'Diseased' Kilmarnock Willow?

Question

I have a 3 metres high Kilmarnock Willow (salix caprea pendula - I think!). Hundreds of the leaves have unsightly 'galls'. Searching the internet tells me that the galls contain larvae of the Sawfly, that "damage is likely to be transient and should not last more than one year" and that "no approved chemical control is available", but not what action to take. So - what do I do? - Do I leave the tree, ugly as it is? Do I prune it very drastically to get rid of the infested leaves? or Do I cut the tree down (which I do not want to do)? Advice from a 'proper' gardener would be much appreciated.

Answer 1

I am afraid all you can do is wait for the sawfly larvae to develop and move on. They will leave you tree looking like a string vest but it will recover and releaf. After a bad infestation the tips of some of the younger branches can suffer die back.

You could remove any leaves that are heavily galled but that would just be for aesthetics. Makes no difference to the tree.

The only thing you can do is encourage birds into the garden. Bluetits among others love sawfly larvae. Possibly put a bird box up somewhere near the tree?

Answer 2

Nematus ventralis - the Willow Sawfly will be in it's second generation around now.  Bifenthrin-based insecticides (Doff All-In-One Garden Pest Killer, Bayer Sprayday Greenfly Killer Plus or Scotts Bug Clear Gun) will control the adult sawflies but as these are active over a six to eight week period in early summer, numerous applications would be necessary to prevent egg-laying. The systemic insecticide thiacloprid (Provado Ultimate Bug Killer Ready To Use) may control the larvae.  The pupae overwinter in the soil so regular turning of the soil at this time around your willow can help to expose them for the birds.  Other than that - it's wait for them to move on - pruning won't help as they are too numerous.  Good luck :-)