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Bacillus thuringiensis: How it works and the pests it targets

Theme: Biocontrol agents

Overview:

While many people only think of bacteria as microbes that cause harmful disease, they can also be beneficial, for example, they play a crucial role in some microbial biopesticides to prevent pests from attacking crops. Bacillus thuringiensis (sometimes shortened to Bt) is a type of bacteria that lives in water and soil. It is an important biological control agent because it can kill many common pests that damage crops, but it is relatively harmless to humans, animals, and non-pest insects. There are many Bacillus thuringiensis varieties, also known as Bacillus thuringiensis subspecies. For example, ‘Btk’ biological insecticides use a subspecies called “kurstaki”.

Bacillus thuringiensis has been used for over sixty years to get rid of pests and protect plants from damage. It is especially important because it does not cause damage to the environment like many chemical pesticides do. This means that it can replace chemical pesticides in dealing with certain pests, and decrease the damaging impact of pesticides on the environment. Today, Bacillus thuringiensis is used all over the globe and is available in different formulations (such as Btk spray) which means it can be used in different contexts such as farming, forestry, and even in urban areas.

In this article, we will provide an overview of Bacillus thuringiensis including what pests it kills and how it kills them.

What is Bacillus thuringiensis?

Bacillus thuringiensis is a bacteria, which is a type of microbe. Down a microscope, it looks tiny and rod-shaped. It was first discovered in Japan, but its first scientific recording happened in a part of Germany called Thuringia. This is where the name Bacillus thuringiensis comes from. It started being used in the USA in the 1950s and in Canada in the 1970s to protect forests from harmful insects like spruce worms.

Bacillus thuringiensis is such an important part of crop protection that techniques used to grow it have been significantly optimized over the last few decades. It is grown in sterilized tanks and important factors for its growth include temperature, oxygen levels, and nutrient type. For spraying formulations, Bacillus thuringiensis is combined with water or mineral oil and other additives (such as stickers) which help it kill pests.

A microscopic view showing the rod-shape bacteria bacillus thurigiensis
Microscopic image of stained, rod-shaped Bacillus thuringiensis spores. Credit: Ansel Oommen via Bugwood.org

How does Bacillus thuringiensis kill pests?

During the spore formation phase of its lifecycle, Bacillus thuringiensis activates important genes called cry genes which allow it to create three-dimensional structures called crystals that are toxic to the larvae of pests. These are called cry toxins and are made of smaller parts called cry proteins.

When a pest eats Bacillus thuringiensis, it enters the gut and releases cry toxins. These toxins bind to the gut wall and cause holes, which causes the pest to die. Below is a more technical explanation of how this happens.

A part of the cry toxin called ‘cry domain III’ is important for allowing the toxins to bind to the cells of pest larvae. Cry domain III swapping can occur between different toxins. This is a bit like swapping Lego pieces to create different shapes and functions. Cry toxin binding happens in the pest gut at locations called brush border membrane vesicles, which are like little bubbles on the surface of some cells. The toxins specifically bind to parts of the cell called cadherin receptors. The amino acid sequence of the toxin determines its ability to bind to the receptor. This is because the amino acid sequence determines the shape of proteins like toxins and cadherin receptors, allowing them to fit together like puzzle pieces.

What are the different pests Bacillus thuringiensis can control?

Bacillus thuringiensis is important for controlling a wide variety of pests and acts specifically during the larval phase of pests. The following pests are excellent Bacillus thuringiensis targets: 

Spruce budworm (Choristoneura fumiferana)

These pests are a type of moth particularly problematic in Canadian forests where they cause significant damage to the leaves of spruce and balsam firs. The larvae of spruce budworms are the ones causing damage to plants.

A larva of the spruce budworm on a branch
Spruce budworm (Choristoneura fumiferana). Credit: Jerald E. Dewey, USDA Forest Service, Bugwood.org 

Gypsy moth (Lymantria dispar)

The larvae of the gypsy moth (also referred to as Spongy moths) are responsible for considerable damage to forests in North America and elsewhere. In large numbers they can cause significant defoliation (loss of leaves) in a variety of tree types, significantly affecting both forests and urban landscapes.

An adult male of the gypsy moth pinned on a board
Male of the Gypsy moth (or Spongy moth) (Lymantria dispar). Credit: Petr Kapitola, Central Institute for Supervising and Testing in Agriculture, Bugwood.org 

European corn borer (Ostrinia nubilalis)

This species of moth causes significant damage to corn crops. Adults lay eggs on the underside of leaves and when larvae hatch, they attack the crop and cause damage. The European corn borer can go through multiple lifecycles in a single year.

A larva of the Europrean corn borer foraging a corn hob
European corn borer (Ostrinia nubilalis). Credit: Mariusz Sobieski via Bugwood.org

Cabbage looper (Trichoplusia ni)

The larvae of these moths attack cabbage and similar crops, and are called “loopers” due to a particular loop movement they make when crawling. Like the European corn borer, adults lay eggs on the underside of leaves which the larvae then feed on resulting in significant damage.

A close-up of a larva of the cabbage looper on a leaf
Cabbage looper (Trichoplusia ni). Credit: Alton N. Sparks, Jr., University of Georgia via Bugwood.org

What are the pros and cons of using Bacillus thuringiensis?

Pros

Targeted pest control:

The benefits of Bacillus thuringiensis mean that it has risen to be one of (if not the most) used biopesticides globally. According to the Environmental Protection Agency, Bacillus thuringiensis offers targeted pest control of a variety of harmful species while being relatively harmless to humans, animals, and non-pest insects. This specific activity is made possible by its mode of action through the production of the cry toxin.

Environmentally friendly:

Bacillus thuringiensis is a naturally occurring microbe in soil and water. This means that it is completely biodegradable and does not cause damage to the environment like chemical pesticides. Its targeted approach means that it does not cause a loss of biodiversity, a common adverse effect of chemical pesticides. Furthermore, its efficiency in dealing with pests means that it has replaced chemical pesticides in many instances. This further increases the positive effect that Bacillus thuringiensis has on the environment.

Multiple strains:

Bacillus thuringiensis is available in different strains which expands its ability to target different pests.

Cons

Cost:

Products that use Bacillus thuringiensis can be more expensive than chemical pesticides in some instances. This means that they may not be a viable option as a standalone treatment. However, using them within an integrated pest management system may prove more cost-efficient.

Resistance:

Long-term use of Bacillus thuringiensis against certain pests has led to those pests developing resistance to cry toxins. This can lead to renewed loss of crops or defoliation in forests. Solutions to pest resistance include swapping to other Bacillus thuringiensis types that use different cry toxins, or to other biopesticides with different modes of action.

Conclusion and future directions

Bacillus thuringiensis plays an important role in modern pest control. It provides an effective and environmentally friendly tool against many damaging pests in agriculture and forestry. Its targeted mode of action means that it can attack pests like the spruce budworm, gypsy moth, European corn borer, and cabbage looper, without harming beneficial insects, wildlife, and humans. These features make Bacillus thuringiensis a powerful tool with significant advantages over harmful non-specific chemical pesticides. Despite these benefits, challenges like resistance and cost requirements can be an issue when using Bacillus thuringiensis, though the multiple strains of Bacillus thuringiensis provide versatility as a solution to resistance. Ultimately, this bacteria remains important for helping farmers and protect their crops and maximize yield while minimizing the impact on human health and the environment.

For more information about environmentally friendly solutions to pests, you can consult the CABI BioProtection Portal resources. To learn more about different types of biological control, including nematodes, visit our dedicated page on the types of biological control.

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