The spread of opuntia was controlled by -

Options

A. Allowing cattle to graze over it
B. Allowing sheep and goats to browse over it
C. Introducing insect
D. Spraying herbicide

Answer

To answer the question about how the spread of Opuntia (prickly pear cactus) was controlled, we can analyze the options provided and the context of the problem.

### Step-by-Step Solution:

1. **Understanding the Problem**:
- Opuntia, or prickly pear cactus, was introduced to Australia and spread rapidly due to the absence of natural predators. This led to it becoming a significant problem, covering millions of hectares.

2. **Identifying Control Methods**:
- We have four options for controlling the spread of Opuntia:
- Option 1: Allowing cattle to graze over it.
- Option 2: Allowing sheep and goats to graze over it.
- Option 3: Introducing insects.
- Option 4: Spraying herbicides.

3. **Evaluating Each Option**:
- **Option 1 (Cattle Grazing)**: While cattle may eat some plants, they are not particularly effective against prickly pear.
- **Option 2 (Sheep and Goat Grazing)**: Similar to cattle, sheep and goats may graze on some vegetation but are not specifically known for controlling prickly pear.
- **Option 3 (Introducing Insects)**: This option is significant because certain insects are known to feed on prickly pear and can effectively control its population.
- **Option 4 (Spraying Herbicides)**: This method is also effective in controlling unwanted plant species, including prickly pear.

4. **Selecting the Best Control Method**:
- Historical records indicate that the introduction of specific insects, such as the Cactoblastis cactorum moth, was a successful biological control method for managing the spread of Opuntia in Australia.

5. **Conclusion**:
- The most effective method that was used to control the spread of Opuntia was **Option 3: Introducing insects**.

### Final Answer:
The spread of Opuntia was controlled by introducing insects.