(Thumbnail image of tobacco budworm on rosebud was taken by DG member azrobin and graciously supplied to BugFiles)

"Safe insecticide"--is that an oxymoron, as seemingly contradictory as "jumbo shrimp?" The post--World War II chemical industry produced a number of highly toxic treatments that spelled doom to some very damaging agricultural insect pests. Heavy-hitting organochlorines and organophosphates, (having the prefix organo- only because they contain carbon) substituted for traditional, less lethal pesticides. Unfortunately, it was later discovered that these toxic products also had unintended victims. Rachel Carson's book Silent Spring helped sound the alarm about toxic chemicals in the environment and boosted a move to find alternatives.

Bacillus thuringiensis is a microorganism that occurs naturally. The organism or its products can be commercially cultured and applied to kill certain insect pests.


caterpillar - a moth or butterfly while in its immature (larval) stage. Caterpillars all have three pairs of stubby pointed legs on segments near the front, and 3 to 5 pairs of round-ended suckery legs near the rear. If there are more or less legs than that, the critter is not a true lepidoptera larva.

larva - the immature stage of certain insects. larvae - plural of larva larval - of larva

var. - a shorthand for the word variety as in Bt var. israeliensis

worm - in plant pest "common name" terminology, worm can refer to larva of various insect families.

Bacillus thuringinesis, or Bt (sometimes BT) was discovered in the early 1900s as a normally occurring organism that could sicken and kill some insects. Specific varieties of Bt cause infection and illness to specific types of insects while they are young, while showing no ill effects to mammals, birds or fish. Of the many subspecies of this Bacillus, the kurstakii strain was the first made widely available, in the 1950s. Often referred to as Btk, and still in use, this product is applied to crops or ornamental plants to kill damaging caterpillars. Btk products may be labeled as "worm killers" although true earthworms aren't harmed by Btk. Dipel and Thuricide, despite having names that sound very "chemical," are Btk products. Now, other strains of Bt are available to control other insects. Bti (Bt variety israeliensis) kills mosquito, black fly and fungus gnat larvae. Bt var. san diego or tenebrionsis sickens Colorado potato beetles, elm leaf beetles and some other tree leaf feeding beetles.

Bt treatments appear safe for people and pets, specific in their action, and effective when used correctly.

One worrisome trait of many widely sold chemical insecticides is their potential to harm other living things, including you, along with your intended insect victims. Those chemicals can affect the biological processes of many creatures and can remain toxic long after the target pest has passed. Bt treatments are quite specific in action and also short lived in the environment when sprayed on plant foliage. (I guess that explains why we have so many butterflies and moths despite Bt 's existence in nature.)

Bt kills essentially by givng its victims a fatal stomachache. You don't have to spray each indivdual insect with Bt, but target insects must ingest (eat) the Bt treatment organism or product. Bt doesn't harm adult insects, only larvae, and young larvae are more susceptible to Bt than older ones. It takes several hours for Bt to sicken larvae enough to stop them from eating, and a few days for the larvae to actually die. Many insects eat all they will need for their short lives while they are larvae and don't eat as adults. This means that if, for example, you wait too long to spray Btk on your cabbage loopers or toss a Bti dunk in your pond, you'll allow the young insects to finish growing and maturing. Those adult insects will either reproduce and perpetuate your problem, or just bite you.

Buy the right Bt for your problem and use it as directed.

Because Bt is so specific in its effects, you may not be able to treat all the bugs bothering you with just one treatment product. Always carefully read product information before purchase and use. Here's a summary of common recommendations for each Bt:

Btk (var. kurstakii) is used to control:

Btk (var. kurstakii) application
  • Cabbage worm (cabbage looper, imported cabbageworm, diamondback moth, etc.).
  • Tomato and tobacco hornworm.
  • European corn borer (granular formulations).
  • Alfalfa caterpillar, alfalfa webworm.
  • Leafroller.
  • Achemon sphinx.
  • Tent caterpillar.
  • Fall webworm.
  • Leafroller.
  • Red-humped caterpillar.
  • Spiny elm caterpillar.
  • Western spruce budworm.
  • Pine budworm.
  • Pine butterfly.
  • Gypsy moth

Note: kills caterpillars which produce pretty butterflies as well

Look for products which name Bacillus thuringiensis in the active ingredient list. Kills caterpillars, which are the larval stage of moths and butterflies. Kills leaf eating caterpillars, but is not effective against caterpillars that burrow inside plant tissue such as apple worms or borers. Sold as a dust or liquid concentrate. Use of "sticking" spray additives may help keep sprays in place. Spraying later in the day helps the product remain effective until eaten. Caterpillars should stop eating within hours and die in two or three days. Reapply after rain or if some caterpillars still appear to be eating. Store unused product in a cool location out of sunlight. Bt sprays are approved for use up until the day you harvest fruits and vegetables for instance.

(Bt var. morrisoni is similarly toxic to caterpillars but I did not find this commercially available while researching.)

Bti (Bt var. israeliensis) is used to control: Bti (Bt var. israeliensis) application
  • Mosquitos
  • Black flies (moving-water breeding black flies in genus Simulium, whose adult females feed on mammals, not just any black colored flies)
  • Fungus gnats (greenhouse and potted plant pest)
Sold as a dry granular or block (dunk) to treat areas of standing water, garden ponds, garden water features, or any place subject to flooding and hatching mosquito eggs. Apply when hatching is expected or soon after flooding. Bti effective up to several weeks. Reapply as directed. For gnat control use as a soil drench. Will also kill some midges which are food sources for wildlife or fish.
Bt var. san diego/tenebrionsis is used to control:
Bt var. san diego/tenebrionsis application
  • Colorado potato beetle
  • Elm leaf roller
  • Other leaf feeding beetles- refer to product specifications
Look for products which name Bacillus thuringiensis in the active ingredient list. Kills grubs, which are the larval stage of beetles. Surface sprays not effective against grubsthat burrow inside plant tissue such as borers. Use of "sticking" spray additives may help keep sprays in place. Spraying later in the day helps the product remain effective until eaten. Grubs should stop eating within hours and die in two or three days. Reapply after rain or if some grubs still appear to be eating. Store unused product in a cool location out of sunlight.

Sources for Bt products:

Planet Natural

ARBICO Organics

A plea from the bugs: Not all feeding caterpillars must be killed! Many gardeners devote whole areas to the propagation of beautfiful caterpillars, moths and butterflies. Please take care to identify your Insect of Interest and also to decide if elimination is necessary. Limited feeding by insects is generally not fatal to garden specimens. When in doubt, call your local government Agriculture Office for identification help, or take a picture and post your question at Dave's Garden's Insect and Spider Identification Forum.

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References and related links

National Pesticide Telecommunications Network Bacillus Thuringiensis General Fact Sheet

National Pesticide Information Center

George W. Ware and David M. Whitacre, An Introduction to Insecticides 4th edition)
Extracted from The Pesticide Book, 6th ed*. (2004)
Published by MeisterPro Information Resources
A division of Meister Media Worldwide, Willoughby, Ohio

University of California Cooperative Extension article Bacillus thuringiensis

Pesticide Action Network North America

How to Get Rid of Black Flies

This article was originally published on April 19th, 2009