Bt (Bacillus thuringiensis) is entirely safe to use and is considered an organic treatment. Safe to use even on veggies you'll harvest the same day.

Bt is effective when caterpillars are less than half-grown -- and that's typically the case when those guys chew through your flower buds.

Realize that Bt is an ingredient rather than a brand name. It's available at all large independent garden centers. The easiest way to find it is to ask for a "safe caterpillar killer" or an "orgaic caterpillar killer."

If you don't want to do that, it's search-and-destroy mission. But lots of luck, this because they assume the color of the flower or leaf they ate.