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Plants that have become weeds in Australia are rarely
invasive and troublesome in their home country. This is
often because the populations of these plants are regulated
by a variety of natural enemies such as insects and pathogens
(disease-causing organisms like fungi, bacteria, etc)
that attack the seeds, leaves, stems and roots. If plants
are introduced to a new country without these natural
enemies, their populations grow unchecked and they often
become weeds.
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The biological control approach makes use of the invasive
plant's naturally occurring enemies, to help reduce its
impact on agriculture and the environment. It simply aims
to reunite weeds with their natural enemies and achieve
sustainable weed control. These natural enemies of weeds
are often referred to as biological control agents.
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It is critical that the biological control agents do
not become pests themselves. This is why considerable
host-specificity
testing is done prior to the release of biological
control agents to ensure that they will not pose a threat
to non-target species such as native and agricultural
plants.
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Not all weeds are suitable for biological control. Developing
a biological control project requires a substantial
investment (i.e. millions) from stakeholders and it is
generally only used when the cost of conventional control
(herbicides, mechanical control, fire, etc) is so great,
both in dollar terms and impact on the environment, that
there is little option than to pursue this avenue.
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CSIRO has been working on the biological control of
weeds since the 1920's starting with the biological control
of prickly pear. CSIRO now has many active biological
control projects for both temperate
and tropical Australian
weeds which cause problems in natural, pastoral and agricultural
ecosystems.
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Best case scenario of the course of events in classical
biological control programs targeting weeds invading habitats
such as rangeland, pasture, or natural ecosystem.
An alien weed is a problem in the introduced range because
its population density fluctuates around an equilibrium that
is above a threshold at which the weed begins to affect the
economic or ecological sustainability of the ecosystem. Following
their introduction and establishment, populations of biological
control agents build up to very high levels due to the abundance
of their host plant. Eventually their attack on the plant
causes a decline in the weed population. This, in turn, leads
to a decline in the numbers of biological control agents until
an equilibrium is reached between the amount of damage caused
by the agents and regeneration by the weed. In a successful
biological control program this new equilibrium is below the
damage threshold that the ecosystem can tolerate.
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