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Current approaches to biological control of weeds 101
although a strain collected from this plant established readily. This programming of an
insect for a particular plant tends to break down in the laboratory and so is best field
tested in the native region.
In contrast to the active dispersal of most insects, the passive dispersal of pathogens
and to a slightly lesser extent certain insects like aphids, results in their deposition on
many plant species which they will frequently attempt to attack. The host range of
pathogens is normally genetically stable so most of the pathogen propagules landing on
other plant species are lost. However, if the genetic capacity (possibly as a result of
hybridization) to attack another plant species exists in a pathogen popUlation, the
occasionally successful individual can, by asexual reproduction, increase rapidly. It is
not a good omen for biological control if dense stands of a desirable plant on which a
pathogen can develop in the laboratory, grow near the target weed.
The advantage of a particular host species to a phytophage depends on its potential for
increase on it. The innate reproductive capacity of the phytophage on various plants can
be measured in the laboratory, but the rate of increase in the field is influenced by
mortality, competition from other consumers, the density of the host, and other factors
that cannot be measured in the laboratory. Thus a good laboratory host is not necessarily at
risk in nature, and a relatively poor laboratory host might be regularly damaged if there
are large amounts growing as a monoculture. This is hard to determine experimentally in
the new habitat without danger that the organism will escape. For this reason the
development of the organism on an introduced crop plant in the laboratory is justification
for rejecting it as a biological control agent; but development on a native wild plant is less
cause for concern. For example, the European moth Plryllonorycter blancardella
(Fabr.) attacks Cratueglls spp., apple, and several related plants in its native range but,
where it has been introduced into Quebec, Pottinger & LeRoux (1971) were unable to
find it on Cratueglls spp., an abundant native, although it was common on the introduced
apple. Similarly Moran (1982, personal communication) found that insect pests on South
African crops derived from native plants were, with rare exceptions, attacked by native
species. On the other hand, introduced crops were attacked by both native and introduced
insects.
There are examples in which introduced insects and pathogens have become pests of
native plants. These phytophages are either polyphagous, like the gypsy moth, so they
readily accept and develop on a wide range of plants; or the native lacks resistance, as in
the case of the chestnut blight, so the organism has a higher reproductive capacity on it
than on the original host. The best indication of the genetic capability of phytophages to
utilize plants in the new habitat is their host range in the native region, and if necessary
desirable plants of the new habitat can be tested there by planting them at their normal
density. Thus a species such as Lema cyan ella L., which in Europe is only found on
Cirs;llm arvense (L.) Scop. (Peschken & Johnson 1979) is unlikely to attack native
North American thistles to any extent even though it will accept them in the laboratory.
Selection of Target Weeds for Classical Biological Control
Classical biological control cannot escape some measure of politics because it depends
on public funding. Also, as an agent will spread over many properties, it has to be
introduced as a matter of public interest.
Originally the priorities for biological control in Canada were largely determined by
popular demand: the squeaky wheel approach. This did not necessarily target the best or
the most urgent weeds for biological control as is evident from some of the examples in
this volume.