Biology Reference
In-Depth Information
species may cause economic damage, reduce populations of nontarget
hosts, or even lead to the extinction of nontarget species.
An additional little-recognized possibility is that the introduced bio-
control agent may carry other organisms that may have unexpected
impacts of their own (Slippers et al. 2001). For example, a wood wasp
(
Sirex noctilio
) associated with northern hemisphere pines was accidentally
introduced to New Zealand and Australia and has subsequently spread to
South America and South Africa. This wood wasp has a symbiotic rela-
tionship with white rot fungus (
Amylostereum areolatum
), a destructive
agent in plantations of exotic pines. A nematode (
Deladenus siricidicola
)
that sterilizes the eggs of the wasp was introduced to Australia as a bio-
logical control agent for the wasp. Subsequently, this nematode was intro-
duced to South Africa and South America from Australia.The introduced
nematode, which also feeds on the fungus, carried a distinct strain of the
fungus, which has now become established in the introduction areas
(Slippers et al. 2002).
An initially effective biocontrol species may select for resistance on the
part of the host, just as the use of chemical pesticides has selected for
resistance by many pest species. Biological control programs have tended
to ignore this possibility, largely because few instances of this have yet
been noted. Holt and Hochberg (1997), however, have suggested that
apparent stability of biological control relationships is real, especially for
biocontrol predators and parasitoids.The low frequency of evolved resist-
ance to such agents may be due to lack of genetic variability by the host,
factors that offset the influence of selection by the control agent, or the
coevolutionary dynamics that tend to balance agent effectiveness and host
resistance.
Ecological Host Shifts by Biological Control Agents
Species introduced for biological control may expand or shift their feed-
ing activity to nontarget hosts (Louda et al. 2003b). Although such shifts
may occur without genetic changes in the biocontrol species, in time,
selection may lead to adaptation to a widened range of hosts or to spe-
cialization for nontarget species. Serious impacts on nontarget species may
occur if high populations of the biocontrol agent shift from the target
host to other hosts, even species of low preference (Lynch et al. 2002). In
extreme cases, this may lead to extinction of nontarget species (see chap-
ter 16).
Many such host shifts have been documented. In some cases, the host
