Bacillus thuringiensis (Bt)

Bacillus thuringiensis BtBacillus thuringiensis (Bt) is a bacteria that is safe for humans but deadly for the larval stage of certain insects. It is fairly easy to find in large garden centers and online.

It is typically sold in a concentrated liquid that is diluted with water prior to application. The diluted liquid is then sprayed on the foliage or other areas of the plant that has been targeted by the larvae.

ALWAYS follow the label directions — more is NOT better! Also, Bt should only be applied when you have a problem, not “just in case” you might have a problem. In the latter case, it is a waste of money and effort. Read through the label thoroughly to make certain the type you have chosen will target your problem critter. 

Remember that Bt ONLY works on the larvae (i.e., worms, caterpillars), and not the adult insect.

If you are applying it to an edible crop, check the label to determine when it will be safe to harvest.

Do not spray it on flowers or plants frequented by butterflies or that desirable caterpillars (i.e., Monarch larvae) are, especially milkweed, Queen Anne’s lace, chervil, dill, parsley or other members of the carrot family.

There are different strains of this bacteria that work on specific types of larvae:

  1. Bt kurstaki (for cabbage worms, tomato hornworms, leafrollers, gyspy moth larvae)
  2. Bt tenebrionis or Bt san diego (for Colorado potato beetle larvae)
  3. Bt israelensis (for fungus gnat and mosquito larvae)

Additional information: Missouri Botanical Garden Bacillus thuringiensis (Bt) kurstaki and israelensis strains and Bacillus thuringiensis Products.

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