'Diseased' Kilmarnock Willow? I have a 3 metres high Kilmarnock Willow (salix caprea pendula -...
I have a 3 metres high Kilmarnock Willow (salix caprea pendula - I judge!). Hundreds of the leaves have unsightly 'galls'. Searching the internet tells me that the gall contain larvae of the Sawfly, that "damage is probable to be transient and should not last more than one year" and that "no approved chemical control is available", but not what action to cart. So - what do I do? - Do I leave the tree, ugly as it is? Do I prune it immensely 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.
Answers: I am afraid all you can do is lurk for the sawfly larvae to develop and move on. They will bestow you tree looking like a string vest but it will recover and releaf. After a unpromising 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 simply be for aesthetics. Makes no difference to the tree.
The only thing you can do is rouse birds into the garden. Bluetits among others love sawfly larvae. Possibly put a bird box up somewhere near the tree?
Nematus ventralis - the Willow Sawfly will be surrounded by 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 full-grown sawflies but as these are active over a six to eight week period contained by 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 larva. 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 continue for them to move on - pruning won't help as they are too numerous. Good luck :-)