Sticking nails into aliens

As a member of the Western Heads/Goukamma conservancy in Knysna we struggle with the ongoing problem of alien tree control, especially pine and black wattle that infests some of the fynbos areas. Cutting and ring barking these two species respectively is as you know, expensive and time consuming for individual landowners.
The question I’d like to pose to people in the know is firstly, what is the world’s most toxic chemical agent to all trees and is this agent able to be infused into or coated over a type of hard metal nail that can be knocked into a tree once, which over time releases the toxins and eventually kills the tree? Imagine a box of say, one hundred nails no longer than 10 cm. each, which in half a day’s work could be hammered into select large alien trees which never have to be either chopped up, removed or cleared but only left to stand and slowly die from the root up and naturally rot away.
I look forward to sharing your readers ideas.
Guy Thesen