It's an organic control directed specifically at caterpillars, and safe to use on food crops even. The active ingredient is Bacillus thuringiensis, a naturally occurring bacteria that when you spray it on plants, makes caterpillars get sick and die when they take a bite of the plant. You buy it as a concentrate at any garden center. I think the common brand name is "Thuricide".
There's also a strain of Bt that is used in mosquito control. You buy little cakes (they're called "dunks") or pellets of it and keep it in any standing water around your house like outdoor plant saucers, rain barrels and such. Kills mosquito larvae in the water before they can become the evil biters.
I haven't had a problem with any caterpillars on my orchids, because they live in my screened pool cage. But I use it all the time on my veggies and brugs. I also keep a constant supply of the mosquito dunks and pellets around because I have 5 rain barrels as well as numerous pots with saucers around.
