How to revive an unhealthy lilly pilly?

[O-J]

What is the most likely cause of the ill-health of this lilly pilly (see photo)?

 

[O-J]

Many thanks @Noelle for your response a couple of weeks ago. I've actually cut a number of the Lilly Pillies right back, because there was more dead wood and damaged growth then healthy growth (see photo). I've done this before and they do come on again. I'm just wondering however, is there any point in spraying the trunk and branches that are left before any new growth even appears, to discourage the caterpillars?!

[MitchellMc]

It looks like some new growth is appearing, so it wouldn't be a bad idea to pre-spray to ensure it isn't destroyed before establishing.

Mitchell

[Noelle]

Thanks, @O-J 

Most of the home garden insecticides available are contact poisons. That is, they will only kill pests when they are sprayed directly on to them.  These sprays generally are not residual (they do not remain active on plants for more than a day or so and are readily washed off by rain) and they are not absorbed by plant tissues (that is, they are not systemic). In my opinion, spraying when there are no pests present probably is not going to do anything much to help the trees. It's important to keep an eye on the regrowth and spray immediately if you see any pests at all.