Biology Reference
In-Depth Information
clover. Lack of genetic variability may thus constrain the evolution of
resistance in the clover aphids.
Other cases involve microorganisms, including the example of myxo-
matosis and rabbits in Australia. Paralleling the evolution of reduced vir-
ulence in the myxoma virus in Australia, described earlier, European rab-
bits rapidly evolved increased resistance following the spread of the virus
through field populations (Richardson 2001). By 7 yr after the initial
infection of field populations, more than 10% of rabbits from field popu-
lations survived infection by the original highly virulent strain that killed
more than 99% of laboratory animals that had not previously been
exposed to the virus. By the late 1970s, the survival rate of animals from
field populations had risen to about 40%.
In the example involving the
Pasturella
bacterium and nematodes dis-
cussed earlier, another consequence of the interaction of these organisms
was that infections by
Pasturella
tended to select for resistant forms of
Meloidogyne
(Channer and Gowen 1992).
Precautions for Introductions of Biocontrol Species
Predicting the ultimate consequences of introducing biocontrol species to
new regions is much more difficult than has been believed (Louda et al.
2003a). Although awareness of many potential problems with introduc-
tion of species for biological control has improved, many releases of
species have inadequate justification.A recent example is the introduction
of parasitoids for control of the brown citrus aphid (
To xoptera citricida
) in
Florida (Michaud 2002). In this case, no evidence exists that the two
introduced parasitoid species exert control over the brown citrus aphid in
other regions of the world.
The first goal of research must be to show that the introduced biolog-
ical control agent will not itself cause damage. This is especially true for
agents that attack plants, since many problem weeds have nonweedy, or
even highly beneficial, relatives. In many cases, impacts on such relatives
do not become evident until 5-10 yr after a biocontrol agent has been
introduced (McEvoy 1996). Improved testing of host specificity is thus
essential. A fly under consideration for use as a biological control of yel-
low starthistle (
Centaurea solsticialis
), for example, was found to have
become established, presumably accidentally, and to attack one variety of
safflower (
Carthamus tinctorius
), a major crop in California (Balciunas and
Villegas 2001).Although it appeared that the risk of serious side effects on
