In a very short time, tiny wiggly worms have eaten the top leaves from a lovely tree.

I cut off as much as possible.

What insecticide or poison can I use to save the tree.

A single defolation is unlikely to kill a tree, several may. This is assuming that the defoliation is not one of many factors. 'the bug that broke the jap's back'
A non-native pest may be more resiliant, it may help to know roughly what your creepy crawly is. Here's an eco-logical story..........
a bilion bugs were born.........................evolution sensibly released the predators slightly later......................but wait theres only 100 predators, so they munch like mad and breed like bugs.....................another billion bugs are born...............but this time there's 1,000 predators, the bugs stand no chance, aaagggh.
If you break the cycle you start at the beginning.