Posted by: lensweb | December 21, 2011

Mistletoe marching………….

Mistletoe image

Mistletoe – Borrowash, Derbys.

For hundreds of years mistletoe Viscum album has been planted on local trees.  There is a thriving mistletoe colony in Chilwell on apple and lime trees, apple trees on Cleveland Avenue and Grange Road Long Eaton.  Recently mistletoe has been seen to spread to wilder habitats.  It is now found on hawthorn by the Erewash Canal in Long Eaton, on lime trees by the railway line in Sandiacre and there is a massive spreading population on hawthorn by the River Derwent at Borrowash.

Below is a précis of an article about mistletoe distribution by Jonathan Briggs which appeared in British Wildlife Volume 23 No 1

Apple trees were and are the mistletoe’s commonest host in Britain. A lot of distribution data was collected in 1990 and the latest data has shown an increase in mistletoe distribution. The new information has weakened the popular belief that mistletoe distribution coincides with apple orchards. The mistletoe needs space around its host, so its normal habitat is probably riversides, open scrub on steep slopes or woodland clearings.  Nowadays it is found on well spaced trees in gardens, orchards and parks.

The commoner hosts are man-made, domesticated apples, poplars and limes although hawthorn is also a common host.  Maple, willow, crab apple and false acacia are occasional hosts but mistletoe can grow on 450 host species. The host and habitat ranges vary across the country with most of the records in wild habitats in the core area Hereford, Worcester, Gloucester, Somerset.

Climatic influences (winter and summer temperature maxima and minima) are the accepted explanation for the mistletoe’s UK distribution. Natural populations are limited to areas below 1000 m in Europe but with other factors the altitudinal limit in the UK is closer to 200 m.

There seems to be a real natural increase in mistletoe colonies in many areas, particular studies in Essex and around the London area have found colonies on young limes and poplars in increasing numbers.

In orchards mistletoe needs to be kept in check each season so that excessive mistletoe growths don’t spread to every branch of the tree. UK mistletoe is not usually a killer but large amounts will eventually overwhelm the host and create water and wind stress.  Not yet subject to thorough data collection and analysis, it seems the current neglect of old orchards has resulted in more mistletoe in biomass terms but the quantity is unsustainable and may accelerate the loss of these old orchards.  This would affect the Christmas crop of mistletoe and there is renewed interest in the management of mistletoe through the Orchard Network.  A new survey looking at how vulnerability varies according to apple varieties is due to start.

The reasons that mistletoe is spreading and becoming more plentiful in some areas seems to be due to several factors; changes in climate; bird vectors spreading the berries and direct planting by man. Climate change studies have confirmed altitudinal and northward trends in Scandinavia but not naturally in Scotland.  In England an eastward trend is possible but the population may ultimately reduce due to warm winter temperatures as the climate potentially became more oceanic.

Few bird species take mistletoe berries, the usual species are thrushes mostly the mistle thrush Turdus viscivorus.  These excrete the seeds as a half digested berry pulp.  The top few seeds can germinate and grow on the host branch but the seeds dangling below will die.  The blackcap Sylvia atricapilla also regularly consumes mistletoe berries.  This warbler separates the seed from the pulp before swallowing, in a distinctive head waggling procedure.  Each seed is then wiped off the bill directly onto the branch making them very efficient mistletoe planters.

Since the 1980s our overwintering blackcap population has increased to many thousands of birds and observational data suggests they may have become significant bird vectors.

Ring necked parakeets Psitticula kameri also take mistletoe berries and their potential role is intriguing.

Man made planting could be a possible factor in distributional change.  Some biodiversity schemes have included limited planting and there has been significant interest in ‘grow your own’ mistletoe kits in recent years – you heard it first on The Archers. There is a long tradition of mistletoe planting in gardens, even in areas way beyond the mistletoe’s natural range.

There are six mistletoe associated insects, four bugs, a weevil and a moth. The species are a psyllid bug Cacopsylla visci, a distinctive mirid bug, Pinalitus viscicola, Hypseloecus visci, a recently discovered mirid bug and Anthocoris visci a predatory flower bug which feeds on the mistletoe psyllid. Mistletoe marble moth Celypha woodiana is a small tortricid moth with a leaf mining larva which flies in July and August this has been known since 1876 but it is only recently that studies have shown it is a species of principal importance in England under the NERC act of 2006. It has been found on mistletoe growing on apple and hawthorn.  Another newly discovered species is the mistletoe weevil Ixapion variegatum which has only been found in Hereford. It seems to be found in mistletoe on stressed apple trees.  The life cycle involves oviposition within mistletoe stems just below the terminal bud with subsequent larval development killing off the terminal bud and leaves.

There is more work to be done to properly understand the mistletoe distribution in Britain.  It may now be vulnerable in orchard situations but seems to be thriving and increasing its range in others.  It seems the importance of orchards as a primary habitat for the species may be exaggerated apart from being the main source of the Christmas crop.  Orchards probably still hold the greatest biomass of mistletoe but this may not be sustainable.  Informed and on-going management is essential for its survival in orchards and as a crop.

Orchard mistletoe may also be critical for associated insects although there had been very little insect recording on mistletoe in other habitats.

Climate change and bird vector changes may give partial explanations for the recent expansion of the mistletoe range.  It is possible that more associated insect species may be discovered or colonise from Europe.  The Mistletoe League is a new project which aims to help orchard and garden mistletoe management by encouraging the recording of  which fruit tree varieties are most susceptible to mistletoe growths and which are less so. See www.british.mistletoe.org.uk.

Marion 21/12/11


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