade habits is the “instant tree/garden syndrome” where developers and the general public are no longer willing to wait several years for their trees to grow; rather they plant semi-mature trees, many of which come from outside the UK and which come with very large root-balls.  It is impossible for the Plant Health and Seed Inspectorate (PHSI) service to check the huge volume of soil associated with these roots and many organisms must be entering the UK unbeknownst to the very over-stretched PHSI.

An often overlooked change that I am certain has contributed to the large-scale invasion of tree pests and diseases, is a result of re-organisation of the Forestry Commission.  Prior to 1990, the Forestry Commission had a localised approach to forest management.  Most forest blocks or amalgamations of them had a Chief Forester or Head Forester in charge of them.  He (very rarely she), lived in the near vicinity and much like the old village Bobby, walked his beat regularly.  Changes in forest health were thus much more likely to be spotted early and a forest pathologist or entomologist from either The Northern Research Station (NRS) or Alice Holt called in to make an assessment as to the cause of the problem.  I worked at NRS during the 1980s and early 1990s so have had personal experience of the effectiveness of this system.  By 1990, the Forestry Commission had amalgamated many forests and the number of District Offices was much reduced with a consequent reduction in the number of foresters living in near to individual forest blocks.  Forest health problems were thus much less likely to be noticed at an early stage.

The other major change was the decision to shift research to amenity forestry and away from commercial production forestry leading to a reduction in the number of entomologists and pathologists employed by the Forestry Commission as budgets were redirected.  There are now no longer enough key personnel in these disciplines to cope adequately with current problems, let alone those likely to arise.  At the same time within the university sector, the way in which government-funded universities was changed  to a system based on the outcome of the notorious publication metric based Research Assessment Exercise.  This disadvantaged academics specialising in niche applied disciplines such as entomology and plant pathology whose research output rarely, if ever, made it into the hallowed pages of Nature and Science.  Recruitment of staff in these areas in the research intensive universities was severely curtailed and retirees replaced by molecular biologists or vertebrate ecologists publishing in so-called ‘high-impact’ journals (Leather, 2009).  Universities have also replaced many specialist niche degrees with more broadly based subjects perceived to be more attractive to students.  As a result, teaching in these areas has also suffered and very few biology undergraduates in the UK today have any experience with whole organismal biology or the field and taxonomic skills needed be able to recognise forest health problems outside in the real world (Leather & Quicke, 2010).  The situation is now very critical, with, as far as I know only two forest entomologists (if you count me) and one forest pathologist teaching in UK universities today.  This is not a healthy situation for the country and we in the Tree health and Plant Biosecurity Expert Taskforce highlighted the need to address key skills shortages in this area as an urgent priority (Defra, 2013).

Worryingly, the problems do not just lie with exotic and invasive pests.  There are a number of long-established native pest species that still need research into their control and management.  The large pine weevil Hylobius abietis, which in the words of the

hylobius2

Hylobius abietis adults

first Forestry Commission entomologist J W Munro writing just ten years after the formation of the Forestry Commission stated “The pine weevil (Hylobius abietis) problem still occupies the attention of the Forestry Commissioners” (Munro, 1929).  The same statement is still as pertinent today although control measures for this insect have evolved greatly from the early use of DDT and organophosphates to more sophisticated, but possibly no more effective, biological control options (Torr et al., 2007).  The pine beauty moth, once a harmless indigenous moth species, rose to become a notorious pest of the introduced Lodgepole pine during the 1970s and still continues to pose a threat to Scottish plantations today (Hicks et al., 2008).   The often over-looked pine looper moth, Bupalus piniarius, may yet cause problems to our native Scots pine (Straw et al., 2002a). The green spruce aphid, Elatobium abietinum  has never gone away (Straw et al., 2002b) and may, if climate change predictions  are correct, make Sitka spruce a non-viable crop in the UK (Straw et al., 2009).

This is a problem we ignore at our peril.  Action needs to be taken, sooner, rather than later. As conventional chemicals are withdrawn and fewer chemicals approved for use in forestry, the emphasis must inevitably shift to biological control methods using classical natural enemies or biopesticide approaches with entomopathogenic fungi or nematodes or microbially derived pesticides such as Bt which was used against the Oak processionary moth in Berkshire in 2013.  We may even be able to develop even more specific methods such as pheromone disruption combined with improved tree resistance (Leather & Knight, 1997).   We need to improve quarantine measures, develop better detection methods and urgently provide more funding to enable the employment and maintenance of an expanded Plant Health Inspectorate as recommended by the Tree Health and Plant Biosecurity Expert Taskforce (Defra, 2013) and by TH0115.  The latter report highlighted the widespread concerns about the lack of undergraduate and even more critically, the lack of MSc and PhD opportunities in forestry and tree health in particular.

A key recommendation of the report is that funding needs to be put in place to support postgraduate level teaching and training support. This is to make sure a new generation of people capable of working in the tree health area, assisting a smoother and more efficient transition from broad-based undergraduate biology degrees to PhD level research.

To staff the proposed new inspectorate and to make sure we have a new cohort of well-trained forest health experts, we need to encourage newly qualified undergraduates to take up the existing training opportunities at post-graduate level, such as the MSc courses run in Entomology, Integrated Pest Management and Conservation & Forest Protection at Harper Adams University by offering government bursaries.  We are planning to launch new MSc courses in Plant Pathology, Plant Nematology and Forestry Management from September 2014.  We also offer undergraduate degrees in Countryside and Environmental Management and Wildlife Conservation and Natural Resource Management, both of which have significant woodland and forest-related elements

In addition, we need to persuade UK universities to employ forest entomologists and pathologists in academic posts by increasing the amount of appropriate whole organism research funding in these areas.  The Forestry Commission’s Forest Research arm also needs to be able to expand its staff in entomology and pathology to enable it to cope with existing and future threats to our forest estate.  Without such capacity building the future of forestry in the UK is uncertain to say the least.

Post Script

At the risk of seeming to blow our own trumpet still louder, another recommendation from the recent Defra report is that a virtual Centre for Tree Health Science should be created. This would be created by linking together those organisations currently active in the field and with appropriate training provision available.  A number of recent key appointments and the newly launched multidisciplinary Centre for Integrated Pest Management (CIPM) mean that we at Harper Adams University are also in an excellent position to undertake research in this area.  We are, as I write, involved in projects on Oak Processionary Moth and Acute Oak Decline.

References

Bevan, D (1987) Forest Insects.  Forestry Commission Handbook 1, HMSO, London.

Crooke, M & Bevan, D (1957) Notes on the first occurrence of Ips cembrae (Heer) (Col., Scolytidae). Forestry 30, 21-28

Defra (2013) Tree Health and plant Biosecurity Expert Taskforce Final Report.  https://www.gov.uk/government/publications/tree-health-and-plant-biosecurity-expert-taskforce-final-report

Forestry Commission (2013) The Oak Processionary Moth http://www.forestry.gov.uk/opm#description accessed 23 October 2013

Hicks, BJ, Leather, SR & Watt, AD (2008) Changing dynamics of the pine beauty moth (Panolis flammea) in Britain: the loss of enemy free space? Agricultural and Forest Entomology, 10, 263-271.

Leather, S.R. (2009) Institutional vertebratism threatens UK food security. Trends in Ecology & Evolution, 24, 413-414.

Leather, SR & Knight, JD (1997) Pines, pheromones and parasites:a modelling approach to the integrated control of the pine beauty moth. Scottish Forestry 51, 76-83.

Leather, S.R. & Quicke, D.L.J. (2010) Do shifting baselines in natural history knowledge threaten the environment? Environmentalist, 30, 1-2.

Munro, JW (1929) The biology and control of Hylobius abietis L. Part 2. Forestry 3, 61-65.

Potter, C., Harwood, T., Knight, J.D. & Tomlinson, I. (2011) Learning from history, predicting the future: the UK Dutch elm disease outbreak in relation to contemporary tree disease threats. Philosophical Transactions of the Royal Society B, 366, 1966-1974. http://rstb.royalsocietypublishing.org/content/366/1573/1966.short

Straw, NA. & Williams, DT (2013) Impact of the leaf miner Cameraria ohridella (Lepidoptera: Gracillariidae) and bleeding canker disease on horse-chestnut direct effects and interaction. Agricultural and Forest Entomology 15, 321-333.

Straw, NA, Armour, H & Day, KR (2002a) The financial costs of defoliation of Scots pine (Pinus sylvestris) by pine looper moth (Bupalus piniaria). Forestry, 75, 525-536.

Straw, N.A., Timms, J.E.L., & Leather, S.R. (2009) Variation in the abundance of invertebrate predators of the green spruce aphid Elatobium abietinum (Walker) (Homoptera: Aphididae) along an altitudinal transect. Forest Ecology & Management, 258, 1-10.

Straw, NA., Fielding, NJ, Green, G & Price, J (2002b) The impact of green spruce aphid, Elatobium abietinum (Walker), on the growth of young Sitka spruce in Hafren Forest, Wales: delayed effects on needle size limit wood production. Forest Ecology and Management  157, 267-283.

Straw NA, Williams, DT, Kulinich O & Gninenko, YI (2013) Distibution, impact and rate of spread of emerald ash borer Agrilus planipennis (Coleoptera: Buprestidae) in the Moscow region of Russia.  Forestry 86, 515-522

Torr, P, Heritage, S, & Wilson, MJ (2007) Steinernema kraussei, an indigenous nematode found in coniferous forests: efficacy and field persistence against Hylobius abietis. Agricultural and Forest Entomology 9, 181-188.

Townsend, M (2013) Oak processionary moth in the United Kingdom. Outlooks on Pest Management 24, 32-38.

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