Biology Reference
In-Depth Information
investigated for nearly 30 yr as a possible biological control for the Euro-
pean rabbit (Richardson 2001), a major alien pest in Australia. In 1950,
before final authorization for release, the virus escaped from trials in field
pen populations in New South Wales, infecting wild populations. Spread-
ing from this area, and from deliberate releases in other locations, the
virus became introduced to rabbit populations throughout Australia.
The virus strain was highly virulent, typically killing more than 99% of
rabbits in pen trials and more than 90% in its initial spread through field
populations.
Ve ry soon after the first epizootics of myxomatosis, less virulent strains
of the myxoma virus appeared independently in many locations
(Richardson 2001). Reduction in virulence was progressive and was doc-
umented by laboratory tests of rabbits from populations never exposed to
the virus. By 1956-57, viral strains that killed less than 50% of rabbits
were common in the field.The initial, highly virulent strain was displaced
almost completely by strains of lesser virulence.
Interestingly, a second viral disease, rabbit hemorrhagic disease (RHD)
was introduced in 1995 to mainland Australian populations of rabbits
(Hayes and Richardson 2001). In a fashion very similar to the escape of
the myxoma virus, the RHD virus escaped from a field test on an island
off the eastern coast of Australia. It has now spread to rabbit populations
throughout Australia.
Evolutionary Counteradaptation of Hosts to
Biological Control Agents
Target plants of biological control have shown indications of evolution of
resistance to control agents. Substantial variability in target plant suscep-
tibility to introduced biocontrol herbivores has been observed with sev-
eral species. In North America, a number of insects have been introduced
for control of leafy spurge (
Euphorbia esula
), a major rangeland weed. Six
species of root-feeding weevils (
Aphthona
spp.) have been introduced to
various parts of the northern Great Plains (Lym and Carlson 2002). Leafy
spurge populations in North America, however, consist of diverse genetic
strains introduced from different parts of Eurasia.These strains show major
differences in susceptibility to
Aphthona
weevils. As a genetically variable,
outcrossing species, leafy spurge has a high potential for evolutionary
response to biocontrol species.
In North America, common St. John's wort (
Hypericum perforatum
) was
the object of one of the earliest classical biological control programs.This
