Ash dieback - a threat to Criorhina and other saproxylic hoverflies

A couple of days ago a friend drew my attention to an article in The Telegraph that highlighted the emerging risk of a new threat to Britain’s trees: Ash dieback caused by the fungus Chalara fraxinea. The story was deeply alarming, suggesting that it had swept across mainland Europe and was killing the majority of Ash trees that it affected.  In Denmark it is reported that 90% of trees are affected! Amazingly, despite the known problems with this pathogen, imports of trees from continental nurseries has yet to be stopped. This is despite known cases of infected trees reaching the UK via the nursery trade. At the moment all known cases involve trees whose health seems to have been monitored, allowing intervention to take place and infected stock to be destroyed. It can surely be just a matter of time before the disease takes a hold unless some urgent action is taken?

Although Ash does not immediately register in the lexicon of important trees for insects, it is actually of considerable interest, especially for the saproxylic diptera that are associated with rot holes and rotting stumps and roots. Woodlands on the borders of Lincolnshire and Northamptonshire are a good example. I never cease to be amazed at the numbers of all four of our Criorhina species in local woods. Two species really stand out: C. asilica and C. ranunculi; neither of which was very abundant in my former stomping ground in Surrey. In northern Northamptonshire they are really quite common and I would expect them to turn up in most long-established Ash woods. I was puzzled by this difference until I discussed it with Alan Stubbs who drew my attention to the key difference in the woodlands.  Those around Stamford support a high density of Ash, whereas in Surrey Oak is much more dominant.

This one small observation makes me very nervous about the future of Britain’s saproxylic hoverfly fauna. Not only would Ash dieback have devastating landscape implications; it would seriously affect one of our most important habitats. The critical issue is that of habitat continuity and in the case of saproxylics that has got to be a key consideration. It is no surprise that ancient woodlands are richer for saproxylics, as they represent the most constant supply of habitat.

What can be done? Probably not a lot, but there is a consultation on the risk assessment by FERA – see http://www.dardni.gov.uk/index/consultations/active-consultations/consultation-by-fera-on-ash-dieback.htm.

This apart, it seems to me that there is an obvious need for urgent measures to survey our Ash woods to determine their real importance for saproxylic invertebrates. This could form a large-scale citizen science project to evaluate Britain's woodlands for the threat Ash dieback poses to one element of our wildlife.