Biocontrol of invasive species sometimes can be achieved through the introduction of a parasite from the region of the invasive species' origin (Table 2), but the danger exists of introducing yet another invasive species, which could create harm to native species. One biocontrol success story is in the control of Opuntia vulgaris in India, a species which was introduced from Brazil. A scale insect (Dactylopius ceylonicus) was released in 1795, and the insect completely controlled the cactus in India. However, in a similar attempt to control Opuntia on the island of Nevia in the Lesser Antilles, disaster occurred with wide-reaching and unintended consequences. In 1957, the cactus moth (Cactoblastis cactorum) was released to control Opuntia on Nevis, but the moth escaped to destroy populations of the rare O. spinosissima in the Torchwood Hammock Preserve in the Florida Keys. Having now reached the Gulf Coast and spread by hurricanes, the cactus moth is spreading across the region, and could eventually threaten populations of cactus in the southwestern United States.
Insects pests of L. salicaria have been introduced from Eurasia to control this invasive in North America. However, these insect species may attack related species of Lythrum native to North America. Furthermore, some studies suggest that the insects do not eliminate L. salicaria but only reduce the biomass of the species as compared experimentally inside and outside of insect exclosures. Other studies suggest that L. salicaria does not necessarily pose as severe a risk to wetland communities, as the release of the insect species. L. salicaria does not reduce the species richness of other species, although the effect on the function of the wetland may be impacted.
The introduction of parasites to control invasive species is not always very successful from the perspective of the survival of the introduced parasite. Sometimes the habitat requirements of the parasite are not met, so that these parasites become extinct shortly after introduction. Biocontrol can fail because the parasite cannot be maintained in the invasive environment.
Many argue that biocontrol is worth the risk that the parasite might damage the environment because the cost of doing nothing may be higher than any potential cost incurred or harm caused by the biocontrol agent. While the benefits of releasing biocontrol agents to the ecosystem may be profound, the potential effects of harmful biocontrol agents are impossible to project. For example, who could have thought that a cactus moth released in the Lesser Antilles would be capable of making its own way across the Gulf Coast of North America? Others use the same reasoning regarding the lack of ability to project the effects of biocontrol agents, to argue that the usage of biocontrol agents is never warranted.
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