Bacillus Thuringiensis (Bt)

Bacillus Thuringiensis (Bt)

Many of my articles reference using Bacillus Thuringiensis (Bt) as a natural and organic means of pest control in your garden.  It is a very safe and effective insecticide that can be combined with other natural pest control solutions to give you a much broader means of protecting your garden from various pests.

What is Bacillus Thuringiensis

Bt is a Gram-positive bacteria that is present in small quantities in the soil.  The bacteria itself contains crystals of a protein called parasporal crystals.  These crystals form around the spores of the bacteria.  These spores then become the insecticidal precursor.  When insects, especially caterpillars, eat these Bt spores, the proteins paralyze the digestive system of the insect.

Bt Dead Cabbage Looper

Bt is available commercially in strains that are beneficial against specific insects.  No matter what strain you choose, it is deadly to caterpillars of all types but affects only certain other insects.  Caution should be used in applying Bt around plants that have been placed to attract beneficial insects or in butterfly gardens.

Safe to Use!

Surprisingly Bt is not harmful to most beneficial insects.  It is also non-toxic to people and animals.  You can use it in your vegetable garden and harvest immediately.  We do suggest that you wash your vegetables before use anyway, to be safe.

Different Strains

Bacillus Thuringiensis (Btk)

Bt can be purchased as a liquid, a powder or as bait.  Different strains of the bacteria are effective against different types of insects.  You should choose a strain that is effective against the pest you need to control.

Bacillus Thuringiensis kurstaki (Btk)

The best choice for controlling tomato hornworms is Btk.  It is also effective against cabbage worms (green loopers) and other leaf-eating caterpillars.  Apply Btk to the leaves on which the caterpillars are eating.  As they ingest it, the toxins will begin to work, and they will eventually die.

Insects that don’t eat leaves will be unaffected by the Btk, which makes it a particularly effective way to control caterpillars.

BTK is effective against:

  • Tomato hornworm
  • Tobacco hornworm
  • European corn borer
  • Corn earworm
  • Gypsy moth larvae
  • Cabbage worms
  • Tent caterpillars (eastern, western and forest)
  • Leaf-rollers
  • Bagworms

Bacillus thuringiensis israelensis (Bti)

Bti Dunks

This strain of Bt is especially effective at controlling flying insect pests whose larval stage is spent in water.   These include fungus gnats and mosquito larvae.  Bti is really useful in controlling mosquitoes in ponds or other standing water.  We always suggest that if you have a pond or water feature that you introduce some mosquito-eating fish.  If you need additional control, Bti is very effective and can be used in the pond with the fish as it is non-toxic to aquatic life. 

Bti can also be used to control black flies and armyworms.  Mixing the Bti with water and using it as a drench on the soil is the best method for controlling these pests.  Using Bti as a foliar spray is not recommended.

Bti is the active part of products known as mosquito dunks.  Many people with rain barrels routinely use mosquito dunks to control the growth of flying insect larvae in their rain barrels.

Bacillus thuringiensis Bt (var. tenebrionis/San Diego)

This strain of Bt was developed to target:

  • Cottonwood leaf beetles
  • Elm leaf beetle
  • Potato tuberworm
  • Potato Beetle

Using Organic Bacillus Thuringiensis

Some manufacturers are selling specific strains of Bt that have been genetically modified (GMO)  We urge you to check the label of the product you are buying and make sure that it is organically grown and is only one of the naturally occurring strains of Bt.  The powdered forms of Bt are the easiest to find in pure non-GMO strains.

How to use Bacillus Thuringiensis

Bt Foliar Spray Application

The secret is in the timing.  Sunlight quickly degrades Bt, so your best bet is to apply it just at sundown. Late afternoon or sundown applications coincide perfectly with the feeding habits of most caterpillars.

Apply the Bt where you have seen caterpillars feeding.  As an example, for corn earworms, apply the Bt to the tips of your ears of corn.  For cabbage loopers, it may be necessary to lift leaves and apply Bt to the underside of the cabbage leaves.  The loopers tend to feed there to stay out of sight of predators and to enjoy the shade provided by the plant leaf.

Never broadcast Bt.  It will kill some beneficial caterpillars if applied in the wrong places.  Many butterfly caterpillars feed only on one or two specific plants so really are not a pest problem in a vegetable garden and the butterflies are good pollinators.

Bt is also affected by watering.  If you are expecting rain within 24 hours, don’t apply your Bt.  It will simply wash away.  The same goes for watering.  Applications of Bt should be repeated every 7 to 10 days until you have resolved your pest problem.

Mixing Bacillus Thuringiensis

Follow the instructions on the container label.  Each manufacturer will have different instructions and they have tested their Bt using those instructions to find the best mixing ratios and application rates.

Mix only as much as you are going to use.  Bt will not store well after it is mixed.  Store the original container away from extremes of heat or cold and protect from direct sunlight.  Extremes in temperature can degrade the effectiveness of your Bt.  Powdered Bt will last for many years if stored properly.


Don’t expect caterpillars to start falling off the leaves as soon as you spray or apply your Bt.  Bt is not a contact insecticide.  It takes time for the insects to ingest the material and for the toxins to work.  Regularity in the application is the key.


Again, we caution you against spreading Bt too generally or broadcasting it widely.  Bt will kill the caterpillars of all the beneficial butterflies that you are trying to attract to your garden.  Avoid using Bt around your butterfly gardens

Links and Resources

For more information about organic gardening, lifestyles, and living, visit our website at West Texas Organic Gardening.

If you found the information here helpful, you might also find these articles on our website of interest.

Organic Integrated Pest Control

The Organic System

Five Tenets of Organic Gardening

If you have more specific questions or problems, you can contact us using the contact form on our website. You can also post your question to our community forum at this page; West Texas Organic Gardening Community Forum.

We have a Facebook page and love your comments, questions, or input. You can find us on Facebook using this tag. @westtexasorganicgardening

Additional and more in-depth information about Bt ad it uses can be found at the Texas A&M website at

Updated: 5 Nov 2019

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