 Re: paperbark sawfly
Malcolm we really need more info...tell us more about the trees and their environment..where they are growing.....what is it that concerns you...the possibility that the sawfly larvae will kill your trees (most unlikely)....the unattractive state of the defoliated canopy (the trees will reshoot very quickly)..the desire to reduce the impact of this insect pest..the fact that the larvae can and do produce unpleasant chemicals for their own defense which can cause allergic reactions in humans?
You see a great many of the symptoms that become evident to tree owners are merely indicators of a less obvious but potentially more serious range of problems.
Sawflies and their larvae are seasonal problems in many parts of Oz, they can defoliate whole trees in a few days if their numbers are great enough, there's no doubt that this has a significant negative impact on the health of the trees in fact in Florida they have been studying the possible use of sawflys along with fungal parasites to control M. quinquenervia.
But enough of that you already know what happening to your trees but perhaps not the why.
Trees that are under stress due for example to alterations to soil and root conditions...trenching, compaction, exposure to chemicals or other injurious liquids, these trees are predisposed to insect and fungal pests. As a trees health and vigour decline very subtle changes take place in the chemistry of their leaves and in the exudates they release into the wider environment, larger more obvious physical injuries will release many chemical markers that are attractive to both insects and micro-organisms.
What am I getting at?............Healthy trees will cope and manage environmental pressures, even plagues of insect herbivores. Not only do unhealthy stressed trees not manage these pressure effectively, they are also attractive beacons to precisely the agents of those negative pressures.
My advice to all tree owners if they present me with the limited info you have is.....improve the health and vigour of the tree, and that is even when there is a registered chemical for the treatment of the insect pest. Spraying or flooding plants with chemicals do much more than kill the target you are trying to eradicate, they can wipe out whole sections of the food web in the soil and on the leaves and bark of the tree.
__________________ Sean  Trees are poems that earth writes upon the sky,
We fell them down and turn them into paper,
That we may record our emptiness.
- Kahlil Gibran
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