Managing weeds with insects and pathogens - Ecological Management of Agricultural Weeds - page 389

Two moth species appeared to be especially good candidates for control
agents: Conservula cinisigna ,which chews fronds of P.aquilinum ,and an uniden-
tified Panotima species, whose larvae mine fronds and then attack the rachis of
the plant.Feeding experiments showed both species were virtually monopha-
gous on the target weed and fed on it early in the season, when the plant is
especially vulnerable to attack. Because the two potential control agents are
ecologically and taxonomically distinct from resident herbivores of P. aquili-
num in Great Britain, Lawton (1988) proposed that they might escape the
effects of predation,parasitism,and disease from indigenous natural enemies.
Nonetheless, despite the many characteristics of the two insects which sug-
gested they might be successful in suppressing P. aquilinum , Lawton (1988)
noted that their effectiveness could be demonstrated definitively only after
field releases took place.
Choosing target weed species and protecting non-target species
At first thought, field release of an organism that may control a
noxious weed seems a worthwhile endeavor that should be pursued as soon as
the suitable agent is identified. Caution, consultation, and serious planning
are advised, however. Organisms used for inoculative biocontrol releases are
chosen because of their ability to reproduce and disperse, and once released
they cannot be recalled. Thus, there is a distinct possibility that susceptible
plant species will be suppressed outside of the areas originally designated for
treatment with biocontrol agents. A very real possibility also exists that,
through evolution, the range of host plants attacked by introduced control
agents may shift or expand to include non-target species (Simberloff &
Stiling, 1996).
In the case of Pteridium aquilinum in Great Britain, the target plant consti-
tutes a major portion of the native vegetation over a large area, and the eco-
logical impact of removing it needs to be considered very carefully before the
release of exotic control agents takes place.The plant clearly has undesirable
characteristics in pasture land, but what benefits would be lost due to its
removal from pastures and other habitats? Are these more or less valuable
than the benefits derived from the plant's suppression? Lawton (1988) noted
that P. aquilinum may provide desirable habitat for native animal popula-
tions, and he proposed construction of an “environmental balance sheet”
that would set out the pros and cons of P. aquilinum before exotic herbivores
were released to control it. Such an assessment process has apparently
worked in favor of the weed. MacFayden (1998) reported that after extensive
agent testing,biocontrol efforts directed at P.aquilinum in Great Britain were
ultimately abandoned because of regulatory requirements for costly field