Info here from a US veterinary site. Citronella is citrus oil and is toxic to cats. Think the cat needs to see a vet.
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Cat Veterinarian: MichelleB717
Essential Oils – CatsEssential oils can pose a toxic risk to household pets, especially to cats. They are rapidly absorbed both orally and across the skin, and are then metabolized in the liver. Cats lack an essential enzyme in their liver and as such have difficulty metabolizing and eliminating certain toxins like essential oils. Cats are also very sensitive to phenols and phenolic compounds, which can be found in some essential oils. The higher the concentration of the essential oil (i.e. 100%), the greater the risk to the cat. It is always recommended that any cat that come into contact with essential oils be taken to the vet for IV fluids, bloodwork to check liver and kidneys, and supportive care. Since just a few drops can kill a cat, immediate treatment is the only thing that improves the chance of recovery.Essential oils that are known to cause poisoning in cats include, but are not limited to, oil of wintergreen, oil of sweet birch, citrus oil (d-limonene), pine oils, Ylang Ylang oil, peppermint oil, cinnamon oil, pennyroyal oil, clove oil, eucalyptus oil, and tea tree oil.Symptoms that develop depend on the type of oil involved in the exposure and can include drooling, vomiting, tremors, ataxia (wobbliness), respiratory distress, low heart rate, low body temperature, and liver failure.Typically you see some of these symptoms within minutes, especially drooling, if the kitty licked it. If kitty stepped in it, it is possible kitty will have some go into the system and it will take longer to see symptoms - by this time, permanent damage might be done; I would wash affected body parts immediately and I would”