Prerelease efficacy assessment, in quarantine, of a tephritid gall fly being considered as a biological control agent for Cape-ivy (Delairea odorata)

Due to the long-standing emphasis on only releasing host-specific agents, classical biological control of weeds has an enviable track record of few direct impacts to nontargets. 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 advisable, 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. Prerelease efficacy assessments (PREA) of potential weed biocontrol agents are not yet common, and are most easily done in the region where both the target and the potential agent are native. We present an example of a PREA performed under strict containment conditions of an approved quarantine facility. A gall-forming fly, Parafreutreta regalis, from South Africa is being considered for release in California to control Cape-ivy, Delairea odorata. We 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 ungalled stems were longer, and had more nodes and larger leaves. These trials confirmed that relatively subtle, sublethal impacts on the target can be quantified, even under strict containment conditions, and this should encourage others to assess, prior to release, the potential impact of prospective agents on their proposed target.