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in seed production by R. conicus may reduce densities of these plants to the
point their persistence is threatened.
McFayden (1998) reported six other cases in which release of weed biocon-
trol agents has resulted in damage to non-target plants. Included within her
list is the release of the moth Cactoblastis cactorum in the West Indies . When C.
cactorum was released in the 1950s to control the native cactus Opuntia triacan -
tha , few problems were anticipated and few objections were raised. Since that
time, however, the moth has spread naturally and through deliberate intro-
ductions throughout the Caribbean basin. In 1989, C. cactorum was found in
the Florida Keys, where it now threatens the survival of the native cactus
Opuntia spinosissima , which was already endangered by clearing and develop-
ment. McFayden (1998) noted that “ C. cactorum is likely to continue its spread
westward into Mexico and the cactus country of the southwest USA, where its
impact may be severe unless it is reduced by the effects of parasitism or by
competition with similar native moths in the genus Melitara .”
The lesson to be learned from these examples is that very thorough investi-
gations of effects on potential alternative hosts are needed before exotic herbi-
vores and pathogens are released to control weeds (Strong, 1997). Weeds
clearly can wreak havoc with existing communities unless controlled effec-
tively, but the biological agents released to control them may also severely
affect the native biota (Simberloff & Stiling, 1996). Information concerning
the full range of potential economic and ecological risks, costs, and benefits is
needed before introductions of weed biocontrol agents can be assessed ade-
quately (Harris et al ., 1985).
Inundative releases of control agents
Unlike inoculative biological weed control, in which herbivores and
pathogen populations are expected to increase naturally and spread consider-
able distances from points of introduction, inundative biological control
methods rely on human intervention to increase and disperse host-specific
control agents onto target weed species. While inoculative methods are
expected to act slowly, often over a period of years, inundative methods are
expected to severely damage and kill nearly all susceptible weeds quickly,
often over a period of a few days.The inundative approach is particularly well
suited to annual cropping systems, in which weeds must be strongly sup-
pressed early in the growing season to avoid crop yield loss due to competi-
tion.
So far, inundative methods for biological weed control have relied almost
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