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Monterey B.t. – An Organic Solution to Your Garden Pests

Monterey B.t. – An Organic Solution to Your Garden Pests

While researching organic gardening methods, you have likely encountered many rave reviews about Bacillus thuringiensis (B.t.), a bacterium also known by its well-known brand name, Monterey B.t. What makes this bacterium so highly regarded?

What is Monterey B.t. good for? Monterey B.t. is a biological insecticide that can control specific caterpillar and worm species without harming plants or beneficial pollinators. It is a safe, nontoxic pesticide that helps reduce chemicals in the environment when used in home or commercial-scale garden applications.

This organic pest control has many good points going for it, but it comes with a few cautions. On that note, let’s look at how to use Monterey B.t. safely, what it’s made of, how quickly it gets to work, and more.

Monterey B.t. Uses

Monterey B.t. can be used domestically and commercially to control worm and caterpillar populations on fruits, cool and warm-season vegetables, and shade and ornamental trees.

Original strains of the Bacillus thuringiensis bacterium specifically targeted caterpillar species, but new strains have been developed to attack flies and other insects.

Given its safe and highly selective pest-control application, B.t. is also used in food-storage facilities according to the plant defense and disease expert Helga George, PhD and is also employed in “greenhouses, non-residential buildings, and aquatic settings” according to the NPIC (National Pesticide Information Center).

What Insects Does Bacillus thuringiensis Kill?

Modern strains of B.t. have been developed to kill:

  • Beetles (but is nontoxic to predatory beetles)
  • Mosquito larvae
  • Black flies
  • Gypsy moths & other moth pests (Lepidoptera sp)
  • Fungus gnat larvae

In addition to these insects, it is also an effective pesticide against:

  • Bagworms
  • Imported cabbage worms
  • Spring cankerworms
  • Tent caterpillars
  • Tomato hornworms
  • Armyworms
  • Elm spanworms
  • Fall webworms
  • Fall cankerworms
  • Cabbage loopers
  • Nematodes

Is Monterey B.t. Safe?

As a naturally-occurring pesticide with a specific target in mind, Monterey B.t. is one of the “safest pesticides around for humans and other vertebrate animals, and reported human infections with the pesticide are extremely rare” according to the ESA (Entomological Society of America).

B.t. is virtually nontoxic with no disease or negative effects observed in fish, birds, shrimp, rats, ducks, or any mammals or wild animals.

Is Monterey B.t. Organic?

Yes, Monterey B.t. is largely considered an organic form of pesticide as it is a non-pathogenic, natural bacterium found in the soil.

It’s important to note that while brands such as Monterey sell B.t. products with naturally occurring strains of the bacterium (OMRI-certified by the Organic Materials Review Institute), some manufacturers may use strains of Bacillus thuringiensis that have been genetically modified.

If you are intending to grow truly organic produce, it’s wise to always read the packaging beforehand (or consult the above OMRI product list) to ensure you are getting a truly organic strain of B.t.

Is Monterey B.t. Harmful to Bees?

In most cases, Monterey B.t. is not considered to be toxic to bees as it targets specific plant pests and not pollinators.

As B.t. products purchased in-store contain other ingredients besides B.t., more research is needed to determine its true effect on bees.

One study, for example, revealed a change in the behavior of Africanized honey bees when they were exposed to products containing the B.t. ‘aizawai’ strain — a subspecies of Bacillus thuringiensis – but these effects could be a result of the additional ingredients in pesticide products.

Will Monterey B.t. Kill Earthworms?

No. Monterey B.t. strains have little to no toxicity in true earthworms, though studies have found that fatal infections have been observed in earthworms after using extremely high doses.

Some B.t. products are misleadingly labeled as “worm killers,” but this refers to the worm-like larvae of caterpillar and moth species the pesticide specifically targets.

B.t. Pesticide Dangers

Potential Human Exposure Risk

Personal protective equipment is always recommended for the safe home garden use of B.t.

According to the NPIC, incident reports collected by the EPA revealed that accidental exposure to products containing B.t. has, in some cases, resulted in the following symptoms:

  • Skin irritation (red rashes)
  • Burning, itchy eyes
  • Watery eyes
  • Laryngitis
  • Breathing issues
  • Headaches

A separate study has found that B.t. toxicity may be enhanced when combined with herbicide products. To avoid exposure, the Minnesota Department of Health advises:

  1. Staying indoors during and 30 minutes after commercial applications of B.t. (state agricultural departments usually notify residents in the area of application time frames).
  2. Washing exposed skin and clothes with soap and water and flushing your eyes with water for 15 minutes.
  3. Allowing the B.t. treatment to dry before touching grass and washing any possible residue off nearby benches, lawn chairs, or equipment.

Diminished Efficacy Risk

Ruth Gonzalez, author of Organic Growers School, explains that the popularity of B.t.-engineered crops in the US has “created increasing insect resistance to the Bacillus thuringiensis bacterium, leaving even GMO scientists scrambling to keep B.t. strains effective!”

What Is the Active Ingredient in Monterey B.t.?

Monterey B.t. contains the active ingredient ‘Kurstaki’, which is a subspecies of the Bacillus thuringiensis bacteria.

It is this biological agent that causes disease in the digestive system of worms and caterpillars and is what the “K” in BTK stands for on some product labels.

Is Monterey B.t. Safe for All Plants?

Yes, Monterey B.t. is safe to use on all plants and not remotely toxic as its specific target is the digestive tract of certain insects.

Can Monterey B.t. Be Used on Food Crops?

Bacillus thuringiensis has “been applied directly to a variety of agricultural crops and home gardens as a living pesticide for over fifty years,” according to the Entomological Society of America.

Many gardeners use Monterey B.t. on their tomato crops to target the tomato hornworm pest and also on their corn, squash, and bean plants.

Just ensure that all fruit, vegetable, and herbs are washed thoroughly before consumption.

Does Monterey B.t. Repel Insects?

The protein mechanism in Monterey B.t. is designed as a pest eliminator rather than an insect repellent.

The odor of B.t. has been likened to both paint thinner and vomit, so it may deter very fussy insects!

When Should I Spray Monterey B.t.?

You should spray our Monterey B.t. pesticide product upon first spotting troublesome worms or caterpillars, applying it thoroughly on the foliage from top to bottom.

As B.t. products can degrade quite quickly in sunlight, Jackie Rhoades at Gardening Know How recommends applying the product “early morning or early evening” for the best results.

How Much Monterey B.t. Do I Use per Gallon of Water?

Grow Organic advises an application rate of 4 teaspoons of B.t. product per gallon of water ( so roughly 2 fluid ounces of product per 3 gallons).

Three gallons of B.t. solution should be able to treat up to 1,000 square feet of garden size.

Can You Use Monterey B.t. as a Soil Drench?

Yes, it’s preferable as a soil drench as this direct application prevents overspray of B.t. onto clothes, skin, and surrounding areas.

Helga George, Ph.D., advocates the specific strain ‘Bti’ or Bti (var. israelensis) as a soil drench to control fungus gnat larvae in the growth of greenhouse plants.

How Long Does It Take for B.t. To Work?

Once applied to the foliage, bacterial spores and toxins enter the body of the larvae pest as it continues to feed on the plant.

Within minutes, the infected pest will stop eating and may appear unaffected for a while before succumbing to the toxin within 4-5 days.

It will therefore take about a week or so to see a significant reduction in your pest infestation.

How Often Do You Spray B.t.?

If your pest problem persists, you can continue to apply B.t. pesticide spray every 7 to 10 days up to and including the day of your plant’s harvest.

Regular applications of B.t. may be necessary every 10 days or so until the infestation has completely cleared, and you will need to reapply the B.t. spray following heavy rainfall as the product will wash off.

Is Spinosad the Same as B.t.?

No. Like B.t., Spinosad is derived from natural soil bacterium, but it is a mixture of the chemical compounds spinoysn A and spinoysn D, which belong to the bacterial species Saccharopolyspora spinosa. 

Spinosad is also a broad-spectrum pesticide that kills many different bug species instead of a targeted group and can be highly toxic to bees when the product application is fresh (not dried), according to the NPIC.

While an application of B.t. typically leaves residue for a few days, Spinosad can last for up to 4 weeks.

You can mix a solution of B.t. and Spinosad, but it may be overkill as it’s more important to identify the specific pests attacking your plants and use the appropriate treatment.

In the case of aphids, for example, all that’s usually needed is a hard spray of water!


B.t. or Bacillus thuringiensis is a safe and highly selective form of pest control that won’t harm your plants or edible crops!

It’s also fast acting, and a little goes a very long way if you’re mixing it from concentrate.

As safe as this organic insect control is reported to be, there’s still much to learn about its health effects, so as instructed on the packaging, always don a safety mask, gloves, and eye protection during use.