Witchweed: A

Parasitic Pest

Witchweed (Striga asiatica) is a parasitic plant that

attacks some of the most important crops in the

United States–corn, sorghum, sugar cane, and rice.

It also parasitizes certain weedy grasses.

Unlike most weeds, which merely compete with

crops, parasites like witchweed do their damage

more directly. They rob nutrients and moisture by

tapping directly into the host’s root system.

Consequently, the host spends energy supporting

witchweed growth at its own expense. While a parasitized host, such as a corn plant, may be less productive (produce fewer seeds), each witchweed plant

produces as many as 50,000 seeds.

The American witchweed is one of many related

parasitic plants native to Africa, India, the Middle

East, and China. In these areas it can completely

ruin agricultural productivity in affected land. No one

is sure how or when witchweed first came to the

United States. However, the slender, red-blossomed

parasite was first identified in 1955 by a graduate student from India who knew it as the pest that had

ruined sorghum production in his country.

Witchweed will grow in the presence of grassy

weeds as well as grass host crops, so cotton, peanut,

or soybean fields–along with home gardens or idle

land–may harbor the pest. Witchweed emerges from

the soil beginning in late June and flowers about 2

weeks later. Swollen seed pods follow, liberating

thousands of microscopic seeds, which can live in

the soil for at least 10 years.

Witchweed’s prolific nature and its potential for

devastating important American host crops led to a

decision to eradicate this pest. Congress first allocated funds for this purpose in 1957. The U.S.

Department of Agriculture’s (USDA) Animal and Plant

Health Inspection Service (APHIS) established a

research station and farm where it developed control

methods.

Since its discovery in North and South Carolina,

witchweed’s spread has been halted, and the

acreage supporting it has been reduced by 99 percent (from 450,000 acres to about 3,400).

Eradication is accomplished in three phases:

• Survey activities find and map all infestations,

• Quarantine activities prevent human spread of 

witchweed beyond the infested region, and

• Control activities seek to prevent existing plants 

from producing seeds and to destroy seeds 

already in the soil.

These activities involve the cooperative efforts of

Federal and State governments as well as the general public.

Survey

APHIS and State cooperators have taken steps

to prevent this dangerous weed from spreading from

infested areas in North and South Carolina. APHIS

is offering a $25 reward to anyone who identifies and

reports the weed. After receiving a report, officials

remove the plant to stop reproduction. Then they

destroy seeds already in the soil.

Finding every specimen of the slender, foot–tall

witchweed plant is not without obstacles. The cooperation of landowners is essential. Scouts are sent

out on foot, in vehicles, and on horseback to find

infested sites. People are asked to check their own

land and to report the presence of witchweed to an

agricultural extension agent or witchweed personnel.

Containment Through Quarantine

Although the tiny witchweed seeds can be

spread by wind or water, people are the chief means

of dispersal. To prevent the spread of this pest, agricultural quarantines specify conditions for moving

soil, plants, or machinery out of infested areas.

Eradication

Eliminating witchweed requires finding and killing

plants before they go to seed and eliminating seeds

already in the soil.

Herbicides are used on fields infested with

witchweed. Extensive field research has provided

information on the best chemical or combination of

chemicals for the given crop, weed species, and field

conditions.

Eliminating the microscopic seeds is another

important part of eradication. Some seeds die of 

natural attrition or sprout only to be killed by an herbicide. However, since witchweed seeds can persist in

the soil for a decade, efficient eradication requires

accelerating the natural rate of seed germination.

Witchweed seeds can be eliminated from the soil

by creating conditions that cause them to germinate

when no host is present. This phenomenon is known

as suicidal germination. Ethylene gas, a natural

ripening agent produced by fruits, vegetables, and

flowers, is injected into the soil under proper environmental conditions. It stimulates seed germination,

but lacking a host, seedlings die.

Soil fumigation is another alternative, albeit an

expensive one. Chemicals such as methyl bromide

are used on occasion to assure seed destruction.

Factsheet

Plant Protection and Quarantine March 2003

APHIS
Eradication Within Reach

The large number of acres involved and the high

seed production of this pest have made eradication a

slow process. However, this parasite has been eradicated from 99 percent of the infested land. In 1995,

the North Carolina Department of Agriculture

assumed responsibility for eradication activities in that

State. APHIS personnel continue eradication activities on the remaining 400 infested acres in South

Carolina. APHIS will continue to provide support to

these States for surveys to verify eradication and for

post–eradication treatments.

Report Witchweed

Report any suspect witchweed plant by calling

1–800–206–WEED. (Outside of the Carolinas, call

APHIS at 1–919–716–5590.)

For more information on witchweed and other

noxious weeds, as well as additional APHIS programs, visit the APHIS Web site at

http://www.aphis.usda.gov. Use the “Comments” link

to send an e-mail request for additional information.

United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture

The U.S. Department of Agriculture (USDA) prohibits discrimination

in all its programs and activities on the basis of race, color, national

origin, sex, religion, age, disability, political beliefs, sexual orientation, or marital or family status. (Not all prohibited bases apply to

all programs.) Persons with disabilities who require alternative

means for communication of program information (Braille, large

print, audiotape, etc.) should contact USDA’s TARGET Center at

(202) 720–2600 (voice and TDD).

To file a complaint of discrimination, write USDA, Director, Office of

Civil Rights, Room 326–W, Whitten Building, 1400 Independence

Avenue, SW, Washington, DC 20250–9410 or call (202) 720–5964

(voice and TDD). USDA is an equal opportunity provider and

employer.