Benefits of Classical Biological Control for Managing Invasive Plants

As a result of the rapid expansion in international travel and trade over the past few decades, invasive plants have become a problem of global proportions. Plant invasions threaten the existence of endangered species and the integrity of ecosystems, and their ravages cost national economies tens of billions of dollars every year. Strategies for managing the threats posed by plant invasions involve three main tactics: prevention, eradication, and control. The effectiveness of prevention, involving enactment of legislation to prohibit the entry and spread of noxious alien plants, has been questioned. Eradication of all but the smallest, most localized weed infestations generally is not regarded as economically feasible. Conventional weed control techniques, such as mechanical and chemical controls, because they are expensive, energy and labor intensive, and require repeated application, are impractical for managing widespread plant invasions in ecologically fragile conservation areas or low-value habitat, such as rangelands and many aquatic systems. In addition, mechanical means of control disturb the soil and may cause erosion; chemical herbicides have spurred the evolution of resistance in scores of weed species and, further, may pose risks to wildlife and human health. Because of drawbacks associated with conventional weed control methods, classical biological control, the introduction of selective exotic natural enemies to control exotic pests, increasingly is being considered and implemented as a safe, cost-effective alternative to address the invasive plant problem. Worldwide, biological weed control programs have had an overall success rate of 33 percent; success rates have been considerably higher for programs in individual countries. Benefits are several-fold. Biological control is permanent, energy-efficient, nonpolluting, and inexpensive relative to other methods. Economic returns on investment in biological weed control have been spectacular in some cases, and range from an estimated benefit/cost ratio of 2.3 to 4000 or more. Although the risks involved in biological control in general are considered unacceptable by some, biological weed control in particular has had an enviable safety record. Since establishment of the stringent standards and regulatory apparatus currently in place in the United States and elsewhere, there have been no reported cases of biological weed control causing significant harm to nontarget populations or to the environment at large.

Keywords:

• biological invasions
• benefit-cost analysis
• natural enemies
• nontarget effects
• nonindigenous species
• weeds

ACKNOWLEDGMENTS

I thank P. Conant (Hawaii Department of Agriculture), E. M. Coombs (Oregon Department of Agriculture), D. Pimentel (Cornell University), G. P. Markin (USDA Forest Service), B. Krissoff (USDA-ERS), L. Garrett (USDA-APHIS, CPHST), and at PERAL, A. Koop, B. Nietschke, K. Colpetzer, and B. Caton for their critical reviews of an earlier draft of the paper.

Notes

¹Subspecies rotundata (DC.) Norl.

^a Benefit/cost ratio calculated by Hill and Greathead (2000) from data in the original work, assuming benefits discounted at 10 percent for 30 years.

^b Benefits discounted at 7 percent for 40 years.