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
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
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
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.
No chemical residue
Low long-term cost (once established)
Good for rugged, inaccessible areas
High initial cost (research)
Slow, low level of results
Won't control multiple species
Potential cross over to native species
Not compatible with disturbance
Does not eradicate weeds