Bacillus thuringiensis: a natural insecticide

Bacillus thuringiensis, often abbreviated as Bt, is a soil-dwelling bacterium that is widely used as a biological pesticide. It produces proteins that are toxic to certain insects, particularly caterpillars, making it an effective tool for pest control in agriculture and gardening. Bt is considered environmentally friendly because it targets specific pests without harming beneficial insects, humans, or animals. It is commonly used in organic farming and is available in various formulations for use on crops and ornamental plants.

To put it simply, Bacillus thuringiensis, commonly referred to by the acronym BT, is a bacterium used as an organic insecticide. A flagship of biological control, this bacterium was isolated and identified in 1901 by the Japanese bacteriologist Ishiwata from silkworm larvae, the famous silk worms.

Ten years later, the German biologist Ernst Berliner isolated the bacterium again from flour moth larvae, discovered in a mill in the Thuringia region. Hence its fitting name…

Today, Bacillus thuringiensis (BT) holds a significant place in the market for organic insecticides, catering to both professionals and amateur gardeners. It is available in wettable powders or liquid concentrates, usable for spraying in organic farming.

However, this bacterium, which takes on multiple forms, is naturally present in soil, water, and air. Indeed, as an aerobic bacterium, it multiplies in the atmosphere around us. Microscopic in nature, it takes on a rod-like shape.

Le Bacille de Thuringe (BT) agit en infectant les insectes, en particulier les chenilles, en libérant des toxines qui perturbent leur système digestif. Une fois ingéré, le BT se transforme en forme active dans l'intestin de l'insecte, provoquant des lésions et entraînant la mort de l'insecte. Ce mécanisme en fait un agent de lutte biologique efficace contre divers ravageurs des cultures.

The Thuringian Bacillus is a curative treatment. Specifically, the larvae of the implicated insects absorb the BT bacterium sprayed on the plants, which secretes toxins in the form of crystals. These toxins act on the intestinal epithelium and develop within the digestive system of the larvae.

The toxic cells of the Thuringian Bacillus induce the formation of pores in the intestinal membrane, leading to the total destruction of the intestinal epithelium and paralysis of the digestive tract. The larvae can no longer feed, while the bacterium continues to multiply. The larvae eventually die from septicaemia. The bacillus can then contaminate other larvae, leading to the complete extinction of the pests.

As for the dead caterpillars, they can be safely consumed by birds without any issues.

Bacillus thuringiensis, an organic insecticide against pests

is a selective insecticidal agent in that it only eradicates caterpillars, leaving beneficial garden insects such as ladybirds, hoverflies, and bees unharmed. However, BT does not differentiate between the larvae of pest butterflies in the vegetable garden and the caterpillars of pollinating and foraging lepidopterans. Its use must be regulated and targeted to reach its intended target.

In any case, it is very effective against most young caterpillars, which are particularly voracious and attack vegetables, fruit trees, and ornamental bushes:

• The box tree moth: this small butterfly is particularly resistant to common natural treatments. Only the strain of Bacillus thuringiensis can eradicate the green larvae of this butterfly with white wings.
• The pine processionary caterpillar also causes considerable damage while being dangerous, as it is urticating to humans and animals. Again, BT is effective, but must be handled with great care.
• The cabbage white: easily recognisable, this small cream-white butterfly gives rise to caterpillars that attack cabbages, as well as all Brassicaceae and Cruciferae. Particularly voracious, they can destroy a patch of cabbages overnight.
• The leek moth (not to be confused with the leek fly, the “baby” of a fly) is a night butterfly. The caterpillar first devours the leaves before hiding in the bole to finish its feast.

Box tree moth, pine processionary caterpillars, cabbage white, leek moth, noctuids, oriental fruit moth

• Noctuids: these are night butterflies that give rise to terricolous and aerial caterpillars, either defoliators or miners, which target carrots, cabbages, spinach, potatoes, apricot trees, or vines, as well as annual flowering plants and certain trees like maple.
• Tortrix moths: these are small caterpillars that thrive in ornamental gardens or orchards. Many varieties can be found on roses, fruit trees (oriental fruit moth), oaks, peas, and more.
• Codling moths of fruit trees: they resemble worms, but are small caterpillars that emerge from a twilight butterfly.

You can find strains of Bacillus thuringiensis against Colorado potato beetles.

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Bacillus thuringiensis is available as a wettable powder. It can also be found ready to use. It is preferable to dilute it in rainwater, strictly adhering to the dosages prescribed on the packaging.

It is a living organism with a lifespan of about one year. It is readily available in garden centres and specialised websites, so it is counterproductive to store it for several years. After purchase, ensure to keep the BT in a dark, dry room at a temperature of around 25 °C.

When and how to spray BT?

Because it is a curative treatment (and not preventive), it is essential to properly identify the enemy to be defeated! Also, knowing the best time of day (when the caterpillars come to feed!) is crucial for the bacterium to be effective. Additionally, understanding the lifestyle of these larvae is important.

Overall, BT is sprayed from March to October, during which several generations of lepidopterans can succeed one another. A pheromone trap can help you detect the presence of males, indicating that mating has begun.

BT is applied with a fine nozzle sprayer, preferably in the late afternoon, and on a dry day. Indeed, Bacillus thuringiensis is sensitive to UV rays and has a low persistence. Furthermore, it is subject to leaching. This is why it is recommended to repeat the spraying two weeks after the first application. Finally, by intervening in the late afternoon, you disturb fewer bees and bumblebees that can continue to forage.