Just wondering if anyone has any advice on this. Its a garden in Perth. There has been something slowly sucking the life out of selected bamboo and conifers in the garden. Have sprayed with imacloprid fortnightly for a few months. Dessimation of the plants has slowed slightly but problem continues. There are two conifers side by side. One is completely uneffected whilst the other is slowly dieing. Two bamboos side by side. One almost completely dead, the other effected but coping reasonably well.

Any advice would be greatly appreciated. Contact details of anyone who has dealt with similar problem also appreciated

I see that you are treating the effected plants with Imidacloprid. What are you trying to bring under control? Imidacloprid is systemic insecticide that is very good for sucking insects but probably won't affect mites if they are present.

First thing to do is identify what is causing the damage if possible.

Are there any insect pests visible on the effected plants?
If not have a bit of a dig around the root zone to see if the roots are healthy and also to see if there isn't and sub-soil grubs having a chew on the root system. If there is evidence of sub-soil activity I suggest a good soil drenching with 350 g/L imidacloprid (Bayer). The uptake through the root system and the Foliage spray that you have already done should knock down any boring/sucking insects as well.