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The rate of success of projects aiming to
achieve biological control is high considering the checks and
constraints placed on biological control. The global success rates for
classical biological control of weeds, estimated by Julien et al.
(1984), using releases of agents from the first deliberate introduction
to 1980 (Julien
1982), were as follows:
 | Of 174 projects 39% were successful.
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| 101 species of weeds were targeted of which 48% were controlled.
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| 178 different species of organisms were released of which 71%
established and 34% were effective.
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| There were a total of 499 releases of control agents of which 64%
established and 29% were involved in successful control of a weed. |
Three factors were identified as positively affecting the rate:
| Organisations such as CSIRO and the Hawaiian Department of
Agriculture, where detailed studies are conducted, had the highest
rates of success. In comparison organisations or regions where
mainly ad hoc releases have been made, had the highest failure
rates.
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| Utilisation of agents already proven successful in another country
provided the best chance for successful control. Repeated use of
proven agents is increasing as more countries increase their
involvement in biological control. For example, the salvinia weevil,
initially successful in Australia, was subsequently released and
controlled salvinia in 13 other countries.
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| Utilisation of native organisms to control weeds showed a far
greater success rate, 62% of releases, compared to 28% for exotic
organisms. These results should be treated cautiously because of
biases in the comparison. The main limitation to using native
organisms is that an ongoing commitment to rearing and making
inundative releases is required. |
One of the difficult aspects of assessing biological control is
defining and describing success or failure. Outcomes of biotic
interactions over the range in which the organisms (weed and agents)
exist are usually variable hence describing the results can be complex.
Determining what effects the success of a project,
and improving the overall (global) rates
of success are considered elsewhere.
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