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100 P. Harris
The number of niches available to insect species is partly a function of the plant
architecture, so a species native to several parts of the world tends to support similar
numbers of leaf chewers, stem borers, and insects in other guilds throughout its range,
even though the insects may be in different taxonomic orders (Lawton 1978). The niches
can be regarded as sites at which the nutrient pool in the plant can be tapped. The pool
normally cannot be drained through anyone tap but they all lower its level. This implies
competition between agents in different niches. For example, Moran & Southwood
(1982) found that there was interaction between chewers and sap suckers on the tree
species studied.
The best targets for classical biological control are introduced plants on which the
consumer pressure is below that found in the native region. It should be possible to raise
consumption to at least the native region level although the number of phytophages that
can be established will be lower as their density will not be depressed by specialized
parasites. Probably each agent established on a weed increases the difficulty of establishing
additional species so those with the most potential should be introduced first.
It is unlikely that native weeds can be controlled by classical biological control unless
their abundance has increased. This has occurred with ragweed, Ambrosia artemis;;folia
L. (Harris & Piper 1970). If the phytophage-ragweed area asymptote is below that
of ragweeds that have not increased since European settlement, it should be possible to
establish additional agents from South America; however the increased consumer
pressure on the weed may not reduce allergenic hay-fever since the symptoms in man
are relatively dose independent. Usually the best hope for the biological control of a
native weed is the periodic application of an agent (probably a native pathogen) to keep it
at an artificially high level. This is called augmentative biological control and is discussed
under the heading 'The Future'. Hall et al. (1980) found no significant difference in
the success of biological control against native and exotic insect pests, but whether the
native pests had increased in adundance with modem agriculture was not considered.
Demonstration that Candidate Agents are not a Threat to Desirable Plants
The use of a plant species as a host by a phytophage depends on its suitability as a
nutrient source and as a place to live, the opportunity of reaching the plant, and the
survival advantage of using it (Harris 1981b). If one of these requirements is
unfavourable, the agent will not reach high levels on it. The strategy is to show that the
organism cannot develop on any plant outside a taxonomic group that does not contain
desirable species (Harris & Zwolfer 1968, Zwolfer & Harris 1971, Wapshere 1974).
Unfortunately in the laboratory, organisms develop on a broader range of plants than
they attack in nature. It is then necessary to show that the organism either does not
have the opportunity of establishing on the desirable plant in nature or that individuals
doing so are at a selective disadvantage. In effect there is a 3 tier system of testing:
desirable plants that fail the first test because they will support development, are
considered at the next tier of opportunity and if this is positive, at the last, and most
difficult, tier of advantage.
The opportunity of an insect to attack a plant within its distributional range normally
depends on the presence in the plant of sensory tokens for attraction and acceptance
by the ovipositing female. This is the result of many generations of selection to
optimize survival and is not easily changed. Indeed, it is often difficult to establish an
agent if the strain or species of the target weed is different from the one on which it was
collected. This is evident in the programme for the biological control of leafy spurge in
Canada. Similarly Goeden (1978) found that the race of R. conicus established on C.
nutans in North America did not attack Silybum marianum (L.) Gaertn. in the field,