If it's caterpillars "Thuricide" will kill all of them. Period!

If it's slugs or snails, sprinkling "diatomaceous earth" along the ground, under the plants that are under attack, will absolutely solve the problem. About every other year I get a major infestation of both snails and slugs.

DE is used in swimming pool filters as a filter media. It's a very fine white powder. If you look at it in a microscope, it is very very spiney with many very sharp edges.

As the pests crawl through the DE, it sticks to them and, (I believe) it slices them wherever it contacts them. I know this sounds crazy but, I don't really know whether it kills them or runs them off. An engineer friend of mine claims it kills them. I've never stayed awake to actually witness what the DE actually does to the pests. I just know I've NEVER had any more damage after DE application!

It amazes me how many people continue to have problems with snails and slugs when, they only need to visit their pool supply store for Diatomaceous Earth.

DE is indeed excellent for snails and slugs, and I think what it does is cut them up so they lose moisture and dry up. I kinda hate to torture things to death, but slugs and snails don't keep me awake nights. You do have to re-apply the DE every time it rains.
As for Thuricide (and other forms of Bacillus thuringiensis) you might want to be careful where you apply it if you are trying to attract butterflies. I really had to laugh at myself when I planted parsley for the butterflies and then in horror started murdering the caterpillars that were devouring it. Managed to catch myself before I killed more than one or two, but I still regret those and wonder what on earth I thought I was doing! Once or twice I found a granulated Bt that was sold for corn earworms and was just right for those [censored] canna leafrollers--stayed put in the cannas so I didn't kill butterfly larva.

Greenelbows,
Forgive me for sounding clueless, but what is granulated Bt? You said that this would help with the canna leafrollers? I have those too.