From out friends over at The Community Gardens website:
Many people are fond of ladybugs because of their colorful, spotted appearance. But farmers love them for their appetite. Most ladybugs voraciously consume plant-eating insects, such as aphids, and in doing so they help to protect crops. Ladybugs lay hundreds of eggs in the colonies of aphids and other plant-eating pests. When they hatch, the ladybug larvae immediately begin to feed. By the end of its three-to-six-week life, a ladybug may eat some 5,000 aphids.