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Number of Phytophages and Weed Control
Current approaches to biological control of weeds 99
depressed by overgrazing or disturbance. Canadian examples are H. perfora/um,
Centaurea diffusa Lam., C. maculosa Lam. in the grasslands of the interior of British
Columbia; Carduus nlllans in the mid-grass prairie of Saskatchewan; Euphorbia esulavirgala
complex in Manitoba; and E. cyparissias L. on limestone soils in
Ontario, North American plants such as Solidago gigantea Ait, behave similarly in
parts of Europe. These plants do not form extensive monocultures in their native region
where they are cropped by phytophages. For example, E. cyparissias normally forms
only a small portion of the herbaceous community on limestone soils in Europe compared
to 25-50% at Braeside, Ontario. The introduction of phytophages against H.
perforalum in British Columbia and Ontario reduced it to a small percentage of its
former abundance at most sites, and C. nlllans has been reduced to less than 10% on
Saskatchewan rangeland. In part, the effectiveness of a control agent depends on the
level of competition from other plants (Harris 1981a) which is normally greatest in
uncultivated habitats and least in disturbed sites.
Specialized phytophages and their hosts normally have a mutual density dependence.
Thus cropping pressure is high when the plant is abundant and low when it is scarce and
vice versa for the mortality of the phytophage. The result is that neither becomes
exterminated. A Canadian biological control example is the cinnabar moth, Tyria
jacobaeae, on Senecio jacobaea L. in British Columbia in which the moth has
tracked the density of its host (Lakhani & Dempster 1981). Similarly Ralph (1977) found
that the highest densities of milkweed (Asclepias syriaca L.) supported the highest
densities of Oncopellus fasciatus (Dall.), and that below a certain density that plant
escaped attack. This has been called 'escape in space' by Feeney (1976).
To summarize, classical biological control of weeds is most appropriate in terms of its
impact and its effects against introduced weed species that dominate large areas of range
or other uncultivated land.
Cropping pressure on a weed can be increased by establishing several phytophage
species on it (Harris 1981a). Even two closely competing seed head flies on C. diffusa
reduced seed production more when together than either did alone (Myers & Harris
1980). Internationally, introductions have tended to continue over a period of years until
an average of four species has been established on the weed (Harris 1979). This process
of adding agents can only go so far as there is a species area-asymptote: the number of
phytophagous insects and pathogens cropping a plant species varies with the log of its
abundance (Lawton & Schroeder 1977, Strong & Levins 1979). That is to say the
number of phytophages that can be supported by a plant species doubles for every
tenfold increase in its abundance. In southern Britain five continental species of Heteroptera
have become established on pine following large plantings. The pines displaced
juniper and as a result one juniper-feeding Heteroptera, which formerly existed in
sizeable colonies, has become extinct (Southwood 1957). Simberloff (1978) achieved
similar results experimentally by altering the size of mangrove islands. In some plants an
expansion of range is matched by an increase in the species consuming it. The recruits
are derived from other plants in the new range or from the host in the native habitat. This
occurred within 50 years on cacao (Strong 1974). With other introduced plants such as E.
cyparissias in North America, there has been negligible recruitment of insect consumers
even after a hundred years. Presumably most native insects are unable to overcome the
physical and chemical defenses of the plant and the infestations are too remote for
specialized insects to reach them from Europe.