Managing weeds with insects and pathogens - Ecological Management of Agricultural Weeds - page 388

and grain fields (Hasan & Ayres, 1990). Initially, introduction of P. chondrillina
resulted in near elimination of the target weed at many locations. In subse-
quent years, however, a resurgence of the weed has taken place. Investigation
of the situation revealed that three biotypes of C. juncea (narrow-leaf, interme-
diate-leaf,and broad-leaf) exist,each with different susceptibilities to the par-
ticular strain of fungus (IT 32) originally introduced from Italy to control the
weed.The intermediate-leaf and broad-leaf biotypes are resistant to IT 32 and
have replaced the formerly dominant, susceptible, narrow-leaf biotype
(Chaboudez & Sheppard, 1995). Consequently efforts have been initiated to
collect additional strains of the fungus in the Mediterranean basin capable of
infecting the resurgent C. juncea biotypes in Australia (Hasan & Ayres, 1990).
As this example points out, adequate attention to genetic variation within
populations of weeds and the organisms that attack them is critical to the
success of biological control efforts.
Use of inoculative control agents against native weed species
In addition to collecting and releasing exotic herbivores and patho-
gens to control introduced weed species,exotic natural enemies can be consid-
ered for release to control native weed species. In essence, this approach
represents exposing a target weed species to herbivores and pathogens it has
not yet encountered in its evolutionary history. In addition, it represents
introducing herbivore and pathogen species where few indigenous natural
enemies may be adapted to attack their populations (Hokkanen, 1986). The
exploitation of evolutionarily new relationships for weed biocontrol remains
largely untested, but it is useful to examine a case where the approach might
be applied.
Lawton (1988) suggested that exotic herbivores might be introduced for
control of Pteridium aquilinum , a fern native to Great Britain, but also to every
continent except Antarctica.The fern is acutely poisonous and carcinogenic to
livestock, serves as a reservoir for ticks that vector viral diseases of sheep, and
aggressively invades pasture land. Costs of control, lost grazing, and stock
poisoning in Great Britain are estimated at several million pounds per year
(Lawton, 1988). While P. aquilinum can be controlled by repeated cutting or
herbicide applications, these measures often are not cost-effective and relaxa-
tion of control may result in rapid resurgence of the problem.
In Britain, 27 native insects regularly attack the above-ground portions of
P. aquilinum but fail to adequately control the plant because they are attacked
by parasites, predators, and pathogens (Lawton, 1988). In an attempt to
subject the weed to greater herbivore pressure, 12 insect and one mite species
that feed on P. aquilinum were located in temperate regions of South Africa.