The buzz on repellents. (Insect Repellents Test).(Buyers Guide)

A good insect repellent can be a lifesaver when a bite could cause not only an itchy welt but a serious disease. We enlisted medical entomologists at an outside lab to see which products best keep bugs at bay. The verdict: Deet remains the best deterrent.

Deet, or N,N-diethyl-meta-toluamide, was developed more than 50 years ago by the U.S. Army and the Department of Agriculture. It doesn't kill bugs, but its vapors discourage them from landing or climbing on you. Deet is generally acknowledged to be the most effective mosquito repellent there is. It has been used by many millions of people worldwide for decades, and it's considered safe when used according to directions.

However, deet poses a very small potential risk of adverse side effects, especially if misused. Generally, products with about 30 percent deet have proven most effective for the longest periods, and they are considered safe for adults and children age 2 months and older. Products with higher concentrations may be appropriate for adults for prolonged protection in an area where bugs are known to carry disease, and if they're used according to directions. Lower concentrations of deet may offer adequate protection for short outings or low-risk situations. (See "Safe Use of Insect Repellents" on facing page.)

We tested repellents with concentrations of 7 percent to 100 percent deet and others with no deet. Some of the products claim to fend off mosquitoes and ticks, others only mosquitoes.

Three testers exposed their repellent-treated arms to 200 mosquitoes for 3 minutes, then counted the number of bugs that bit them. If none bit, the testers repeated the process every half-hour until at least one mosquito bit during two successive exposures. To judge effectiveness against ticks, the testers put three active deer-tick nymphs on their untreated wrists and waited to see whether the ticks would climb onto a repellent-treated section of their forearm. The test was repeated every hour with new ticks until at least one tick ventured onto the treated area and stayed there. (None of the bugs carried disease.)

Of the repellents we tested, the two most effective at deterring both mosquitoes and ticks were 3M Ultrathon, $1.50 per use, and Off! Deep Woods for Sportsmen, $1.67 per use. Both kept mosquitoes from biting for about 13 hours. Ultrathon protected from ticks for 10 hours, Off! for 7 hours. (Results vary by individual.) Although they offer similar protection, Ultrathon has 33 percent deet-one-third the amount in Off! Deep Woods. If you don't need daylong protection, consider BugOut, a good buy at 11 cents per use. It contains 15 percent deet.

Don't expect much from nondeet repellents. Avon Skin-So-Soft Bug Guard Plus 1R3535 with Sunblock SPF 30, which uses a chemical called IR3535, deterred mosquitoes for only 1 hour and ticks for 4 hours. We're skeptical of products using plant oils such as citronella, soybean, coconut, and geranium. We've found that they provided little if any protection.