Bacillus thuringiensis: a natural insecticide

What exactly is Bacillus thuringiensis?

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

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

Today, Bacillus thuringiensis (BT) has a significant place in the biological insecticide market, serving both professional growers and amateur gardeners. It is available as wettable powders or liquid concentrates, suitable for spraying in organic farming.

Even so, this bacterium, which takes many forms, is naturally present in soil, water and air. As an aerobic bacterium, it multiplies in the atmosphere around us. Microscopic in size, it has a rod-like shape.

How does Bacillus thuringiensis (Bt) work?

Bacillus thuringiensis is a curative treatment. In practical terms, the larvae of the insect pests concerned absorb the BT bacterium sprayed onto the plants, which secretes toxins in the form of crystals. These toxins act on the intestinal epithelium and develop within the larvae’s digestive system.

The toxic cells of Bacillus thuringiensis induce the formation of pores in the intestinal membrane, resulting in 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 bacterium can then infect other larvae, until the pests are completely eradicated.

As for the dead caterpillars, they can be eaten by wild birds without any problem.

Bacillus thuringiensis, an organic insecticide against pests

Bacillus thuringiensis is a selective insecticide in that it only eradicates caterpillars, leaving beneficial garden insects such as ladybirds, hoverflies and bees unharmed… However, BT does not really distinguish between the larvae of vegetable garden pests and the caterpillars of pollinating and nectar-gathering moths and butterflies. Its use must be carefully controlled and targeted to ensure it reaches the intended pest.

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

• Box tree moth: this small moth is particularly resistant to common natural treatments. Only the Bacillus thuringiensis strain is capable of eradicating the green larvae of this white-winged moth
• The pine processionary caterpillar also causes considerable damage while being dangerous, as it is urticating to humans and animals. Here again, BT is effective, but it must be handled with great care
• Cabbage white: recognisable among all others, this small creamy-white butterfly gives rise to caterpillars that attack cabbages, as well as all Brassicaceae and crucifers. Particularly voracious, they can destroy a patch of cabbages in a single night
• Leek moth (not to be confused with the leek fly maggot, the “baby” of a fly) is a night-flying moth. 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, noctuid moth, oriental fruit moth

• Noctuid moths: these are night-flying moths whose caterpillars are soil-dwelling or aerial, defoliating or mining, and target carrots, cabbages, spinach, potatoes, apricot trees and grapevines, annual flowering plants and certain trees such as maples
• Tortrix moths: these are small caterpillars that thrive in ornamental gardens and orchards. Many varieties can be found on roses, fruit trees (oriental fruit moth), oaks and peas…
• The codling moths of fruit trees: they look like worms, but are small caterpillars produced by a crepuscular moth.

Strains of Bacillus thuringiensis can also be found commercially for controlling Colorado potato beetles.

What does Bt look like?

Thuringian bacillus is available as a wettable powder. It can also be found ready to use. It is best to dilute it in rainwater, strictly following the dosage instructions on the packaging.

Bacillus thuringiensis is a living organism with a lifespan of around one year. It is readily available from garden centres and specialist websites, so storing it for several years is counterproductive. After purchasing it, keep the BT in a dark, dry room at a temperature of around 25 °C.

When and how should you spray BT?

Because this is a curative treatment rather than a preventive one, it is essential to correctly identify the pest to be dealt with! You also need to choose the best time of day—the time when the caterpillars sit down to eat!—to ensure that the bacterium is effective. It is also important to understand the lifestyle of these larvae.

Overall, BT is sprayed from March to October, when several generations of lepidopterans may follow one another. A pheromone trap can help you detect the presence of males, indicating that mating is beginning.

Apply BT with a fine-nozzle sprayer, preferably at the end of the day, and on a dry day. In fact, Bacillus thuringiensis is sensitive to UV rays and has little residual action. It is also subject to leaching. This is why it is recommended that you repeat the spray treatment two weeks after the first application. Finally, applying it at the end of the day disturbs bees and bumblebees less, allowing them to continue foraging.