Biological control is a developing field of study that is steadily growing. It involves using insects, disease organisms, or animals to reduce the dominance of weedy plants and bring them back into balance with native plants. This method of control will not eradicate the invasive plants from a site, but can reduce their numbers and density. This is accomplished by stressing or weakening the plant and reducing its ability to compete, preventing or eating new seeds, and killing individual plants.

          Biological weed control efforts begin with a researcher returning to the weed's native land and monitoring what eats, hurts, or kills the plant there. These control agents (usually insects and pathogens) are then collected and studied under secure conditions for at least 10 years. During this time, these agents are introduced to a number of different native plants and animals to see if they will eat or somehow affect them. Most of the agents fail this test (because they are not host-specific) and are not introduced into America. Those that do pass the test are grown and distributed into existing weed sites where they perpetuate themselves through natural reproduction. Many of these agents die because of the different climate or habitat. Some survive and are effective on some sites, but not on others. A few have noticeably impacted the weed populations. For example, a fifty acre plot was tested primarily with the Gallerucella beetle, and showed a 75-80% reduction of purple loosestrife.

          It usually takes the combined efforts of a number of agents to reduce a weed population. For example, one group of agents is insects. Some of these eat the leaves, a few eat the seeds in the flower, and others will burrow down and eat the stem and roots. Other agents may be disease organisms or fungi that stunt or slowly kill the weed.

          Biological control agents may also include mammals, fish, and birds. These typically require more management, but are very effective at times. For example, sheep have been used to control leafy spurge, spotted knapweed, tall larkspur, and other weeds. Goats have also been found to be effective against some weedy species.