Dear Eve:
Bt is bacteria that affects the digestive track of insects that have a moth as an adult stage. Examples would be cabbage worms, tomato hornworm. If your white worms are the larval stage of a moth, then Bt will control the worms. Bt products (there are several strains available) can be found at any good garden center. These are typcially a dust that is applied to the foliage of the plant. Please read the label for any resitrictions or special uses.
Good Luck, Richard
