United Kingdom 
How to treat Berberis sawfly larvae damage? Assuming I have made the right diagnosis. This happens to my Helmond's Pillar every year. Should I spray with insecticide? I have not been able to detect any grubs on the leaves, but only this leaf damage. The young bush is about a third defoliated and I hope to stop it getting worse. Advice much appreciated.
16 Jun, 2011
Answers
Hi Spritzhenry, No good I'm afraid. My computer has software on it to block porn, but which unfortunately restricts access to the web. It enables me to visit 60 or so specifically named sites, but does not give me the full run of the whole internet. It gives wonderful benefits to my life, but I am not able to, say, follow a link to the RHS. It is still the best arrangement for me.
16 Jun, 2011
OK, is this any use? :-)
Chemical control
The larvae can be controlled by spraying the shrub with pyrethrum (Py Spray Garden Insect Killer, Doff All in One Bug Spray, Scotts Bug Clear Gun for Fruit & Veg), deltamethrin (Bayer Sprayday Greenfly Killer), lambda-cyhalothrin (Westland Plant Rescue Fruit & Vegetable Bug Killer) or thiacloprid (Bayer Provado Ultimate Bug Killer).
Avoid spraying plants while they are in flower.
16 Jun, 2011
Thanks Spritzhenry. I think I will go for the Pyrethrum, seeing as I have recently bought some. I tried using Provado on something recently, in sunlight, and it damaged the plant. No sign of the bug, but I guess it may be a very inconspicuous thing. I presume the pest may at times attack whole berberis hedges. Unlucky for some. Does one application per year deal with it, or might it need furhter treatments later in the season?
16 Jun, 2011
I'd have to go back to the RHS advice for that. I'd say watch it and treat it again if it needs it!
16 Jun, 2011
Hi Jonathan - try the RHS advice on this.
http://apps.rhs.org.uk/advicesearch/Profile.aspx?pid=645
I hope it helps. :-)
16 Jun, 2011