Managing weeds with insects and pathogens - Ecological Management of Agricultural Weeds - page 390

cage experiments and doubts over the wisdom of controlling a native
species.
The difficulty and necessity of resolving social issues related to weed bio-
control are further illustrated by the controversy associated with release of
several insects for control of Echium plantagineum in Australia (Tisdell, Auld &
Menz, 1984) . Depending on one's perspective, E. plantagineum may be called
either “Paterson's curse”or “Salvation Jane.”For Australian grain farmers and
orchardists,the plant is a noxious weed.In contrast,for livestock producers,it
is a useful (though cumulatively toxic) source of sheep fodder during drought
periods.For honey producers,it is an important nectar source for bees.Failure
to reconcile these opposing perspectives led farmers and others who derive
benefits from E. plantagineum to obtain a court injunction and restraining
order against the Commonwealth Scientific and Industrial Research
Organization, the Australian government agency responsible for biocontrol
releases . This type of conflict is expensive, time-consuming, and should be
avoided if weed biocontrol programs are to gain strong public support. Open
discussions of potential losses and gains are necessary to insure that the goals
of weed biocontrol efforts are shared by all parties holding an interest in the
targeted species.
The potential for weed biocontrol agents to have unintended impacts on
non-target species is illustrated by the case of Rhinocyllus conicus , a weevil col-
lected from France and Italy and released in the USA and Canada, starting in
1968, to control exotic thistles (Louda et al ., 1997). Originally targeted were
weedy Eurasian species in the genus Carduus. Although feeding preference
trials conducted before release of R. conicus indicated that its range of hosts
included the native North American genera Circium , Silybum , and Onopordum ,
the weevil's stronger oviposition preference and more successful larval devel-
opment on Carduus spp.were supposed to limit its use of native plants.
The real outcome of releasing R. conicus differed markedly from this expec-
tation, however. The weevil fed on both native and exotic thistle species.
Measurements made in national parks and conservation land in Nebraska,
South Dakota, and Colorado indicated that seed production by five native
thistle species dropped precipitously because of attack by the weevil (Louda et
al .,1997).For the native,sparsely distributed species Circium canescens ,the per-
centage of flowerheads per plant infested by R.conicus rose at one location from
none in 1992 to 58% in 1996.The average number of viable seeds produced by
C. canescens flowerheads infested by weevils was only 14% of that produced by
similar heads without insects or with exposure only to native insects. Because
regeneration of native thistle populations is seed-limited, sizable reductions