Problems with biological control
by Bob Hartzler

August 1997 - Biological control has long been looked at as a safe, economical alternative to herbicides for managing weeds. Two articles in the 22 August 1997 issue of Science (Strong, D. R. 1997. Fear no weevil. Sci. 277:1058-1059; Louda, S.M., D. Kendall, J. Connor, and D. Simberloff. 1997. Ecological effects of an insect introduced for the biological control of weeds. Sci. 277: 1088-1090.) focused on one of the limitations of biological control - host specificity.

Classic biological control of weeds is based on the principal that many weed species are introduced plants that become pests due to the lack of natural enemies (insects, diseases, etc.). Scientists go to the origin of the plant and search for organisms that attack the plant and keep populations in check in its native region. Advantages of this approach are that effective biocontrol agents can be very economical, long lasting, and eliminate or reduce the need for pesticides that might have negative environmental impacts.

The articles describe problems associated with a biological control agent that has moved from its target plant to attack native plant species. Musk thistle weevil (Rhinocyllus conicus Froeh.) has been introduced across the United States to control musk thistle (Carduus nutans), a serious weed of pastures, rangeland and other non-cultivated areas. The adult weevil lays eggs on young flowers and then the larvae feed on developing seeds, therefore greatly reducing seed production. The reduction in seed production results in a slow decline in thistle populations, and has been successful in many areas at reducing musk thistle infestations.

The musk thistle weevil has been found to expand its host range to five native thistle species in the U.S. National Parks and Nature Conservancy preserves. The weevil was first found on Sandhill Prairie thistle is 1992, two decades after its initial release in the region. The expansion of the host range is not surprising since early testing found that the weevil could survive on many Carduus species.

The authors conclude that greater caution to environmental costs must be considered when evaluating biological control agents. "The breadth of diet, potential host range, and ecological effects need to be investigated and then carefully weighed against the environmental costs of the pest and of alternative management options. Intensified follow-up monitoring of species that have already been released is a key step in assessing environmental costs and improving the predictability of biological control."

The article illustrates that there are no free lunches when it comes to weed control.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University