Herbal Flea Collar

Take the pet's collar or neckerchief and put on drops of Eucalyptus oil, tea tree oil , citronella, lavender, or geranium. This will need to be done weekly.

• Note: Do not use essential oils on cats! While cats are extremely sensitive to Eucalyptus and tea tree oils, a cat's ability to metabolize any essential oils inhaled, ingested or absorbed through skin is considered to be weak and there is always a risk of overloading a cat's system.
  
• With tea tree oil, be especially careful to use products with 1% or less of this essential oil. It can cause nausea, vomiting, lethargy and general weakness, and should be used with caution. With this oil, more is not better.

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Source: http://www.wikihow.com/Get-Rid-of-Fleas-Naturally