Taken from my Greasy Rider column for Outside magazine's web site:
Q: Is there a DEET alternative I can really trust when I go hiking in the Costa Rican jungle?
A: I can't speak for Costa Rican insects, but I can tell you what the mosquitoes and no see 'ems in my back yard don't like—and my back yard happens to hold the unofficial title as Buggiest in Asheville, North Carolina. Last summer, whenever I let my five-year-old daughter and four-year-old son run out the back door for more than five minutes without bug spray on them, they'd come back inside covered with welts. Forget DEET, I was ready to dip them head-first into a pool of DDT—like the baby Achilles in a chemical River Styx—if it meant that their arms and legs would no longer make them look like smallpox victims. But Dr. Wife, MD, is much heavier into the whole "organic" thing, and wants our kids' DNA to remain relatively humanoid, so she instructed me to find a natural alternative.
The National Coalition Against the Misuse of Pesticides—not a fan of DEET—recommends an herbal repellent like All Terrain’s pleasantly scented Herbal Armor, which is sold at many natural food stores. It employs citronella, peppermint, soybean oil, and lemongrass to scare off bugs. On the other hand, the Centers for Disease Control approves of only one natural remedy for mosquitoes: oil of lemon eucalyptus, like what’s found in the pungent and aptly named Repel Lemon Eucalyptus. (The CDC recommends that kids under three years old shouldn’t be exposed to Lemon Eucalyptus, though.)
After buying these two products, my logical next step was to perform a controlled test on the kids. So I sprayed one arm and leg on each of them with Herbal Armor and the other arm and leg with Repel. (Neither had any idea what was going on. Kids are so trusting.) The results: After a half hour of solid protection, the bugs began devouring the Herbal Armor-covered side like entrants in a Coney Island hot dog eating contest, while nearly three hours passed before Repel began losing its potency. When Dr. Wife, MD, came home that evening, she was mystified to see that the kids were suffering from bug bites over only half of their bodies. ("You’re right, honey, that's awfully strange. I have no idea why that happened.") As a result, my anecdotal conclusion is that a lemon eucalyptus-based solution like Repel will probably be effective for you in Costa Rica, as long as you’re more than three years old, and you reapply after a couple of hours. Though to be safe, I'd carry along a something with DEET in it as well.