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Success of the Programme
Chapter 26
Current Approaches to Biological Control
of Weeds
P. HARRIS
A total of 30 agents has been released against 14 weed species in Canada. A third of
the agents have failed to become established, a third are established but inflict
relatively minor damage to the weed, and a third inflict major damage. This gives a
crude measure of the success of the programme, but it is not adequate to determine
whether investment in biological control of weeds is justified in economic terms. The
opportunity is taken in this chapter to discuss this and other current issues.
The success of any pest control programme depends on a combination of its
effectiveness and its cost. In biological control both of these vary greatly with the
agent selected. It is easiest to predict the cost of an agent; so other things equal, the
least expensive agent is the best one. Indeed, van Lenteren (1980) suggested that there
are too many variables and unknowns to enable an effective agent to be selected on a
scientific basis. Scientific or not, there are rules of thumb for increasing the chances of
success: matching the climate of the collection and release area (Wapshere 1980),
collecting from the host plant species or strain on which the agent is to be released, and
Harris (1973) suggested a number of desirable agent characteristics. These approaches
can now be tested and modified against the record in Julien's (1982) world catalogue of
releases. The procedures that work in practice can then be followed, even if the
reasons for success are not understood.
Biological control of weeds, in common with government sponsored chemical weed
control programmes such as those against leafy spurge and knapweed in Canada, has
been derelict in not evaluating the benefits of the programme in economic terms.
Without economic cost:benefit data it is difficult to determine whether a particular
strategy against a weed makes economic sense or it is being pursued for political,
philosophical, or historical reasons. Several chapters in this section represent initial
evaluation of the prospective cost:benefits of biological control of possible target
weeds. Hopefully, this is a step towards substantiating the intuition that biological
control is economically superior to any other method of control for certain types of
weed problems.
The usual practice in biological control is to rate individual agents as failures, partial
or complete successes depending on their impact on the density of the host (see Laing
& Hamai 1976). This is neat, but a problem, because it is sometimes the combined
cropping pressure of several agents on a weed that achieves its density reduction; also
the effects on weed density are often delayed by the seed accumulated in the soil so
that the project cannot be rated until several years after the agent has achieved its
maximum effect. I suggest that it is more useful to rate biological control agents of
weeds in terms of the proportion of the annual production removed or destroyed. The
agents are similar in effect to cows in a pasture: if consumption is below a certain level,
future yield is not reduced. Thus the good farmer restrains utilization of grasses to
40-60% of annual production depending on the species and area. The good biological
control worker, on the other hand, must try to exceed the threshold for the target weed
(which might be determined by clipping tests). Most forbs are less tolerant than grasses
to utilization (Jameson 1963), but many shrubs will tolerate browsing up to 75% of
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