Cover young watermelon seedlings with row covers to keep them out of bugs' reach. Remove the covers once the watermelon plants begin to develop flowers.
Manually remove bugs off of watermelon plants by spraying the surfaces and undersides of leaves as well as the fruit with a gentle yet steady stream of water from a garden hose. Wear gloves and pick the pests from the plants. Drop the bugs in a bowl full of soapy water to kill them.
Release groups of praying mantis and lady bugs into your garden at night near the watermelon plants. These bugs are natural predators of mealy bugs, aphids, caterpillars, slugs and leaf worms.
Plant radishes around your watermelon plants to deter cucumber beetles. The smell and taste of radish plants will deter the beetles from coming near your watermelon plants.
Spray insecticidal soap onto your plants according to package directions to control aphids. Read the package warnings and directions before making the application.
Spray a rotenone or a pyrethrum-based insecticide on the plants to control adult cucumber beetles. Make the application at dusk to prevent hurting beneficial insects such as honey bees, lady bugs or praying mantis. Read and follow all package directions when applying these insecticides.