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Pros and Cons of Biological Control 407
these cases constitute the great majority. By contrast, the evidence for population
reduction or extinction is fairly weak in many cases. Those cases where
more serious effects were observed have virtually always happened on
islands. For instance, the introduction of predatory land snails (especially
Euglandina rosea) for control of the alien giant African snail into Hawaiian
Islands in the 1950s – and later even into other countries – had disastrous consequences
for the non-target mollusc fauna (Howarth 1991). There is good
evidence that the extirpation of endemic tree snails is caused by this introduced
predator. In addition to this, more cases of non-target effects were documented
and reviewed by Howarth (1991) and Hoddle (2004).A common feature
of these cases is that polyphagous predators are largely responsible for
the observed effects, as demonstrated in several projects conducted early in
the 20th century. Some of the most serious effects originated from the introduction
of vertebrate predators (e.g. mongoose against rats) – conducted not
by biological control experts but rather by other stakeholders. Nevertheless, a
critical question still is whether these known negative reports represent only
the tip of an iceberg – many non-target effects may simply have escaped our
attention (Howarth 1991; Simberloff and Stiling 1996).
In an attempt to address this concern, several post-release studies were
conducted recently with a focus on those biological control projects where
population declines of non-target species have been observed or on projects
where the potential for non-target effects have been judged high due to the
polyphagous nature of the biological control agent. Obviously, such a selection
is strongly biased and, thus, can not be representative. In one of these
cases, Barron et al. (2003) evaluated parasitism by the introduced biological
control agent Pteromalus puparum on the New Zealand red admiral butterfly
Bassaris gonerilla, in comparison to other mortality factors. From an extensive
dataset, Barron et al. (2003) constructed a partial life table and concluded
that the level of mortality caused by P. puparum is low relative to egg parasitism
by Telenomus sp., and also low in comparison to larval disappearance
and pupal parasitism caused by the accidentally introduced ichneumonid
Echthromorpha intricatoria.
Similarly, Benson et al. (2003) tested whether the introduced parasitoids
Cotesia glomerata and C. rubecula may have been responsible for the decline
of the native butterfly Pieris virginiensis in New York and Ontario.Although P.
virginiensis is an acceptable and suitable host, Benson et al. (2003) concluded
that populations of this butterfly do not appear to be at risk because both C.
glomerata and C. rubecula do not forage in forested habitats, even when they
are locally present in adjacent meadows.
Extensive studies have been carried out on potential risks of the polyphagous
egg parasitoid Trichogramma brassicae, which is being massreleased
against the European corn borer in many countries.Although T. brassicae
did parasitize various butterfly species (including rare ones) under field
cage conditions (Babendreier et al. 2003a), subsequent studies have demon-