Bacillus thuringiensis (Bt) is a kind of beneficial microorganism that exists widely in nature. It produces proteins that are toxic to immature insects (larvae).
There are many types of Bt bioinsecticide. Each type targets a different group of insects. Target insects include beetles, mosquitoes, root aphids, nematodes, caterpillars, and grub.
Currently, there are more than 180 registered bacillus thuringiensis products on the market. Bt insecticide products are used in crops and ornamental plants. These products are usually aerosols, dust, particles. Some of these products have been approved for organic agriculture.
The colony of Bacillus thuringiensis on the NA medium is round or oval, light yellow, with irregular edges, opaque, and slightly raised, and waxy.
Bacillus thuringiensis cells are purple elliptical rod-shaped under the microscope, with a size of (1.2～1.8)μm×(3.0～5.0)μm, arranged in short or long chains. The spores are elliptical and grow near the middle.
How does Bacillus thuringiensis (Bt) work
Bacillus thuringiensis produces a protein that is toxic to immature insects (larvae) called parasporal crystals. Target insects beetles, mosquitoes, root aphids, nematodes, caterpillars, and grub.
When these insect larvae gnaw on the leaves with the bacteria, the parasporal crystals will be dissolved and released in the insect’s highly alkaline intestines, preventing the larvae from further eating plants, loss of appetite, endotoxins destroying the intestinal cell wall, and finally breaking the intestine, leading the toxins to enter the larvals’ blood system, causing systemic poisoning. Infected insects stop foraging within a few hours and die within a few hours to a few weeks (usually 2-3 days).
This mechanism can make Bt harmless to fish and mammals because their acidic intestinal conditions inhibit bacterial toxin dissolution.
Due to its non-chemical pollution and low toxicity, Bacillus thuringiensis has been widely recognized and used since the 1950s, and is often used as a biological pesticide, biocontrol agent in organic agriculture.
Application in Agriculture
As a biopesticide, Bacillus thuringiensis (Bt) has many advantages compared with chemical pesticides.
- It is non-toxic to mammals and safe to use. Since the intestinal environment of mammals is acidic, parasporal crystals have no effect in the intestines of these animals.
- It has strong selectivity and specificity and does not harm other beneficial insects. Bt bacteria only specifically infect Lepidoptera insects.
- Strong sustainability. Bacillus thuringiensis is a microorganism that can multiply in nature without destroying the soil structure.
- Naturally, control pest density.
- The control of pests helps prevent the widespread of plant pathogens or viruses.
Because it is a microbial agent, it has high environmental requirements and is toxic to animals such as silkworms, so its use has certain limitations. But this natural insecticidal mechanism meets the environmental requirements of organic agriculture.
How to apply Bacillus thuringiensis (Bt)
Take 50000IU/MG Bacillus thuringiensis as an example, the dosage is about 50-100G per 30L water.
Bt will quickly inactivate under the ultraviolet light (sunlight), but it will survive longer at night, under cloudy or humid weather. The application of Bt insecticide does not continue to control insects through the reproduction of bacterial cells. The application of Bt is similar to chemical insecticides, using its protein toxins to kill insects. After preparing the Bt solution, it should be used immediately, especially when the pH value of the water for preparing the solution is greater than 7 (alkaline), the toxin is easy to dissolve, so use it in time to avoid inactivation.
In addition, after two or three years of storage, the effectiveness of Bt products may be reduced. Dry powder lasts longer than liquid formulations. Bt products should be stored in a cool and dry environment, away from direct sunlight.
In tea, corn, vegetables, and other fields, there will be pests such as spodoptera frugiperda, armyworm, beet armyworm, corn borer, etc. When the larvae of these pests are found, microbial agents such as bacillus thuringiensis should be applied as soon as possible and used repeatedly. It will reduce the density of pests and achieve the purpose of avoiding them.
In recent years, some scientists have used bacillus thuringiensis to control root knots and cyst nematodes by using strain screening, strain improvement, and secondary metabolite collection.
In short, Bt is a microbial biological pesticide, which is very specific to lepidopteran insects. It causes the insects to stop foraging within a few hours and usually kills the insect larvae within a few days. However, due to environmental factors and rain washing, it cannot last for a long time on plants, so it needs frequent use. It is considered organic to agriculture and non-toxic to mammals.
Finally, the main control objects of Bacillus thuringiensis are attached:
Lepidopteran pest larvae; Pieris rapae, Spodoptera frugiperda, Plutella xylostella, Spodoptera exigua, Spodoptera litura, Brassica napus (moth), Tobacco caterpillar, Corn borer, Rice leaf roller, Chilo suppressalis, Pine Caterpillars, tea caterpillars, tea loopers, corn armyworms, bean pod borers, silver-striped armyworms, and other pest larvae.
It also has a certain control effect on root-knot nematodes, mosquito larvae, leek maggots, beetles, and other pests.