DO GOOD: Volunteer and get outside at the same timeIf you love snorkeling for scallops or tromping through the woods, you can put your outdoorsy ways to good use by volunteering to help keep nature preserves as pristine as possible. Here's some opportunities just this week:Great Bay Scallop Search: This Saturday, Tampa Bay Watch is looking for volunteers with shallow draft boats to help monitor the progress of the Tampa Bay area scallop populations. Snorkelers will count scallops along each side of weighted 50-meter transect lines. Registered volunteers must meet at the Fort De Soto boat ramp in Tierra Verde to receive survey equipment and instructions. Positions are also available for volunteers with canoes and kayaks. A limited number of snorkelers without boats will be pair up with boaters who have room aboard their vessels 9 a.m.-1 p.m. Saturday. Call (727) 867-8166, ext. 233. Fort De Soto Park, 3500 Pinellas Bayway S, Tierra Verde. Return the Preserve Day: Join Friends of Brooker Creek and help rid the preserve of exotic, invasive vegetation and nuisance plants. Bring work gloves and yard tools 8-11 a.m. Saturday. Water and snacks provided. Best for ages 12 and older. Brooker Creek Preserve, 3940 Keystone Road, Tarpon Springs. (727) 453-6800. Native Grass Planting: Volunteers are needed 9 a.m.-noon Friday to help plant high marsh native grasses at the Rock Ponds Borrow Pit in Ruskin, the site that was historically agriculture fields, shell mining pits and surrounded by nonnative nuisance plants. The Rock Ponds project is the largest coastal ecosystem restoration project in Tampa Bay. The site has been partially filled and reshaped to provide more than 2 miles of new Tampa Bay shorelines and islands in need of plants. Go to tampabaywatch.org/volunteer to register and get driving directions. Rock Ponds Borrow Pit, 4235 Lavender Road, Ruskin. (727) 867-8166. HEADS UP: Free entry to national parksTo celebrate the National Park Service's 100th birthday, all national parks will be free Aug. 25-28. So it's time to plan a road trip! While most of the 400 parks in the system are free, 127 of them charge entry. You can find a complete list of national parks by state at nps.gov. The Florida parks that normally charge for entry are Florida Canaveral National Seashore ($1-$5), the Castillo de San Marcos National Monument in St. Augustine ($10), Dry Tortugas National Park ($10), Everglades National Park ($20 per vehicle) and Gulf Islands National Seashore ($15 per vehicle). THE BUZZ: Best mosquito repellents With news of the spread of the Zika virus, transmitted mainly by mosquitoes, health authorities are warning that avoiding mosquito bites requires more than just wearing long-sleeved shirts and pants when outdoors. Mosquito repellents are essential. Consumer Reports recently released for free its test results and ratings of mosquito repellents, including those that will protect you best against Aedes mosquitoes, the type that carry Zika. "Our brave testers stuck their arms into cages full of disease-free female mosquitoes in need of a blood meal to lay their eggs," the consumer watchdog said. They recorded bites for five minutes every hour. A repellent failed if a tester was bitten two or more times in one five-minute session. The results found products containing DEET and picaridin, a synthetic repellent modeled after the compound that occurs naturally in the black pepper plant, were the most effective. But concentration matters. "A 20 percent picaridin product was our top repellent overall — and the only one to ward off both species of mosquitoes plus ticks for at least eight hours," the report said. "Another picaridin product, this one just 5 percent, was our second-lowest scoring insect repellent." Sawyer Picaridin and Natrapel 8 Hour, which each contain 20 percent picaridin, and Off! Deepwoods VIII, which contains 25 percent DEET, earned top ratings, keeping mosquitoes away for at least eight hours. Ben's 30 percent DEET Tick & Insect Wilderness Formula kept Aedes mosquitoes away for 7 1/2 hours, and Repel Lemon Eucalyptus, containing 30 percent lemon eucalyptus, stopped them for seven hours. The testers got pretty poor results from products made with natural plant oils such as citronella, lemongrass oil and cedar oil. EcoSmart Organic, which includes geraniol, rosemary oil, cinnamon oil and lemongrass oil, did not last for more than one hour against Aedes mosquitoes, and some failed almost immediately.