Submitted to: Biological Control Theory and Application in Pest Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 14, 2006
Publication Date: December 1, 2006
Citation: Balciunas, J.K., Smith, L. 2006. Pre-release assessment, in quarantine, of a tephritid gall fly being considered as a biological control agent for Cape ivy (Delairea odorata). Biological Control Theory and Application in Pest Management. 39(3):516-524. Interpretive Summary: Invasive weeds degrade natural areas, cause billions dollars worth of losses in agriculture, and attempts to control them account for more than half the pesticides used in the United States. Classical biological control, the release of carefully selected and tested insects and other natural enemies from the native home of the weed, is a proven strategy for reducing the impacts of invasive weeds, and reducing the use of herbicides. To avoid direct impacts on crops and beneficial native plants, prior to release, the intended agents are screened to assure that they are host-specific, and will not damage non-targets. However, even agents that are restricted to their target, can cause indirect impacts to other organisms, especially if they become numerous. This paper discusses the need for also testing candidate agents, prior to release, to assure that they have the potential to control their target weed. It offers an example of pre-release tests to quantify the impact a gall-making fly has on Cape ivy, a serious weed in coastal California. Similar tests by other scientists would allay the fears of ecologists and other critics of classical biological control, and help assure that this control methodology continues to be available to assist in managing invasive weeds.
Technical Abstract: Due to the long standing emphasis on releasing only agents whose host specificity has been ascertained, classical biological control of weeds has an enviable track record of few impacts to non-targets. However, even an agent whose host range is restricted solely to the target weed can have indirect impacts. Such indirect impacts are most likely if, after release, the populations of the agent build up to high numbers without causing accompanying declines in the populations of the target weed. Therefore, it is imperative, prior to release, to demonstrate that the candidate agent is not only host specific, but that it has clear potential to depress populations of the target weed. Pre-release assessments of potential impact on the target weed are not common, but I present an example for a gall-forming fly, Parafreutreta regalis, that is being considered for release to control Cape ivy, Delairea odorata. Under strict containment conditions of an approved quarantine facility, I conducted two trials exposing test Cape ivy plants to two different densities of this fly, and, after approximately two months, comparing the growth of the galled vines to similar vines that had not been exposed to flies. Under both the high density (10 pairs of flies/ plant) and low density (2 pairs/plant) treatments, the galled vines exhibited visible stunting, and the non-galled plants were longer, and had more nodes and larger leaves. These trials confirmed that relatively subtle, sub-lethal impacts on the target can be quantified, even under strict containment conditions, and this should encourage others to more routinely, prior to release, to assess the potential impact of prospective agents on their proposed target.