Tuesday’s Tips — Bt for Caterpillar Control

We are deep into spring and on our way to summer though it sure doesn’t feel like it in Southern California. We’ve got cool temps, wind and rain which is unusual for this time of year. What’s not so unusual are the garden pests that have started to make their appearance in my garden and which will soon be plaguing Mary Beth and the rest of you in somewhat cooler zones. So we thought that for the next few weeks we’d look at organic ways to deal with the bugs and diseases that bother us in the garden.

We’d like to point out first of all that some nibbling of leaves, flowers and fruits is normal. Bugs are supposed to be in your garden. Live with it. Insects are part of the whole wonderful cycle that makes this all work. We are not striving for a bug-free zone, we are looking to create a balance in the garden. That said there are times when you need to get the upper hand especially when something is devouring your herbs or, perish the thought, your beautiful tomatoes.

This week we’ll be looking at Bt or Bacillus thuringiensis. The most commonly used form of Bt, the one you’ll likely see at your nursery, is Bacillus thuringiensis kurstaki. It is used for controlling leaf-eating caterpillars like cabbage worms and tomato hornworms (although they are called worms they are really caterpillars). Another form, Bacillus thuringiensis israelensis, is used to control certain fly larvae like mosquito larvae which can be a problem in ponds.

Here’s what Colorado State University Extension has to say about Bt:

“The most commonly used strain of Bt (kurstaki strain) will kill only leaf- and needle-feeding caterpillars. In the past decade, Bt strains have been developed that control certain types of fly larvae (israelensis strain, or Bti). These are widely used against larvae of mosquitoes, black flies and fungus gnats.

More recently, strains have been developed with activity against some leaf beetles, such as the Colorado potato beetle and elm leaf beetle (san diego strain, tenebrionis strain). Among the various Bt strains, insecticidal activity is specific. That is, Bt strains developed for mosquito larvae do not affect caterpillars. Development of Bt products is an active area and many manufacturers produce a variety of products. Effectiveness of the various formulations may differ.

Insects Controlled by Bt

Kurstaki strain (Bonide Thuricide, Safer Caterpillar Killer, Greenstep Caterpillar Control, etc):

  • Vegetable insects
    • Cabbage worm (cabbage looper, imported cabbageworm, diamondback moth, etc.)
    • Tomato and tobacco hornworm
  • Field and forage crop insects
    • European corn borer (granular formulations have given good control of first generation corn borers)
    • Alfalfa caterpillar, alfalfa webworm
  • Fruit crop insects
    • Leafroller
    • Achemon sphinx
  • Tree and shrub insects
    • Tent caterpillar
    • Fall webworm
    • Leafroller
    • Red-humped caterpillar
    • Spiny elm caterpillar
    • Western spruce budworm
    • Pine budworm
    • Pine butterfly

Israelensis strains (Vectobac, Mosquito Dunks, Gnatrol, Bactimos, etc.)

    • Mosquito
    • Black fly
    • Fungus gnat

San diego/tenebrionis strains (Trident, M-One, M-Trak, Foil, Novodor, etc.)

    • Colorado potato beetle
    • Elm leaf beetle
    • Cottonwood leaf beetle”
We (back to Bees and Chicks now) have used Bt for years and find it very effective and safe when used according to package directions. It is not harmful to humans, pets, or beneficial insects like bees. It can be used up to the day before harvest (I’d still want to wash sprayed fruit before eating).
Nonetheless, you shouldn’t go spraying everything willy-nilly. Be sure first of all that a caterpillar is what is eating your plant. Some insects that look like caterpillars are really worms, most notably rose slugs, and Bt will not kill worms. (Note that these are called slugs, but are really worms! For info on how to identify and control rose slugs click here.)  If it is a caterpillar eating your plants, spray just the affected plants/area. You must be very thourough because the caterpillar needs to eat the BT for it to be effective. You may need to repeat the application every 5 – 7 days while the insects are active. We have usually gotten good results with just a couple of sprays.
A note of caution here: Bt will kill good caterpillars as well as bad. If you have a butterfly garden or plants that attract butterflies, DO NOT spray when these caterpillars are feeding.
Bt is but one tool in the organic arsenal. Next week we will take a look at more.

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