The best substance yet found to block mosquitoes' powers of detection is DEET, and while a few repellents are equal over a short term, nothing even comes close in terms of residual repellency; DEET's effects can last for several hours, whereas even the best alternative repellents last at most about 2 hours, maybe 3, if you're lucky and the conditions are right. Garlic is only one of a list of plant compounds with similar "repellent" effects, including citronella, cedar, verbena, geranium, lavender, pine, cinnamon, rosemary, basil, thyme, allspice, and peppermint. But again, these substances have a very limited efficacy, in terms of duration, and if you are going to be exposed to mosquitoes for more than an hour or two, you should get a good DEET-based spray or lotion and stick with it.