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Pros and Cons of Biological Control 405
23.3 Cons of Biological Control
As mentioned above, biological control can work forever. Nevertheless, a quite
common problem is that, in very many cases, biological control simply fails to
control the pest sufficiently. In classical biological control of insects, for
instance, only about one third of the introduced agents have been able to
establish, from which again about one third is able to suppress pest populations.
Though success rates are somewhat higher for weed biological control,
this means that numerous exotic natural enemies have been added to the
native fauna without noticeable benefits.
Despite the fact that both classical biological control and augmentative
biological control were regarded safe for much of their history, concerns
about detrimental effects of introduced exotic species on the native fauna
have been increasingly expressed over the last two decades. This development
led to the release of several papers which reviewed the impact of biological
control agents on non-target species and, more importantly, reported that
many effects may have passed unnoticed because no study had been conducted
(Howarth 1991; Simberloff and Stiling 1996; Samways 1997; Lynch et
al. 2001).
Potential risks concerning the introduction of exotic biological control
agents include those to human health, to the economics and to the environment.
No serious health risks are known for any macro-organisms, though
some cases of allergy in the mass-production of predatory mites or nematodes
may occur. Economic issues are dealt with in Chap.18 and I thus would
like to focus on environmental risks and non-target effects here.Although the
term non-target effect is not very well defined, it clearly encompasses a large
spectrum varying from very small effects, e.g. 2 % parasitization of a parasitoid
biological control agent on a non-target insect, to massive effects at the
population or even ecosystem level. Until now, there is no general agreement
on how to judge the magnitude of non-target effects, and whether these
effects can be tolerated or are unacceptable. Clearly, the most serious negative
aspect of biological control would be the displacement of non-target species
on a large, geographical scale or even globally, and the change of complete
ecosystems.
23.3.1 Weed Biological Control
In weed biological control, the most serious concern is that an exotic introduced
herbivore would be able to feed on crop plants, thereby becoming a pest
itself. As a consequence, the assessment of the candidate’s host range became
routine already several decades ago, and only herbivores with a narrow host
range are considered for release. Compared to agents released against arthro-