Pest management - Ecological Applications: Toward a Sustainable World - page 160

short-term control by inundation. Nematodes are available commercially and can
be applied to crops such as mushrooms to control fungus gnats ( Sciara spp.).

6.3.5 When
biological control
goes wrong

Successful importation biological control can provide a permanent solution because,
unlike pesticides, the agent reproduces and maintains itself into the future. But this
also has a downside - if the control agent turns out to pose an environmental threat,
it may last forever.
As with pesticides, unwanted outcomes of biological control occur and some are
spectacular in their extent. Take for example the introduction in the 1970s and
1980s to the Society Islands (French Polynesia) from Florida of the predatory snail
Euglandina rosea as a biological control agent for a previously introduced crop pest,
the giant African snail Achatina fulica . These Polynesian islands were once famous
for the dramatic diversity of their tree snails (family Partulidae). But the failed bio-
logical control agent Euglandina rosea has instead driven all but fi ve of the Society
Islands' 61 species of tree snails extinct in the wild (Coote & Loève, 2003). (Fifteen
species still exist in captive collections in Europe and North America.) The loss of
Partula varia and Partula rosea on the island of Huahine had signifi cant economic
and social costs too. These species were used for making shell jewellery, and with
their demise many women lost their livelihoods and the artisan's association was
closed down.
Euglandina rosea should never have been contemplated as a control agent, because
of its generalist diet and the precious biodiversity that it threatened. However,
examples have also come to light where even carefully chosen and apparently suc-
cessful introductions of biological control agents have impacted on nontarget species.
Thus biocontrol by the myxoma virus of European rabbits in the UK unexpectedly
resulted in extinction of the large blue butterfl y Maculina arion . This happened
because the butterfl y uses nests of the ant Myrmica sabuleti to rear caterpillars. The
ants, in turn, depended on rabbit grazing to maintain the open habitat where they
nest. Another example concerns a seed-feeding weevil ( Rhinocyllus conicus ). Intro-
duced to North America to control exotic Carduus thistles, the weevil also attacks
more than 30% of native thistles (of which there are more than 90 species), reducing
thistle densities (by 90% in the case of the Platte thistle Cirsium canescens ), w ith
consequent adverse impacts on the native picture-winged fl y ( Paracantha culta )
which feeds on thistle seeds (Louda et al., 2003a).
Sometimes such indirect food-web effects may even compromise human health.
In the early 1970s two species of gallfl y ( Urophora spp.) were introduced to control
knapweeds ( Centaurea spp.) in western North America. The gallfl ies established and
remained host-specifi c, but they did not control the target plants and became super-
abundant. Now a rich food source in areas invaded by knapweed, the gallfl ies make
up 85% of the diet of the deer mouse ( Peromyscus maniculatus ), whose population
has increased two- or three-fold. So how might this affect human health? Deer mice
are also the primary vector of the deadly Sin Nombre hantavirus (see Pearson &
Callaway, 2003). More mundanely, the increased mouse populations might, through
seed and insect predation, alter biodiversity, compete with other small mammals
and, as prey, affect the populations of larger predators (Figure 6.11).
Louda et al. (2003b) reviewed ten biological control projects that included the
unusual but worthwhile step of monitoring nontarget effects and concluded that
relatives of the target species were most likely to be attacked, with rare native species