|What can I do to prevent this?
|You weren't very specific, or very descriptive of the green bugs on your lettuce, but considering it's very early in the growing season, you're probably dealing with aphids. They seem to be the first insects out each spring, and they love tender new foliage. You can dislodge them with a strong stream of water from the hose. They'll crawl or fly back up when they dry out, but while they're recovering from the water spray, they won't be eating. Just keep after them!