Bt, Bacillus thuringiensis: how to use this organic caterpillar killer

Today, chemical insecticides are no longer an option for gardeners. And that’s a good thing! But pests haven’t gotten the message that they’re not welcome… An invasion of pests can destroy a crop in a few days or weeks, to the gardener’s great despair. One fully organic solution in particular is very effective against fluttering insects like butterflies and moths: bacterium Bacillus thuringiensis. And Bt is harmless to beneficial insects, too!

→ Also read:

• Diatomaceous earth, an insecticide against garden pests

What exactly is Bacillus thuringiensis?

Bacillus thuringiensis is the scientific name of a bacterium commonly known as “Bt”. It is a bacterium used for biological pest control in vegetable gardens. Bt was identified in 1901 by a Japanese bacteriologist named Shigetane Ishiwata, who was working with silkworms. This bacterium kept infecting and killing his silkworms.

About ten years later, Ernst Berliner, a German scientist, managed to describe and identify this bacterium, which he named Bacillus thuringiensis. However, it was not until the 1940s that people realized Bt could help as a biological insecticide. This prompted commercialization to professional growers and farmers at first.

It is a ubiquitous bacterium, meaning that it is found everywhere in nature, both in soil and water, in the air, and on the foliage of plants, in a completely natural way. It is also said to be aerobic, which means that it develops in the ambient air.

When observed under a microscope, Bacillus thuringiensis looks like a rod-shaped bacterium with flagella, measuring a few micrometers in length.

How does Bacillus thuringiensis work?

To put it simply, Bacillus thuringiensis produces, through sporulation, toxic crystals containing multiple associated proteins. These proteins are toxic when ingested by certain insects of the lepidoptera class (butterflies and moths) and a few coleopterans (beetles). Specifically, these toxins attack cells in their intestine, causing lesions. The intestine becomes paralyzed. Insects, especially larvae, are no longer able to feed properly and die 24 to 48 hours after spraying. They are eventually entirely consumed by the bacterium.

When to use Bacillus thuringiensis

Without delving into (too) scientific details, there are different strains of Bacillus thuringiensis.  Each is better at targeting specific classes of insects, especially lepidoptera such as butterflies. The bacterium primarily eliminates caterpillars which usually attack numerous produce in the vegetable patch, orchard and eat up ornamental plants and shrubs:

• Tortrix moths that can be found on peas, roses, fruit trees (particularly peach trees)
• Cutworms attack many vegetables, from carrot to cabbage, including spinach and potato. They also target fruit trees such as apricot trees or grapevines.
• Cabbage white butterfly
• Codling moth of apple and pear trees
• Leek moth
• Oak and pin processionary caterpillar
• Boxwood moth
• Pear tree borer

Bacillus thuringiensis also works great against the Colorado potato beetle.

How to prepare Bacillus thuringiensis?

This bacterium, which is a living organism, is dormant in the form of a powder to dilute in water. You can find it in specialized stores, garden centers, and websites dedicated to plants. Since it has a shelf life of approximately one year, it is important to check the expiration date on the package. Store it in a dry, dark room at a temperature of around 68 to 77 °F (20 to 25 °C), and out of reach of children.

Dilute the Bacillus thuringiensis powder in water (preferably rainwater) to revive it and make it easy to spray onto plants. Once again, it is crucial to strictly follow recommended doses. Likewise, it isn’t a good idea to mix it with other products such as Bordeaux mixture or pyrethrum.

When applying the treatment, it is better to wear gloves, a mask, and goggles. Even though Bacillus thuringiensis is used in organic farming, ingesting or getting covered with high doses of the bacteria isn’t comfortable.

Spraying Bacillus thuringiensis

Spray the solution on topside and underside of leaves and on stems infested by caterpillars.

• Use a very fine nozzle for spraying, misting if possible
• Do it on a calm, rain-free day, preferably in the evening, at an ambient temperature of around 60 °F (15 °C)
• Clean the sprayer and dispose of cleaning water in infested areas (might as well maximize those rinsed-out bacteria)
• Repeat spraying about ten days after the first application to eliminate new batches of hatching larvae
• Intervene before flowering to avoid disturbing pollinating and beneficial insects. Even though Bacillus thuringiensis is not toxic to bees, hoverfly, ladybugs, etc., it still upsets the balance of things for a while
• Spraying is usually done between the March and October, depending on the pest species

Note that Bacillus thuringiensis is a curative product, not preventive. Therefore, there is no need to apply it to non-infested vegetable plants. Use pheromone traps to pinpoint the time when insects mate. With this knowledge, you can schedule when the first caterpillars are expected to hatch. That’s when to start treating with Bt.

Similarly, remember that the toxin secreted by this pathogenic bacterium has a low persistence. This means that ultraviolet rays destroy it within a few hours after spraying.

To learn more:

• Beneficial Insects: Nature’s Little Helpers in the Garden