Biology Reference
In-Depth Information
St. John's wort or Aaron's beard (
Hypericum calycinum
). Reproductive suc-
cess of
C. quadrigemina
was much greater on creeping St. John's wort than
on common St. John's wort, so in a short time a strain of this beetle
adapted to this host evolved (Harris 1988).
C. quadrigemina
has also been
observed to attack a native plant known as goldwire (
Hypericum concin-
num
). Two other biological control insects introduced for common St.
John's wort have also been found to attack goldwire. This case demon-
strates that biological control agents have the potential to shift to nontar-
get hosts and eventually to evolve forms specific to these hosts.
Biological Control with Introduced Enemies
Biological control involves a variety of strategies, including the augmen-
tation of existing natural enemies of pest species, the release of sterilized
insects to interfere with reproduction of field populations of the same
species, and the use of various microbial agents dispersed in the manner
of chemical pesticides. Traditional biological control, however, involves
the introduction of predators, parasites, or pathogens that are hoped to
reduce the population of a pest to a level at which it does not cause eco-
nomic damage. Classic biological control uses one or more species that
were associated with an alien pest in its native region—the hope being
that it will reduce the pest as much as, or more than, it did in its native
situation. A potential biocontrol agent, introduced without its own pred-
ators, parasites, or diseases, might be expected to be more effective than it
was in its native area.
Many biocontrol efforts, however, involve new associations between
potential biocontrol species and pests (Hokkanen and Pimentel 1989).
Myxomatosis, the disease introduced to Australia as a biocontrol agent for
European rabbits (
Oryctolagus cuniculus
), for example, is a disease of New
World cottontail rabbits (
Sylvilagus
spp.). Cottontails are only mildly
affected by the disease, whereas it was initially highly lethal to European
rabbits. Some biocontrol scientists believe that new associations like this
offer the greatest potential for effective control of pests.
In either case, introducing species for biocontrol purposes carries sev-
eral risks. In some cases, biocontrol species can use nontarget species
closely related to the target species. In other cases, variation in host pref-
erence may exist in the introduction stock, particularly if the introduced
organisms come from different geographical areas where host differences
exist. In still other cases, an evolutionary change may enable an intro-
duced biocontrol agent to use nontarget hosts. In such cases, biocontrol
