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2.3.2 Beneficials or Biological Control Agents
W. Nentwig
2.3.2.1 Vertebrates
Using vertebrates as biological control agents is associated mainly with the
dark and pre-scientific period of biological control. All the foxes and
weasels, dogs, cats and toads which were released to control pest species
soon became pests themselves because they predated on everything, except
the target pest. One of many typical examples concerns the ermine Mustela
erminea, now also ranked among the 100 world’s worst invaders (ISSG 2006),
native to the Holarctic region north of the 40th parallel. It was introduced
into New Zealand and into some small European islands because it was
believed that it could control rabbits. As a predator specialised in small
mammals and birds, in New Zealand M. erminea preys upon a variety of
native species, particularly kiwi chicks and hole-nesting forest birds.
Ermines spread easily over long distances on land and also reach small offshore
islands unaided.
Vertebrates have also been used as herbivores. The nutria Myocastor
copyus has been introduced into Texas as a “cure-all” for ponds with dense
vegetation. It reduces many kinds of aquatic plants but, when nutrias get
established, overpopulation soon results and the animals move into places
where they destroy vegetation which is particularly valuable, e.g. for waterfowl.
In 1963, the grass carp or white amure Ctenopharyngodon idella was
imported into the USA from Malaysia as a biological control of aquatic
macrophytes. It was introduced into selected ponds and rivers but escaped.
Since grass carps are capable of moving well beyond areas intended for plant
control within a single season, within a few decades they had spread
throughout the USA (Guillory and Gasaway 1978). Grass carps were also
widely introduced into Europe and Central Asia. C. idella consumes all types
of aquatic plants, including reeds, reed sweet-grass, reed-mace, sedges, bulrushes
and horsetail, thus destroying essentially all aquatic vegetation. This
has also indirectly reduced native invertebrate and fish populations to
extinction (Maeceina et al. 1992; Bain 1993; Crivelli 1995). Shipments of
grass carps were not always controlled, so that they also contained foreign
species, such as the stone moroko (Pseudorasbora parva), now widely spread
in Europe. In addition, grass carps are hosts of the Asian tapeworm Bothriocephalus
opsarichthydis which became established in North America,
Europe, Asia and New Zealand. It now parasites indigenous fish, has the
potential to reduce local biodiversity, and causes considerable economic
harm (Köting 1974).