Biological Methods

Biological weed control methods use weeds’ natural antagonists or enemies as control agents. Typically biological weed methods have involved the use of plant pathogens or insects for the control of a problem species. Unlike tillage, mowing, fire and chemical control methods, most biological control agents are single plant species specific. For example, two insects are currently being used in Georgia for musk thistle (Carduus nutans) control. The thistle head weevil (Rhinocyllus conicus) and the rosette weevil (Ceuthorrhychidius horridus) have been released at numerous sites in north and central Georgia for the control of musk thistle. The thistle head weevil feeds primarily on musk and plumeless thistles and will not feed on other plants. Research and on-farm demonstrations has shown that these weevils will survive in Georgia and over a five to seven year period reduce musk thistle populations. The objective of biological control is not weed eradication, but rather the reduction of the population below a level of economic or aesthetic injury.
A successful biological weed control agent includes the following attributes:

1. it weakens or kills the weed,
2. it controls only the target species, and does not affect the growth of desirable plant species,
3. be mobile enough to find the weed,
4. reproduce faster than the weed,
5. be adapted to the weeds environment, and
6. be free of predators or pathogens.

In addition to the use of insects and plant pathogens for biological weed control, various types of animals are commonly used to control nuisance vegetation. An effective and commercially available biological control agent for aquatic weeds involves the white amur, or grass carp (Ctenophryngodon idella). This fish feeds mostly on filamentous algae, Chara, submersed weeds and duckweed. It does not, however, feed extensively on emergent vegetation or large free-floating weeds such as water hyacinth. Stocking rates range from 5 to 20 fish-per-acre. In many pastures and noncropland areas such as automobile junkyards, goats are commonly used to control both herbaceous and woody vegetation. Goats forage on a diversity of plant species rather than eating only one or two species.
Although some outstanding successes have been achieved with biological agents on some sites, additional research is needed to identify additional biological weed control agents for noncropland areas. Through wide publicity, the general public has incorrectly viewed biological control as a viable, practical and immediately available alternative for the control of all types of nuisance vegetation.
Links to more information on biological weed control methods:

Biological Control: A Guide to Natural Enemies in North America

Center for Aquatic and Invasive Weeds

Biological Control of Eurasian WaterMilfoil

Biological Weed Control in British Columbia

Biological Weed Control Grazing

Biological Weed Control