There are several essential oils that can be used to kill fleas. Here are some of the most effective ones:

1. Lavender Essential Oil: This is one of the most effective essential oils available to repel fleas and keep them at bay. It also has calming properties that can help alleviate stress and skin irritations caused by flea bites.

2. Citronella Essential Oil: This oil is widely known for its mosquito-repelling abilities, but it is also effective against fleas and ticks. However, it is quite toxic for pets, so it should always be used in diluted form and in very small quantities.

3. Rosemary Essential Oil: This oil has insecticidal properties that can kill fleas. It can also be used to repel future fleas.

4. Peppermint Essential Oil: Peppermint oil can kill fleas and serve as an effective repellent. It can also relieve skin irritation and inflammation caused by flea bites.

5. Lemongrass Essential Oil: This oil is well known for its antifungal and insecticidal properties. It can be used to repel fleas, ticks, mosquitos, and other bugs.

6. Cedarwood Essential Oil: Cedarwood oil not only repels but also kills fleas, making it great for use on pets and in the home. It is considered to be the best essential oil for fleas.

Its important to note that essential oils should always be diluted before applying them to pets, and they should never be placed anywhere your pet can lick at them. Additionally, some essential oils can be harmful to pets, so its important to use them with caution.