Leaves of Castor plants are not used for the rearing of silkworms because they are toxic and can be harmful to silkworms.

Silkworms are the larvae of silk moths and are primarily reared for the production of silk. They feed on the leaves of certain plants, and the most commonly used plant for silkworm rearing is the mulberry plant.

- Mulberry:
Mulberry leaves are the primary food source for silkworms. They are rich in nutrients and provide the necessary sustenance for the growth and development of silkworms. The leaves of mulberry trees are highly preferred by silkworms.

- Castor:
Castor plants, on the other hand, contain a toxic compound called ricin. Ricin is a protein that is poisonous to many animals, including silkworms. If silkworms are fed with castor leaves, they can suffer from toxicity and even die. Therefore, castor leaves are not used for the rearing of silkworms.

- Oak:
Oak leaves are not commonly used for silkworm rearing either. While oak leaves are not toxic to silkworms, they are not as nutritionally rich as mulberry leaves. Silkworms require a diet that is high in protein, and oak leaves do not provide the same level of nutrition as mulberry leaves.

- Teak:
Teak leaves are also not used for silkworm rearing. Similar to oak leaves, teak leaves do not offer the same nutritional value as mulberry leaves. Therefore, they are not suitable for feeding silkworms.

In conclusion, the leaves of castor plants are not used for the rearing of silkworms due to their toxicity. While other plants like oak and teak may not be toxic, they do not provide the necessary nutritional requirements for silkworms. Mulberry leaves remain the most preferred and widely used food source for silkworms.