Mosquitoes aren't simply biting pests. They can transmit several serious diseases to your dog. However, you have quite a few options for effective mosquito repellents for dogs, both commercial formulas and some natural deterrents.
There are almost 3,000 types of mosquitoes throughout the world and several hundred species in North America. Most lay eggs and mature in stagnant water and are more prevalent in the humid southern and midwestern parts of the United States. Many of these can transmit diseases---including heartworms, West Nile virus, encephalitis and yellow fever---to dogs.
Commercial Mosquito Repellents
You can use some mosquito repellents intended for humans on dogs, but read the label to be sure they're approved for pets, and always double-check with your veterinarian.
Never use any product containing DEET on dogs. In addition to exposure through the skin, dogs are likely to ingest the chemicals while grooming themselves.
Several formulas are available specifically for animals from veterinarians and pet and farm supply stores. Pyrethrin-based flea spray (use as directed) is also effective as a mosquito repellent. A veterinarian-approved topical treatment, K9 Advantix by Bayer, is currently the only monthly mosquito repellent on the market. All the others are sprays or shampoos, with effectiveness varying from several hours to nine days, depending on the product.
Natural Mosquito Repellents
If you don't want to routinely spray synthetic chemicals on your dog, you can try gentler repellents, though they may not be as effective. Wiping your dog's fur down with rubbing alcohol, dryer sheets or diluted Avon Skin So Soft bath oil (a favorite of hunters) can repel mosquitoes. Some people recommend a spray made up of white vinegar and water mixed in equal parts, with a single drop of either cedar, lavender or lemon oil added to a quart of the vinegar-water mix.
The most natural repellent is, whenever possible, to keep your dog indoors when the mosquitoes are most active, in the late evening.
Repellents Not Guaranteed
Bear in mind that no repellent is a guarantee against a bite by an infectious mosquito. Your vet can tell you which mosquito-borne, preventable diseases are present in your area. Never rely on repellents alone to protect your dog from a lethal disease like heartworms: Your veterinarian will carry safe, monthly heartworm preventatives. Use them.
There are many mosquito repellents posted online using various blends of essential oils. While some essential oils are quite safe to use on dogs, you should heavily dilute most, and several can cause illness and even death. Pet stores sell sprays and shampoos containing oils like citrus or tea tree oil, but these are highly diluted. Never put essential oils on an animal without being absolutely sure it's safe. Unlike humans, dogs and cats lack the enzymes to break down some of the chemicals in these oils, and they can lead to organ toxicity and neurological damage.