Which is a microbial insecticide ?

Options

A. Bacillus thuringiensis
B. B. subtilis
C. B. polymixa
D. B. brevis

Answer

**Step-by-Step Solution:**

1. **Understanding Microbial Insecticides**: Microbial insecticides are defined as microorganisms or products derived from microorganisms that can attack or kill pest insects. They serve as a method of biological control in agriculture.

2. **Identifying the Options**: The options provided are:
- A) Bacillus thuringiensis
- B) Bacillus subtilis
- C) Bacillus polymyxa
- D) Bacillus brevis

3. **Evaluating Each Option**:
- **Bacillus thuringiensis**: This is a well-known microbial insecticide. It produces toxins that are harmful to certain insect pests, making it a popular choice in agricultural practices. It is also used in genetically modified crops like BT cotton, which are engineered to be resistant to specific pests.
- **Bacillus subtilis**: This bacterium is not recognized as a microbial insecticide. It has other beneficial uses, such as in probiotics and soil health, but it does not specifically target insect pests.
- **Bacillus polymyxa**: Similar to Bacillus subtilis, this bacterium does not have a role as a microbial insecticide. It is primarily known for its role in soil health and plant growth promotion.
- **Bacillus brevis**: This bacterium also does not function as a microbial insecticide. It is used in other applications but not for pest control.

4. **Conclusion**: Based on the evaluation, the only option that qualifies as a microbial insecticide is **Bacillus thuringiensis**.

**Final Answer**: The correct answer is **Bacillus thuringiensis**.

---