Prospects for the biological control of invasive Pinus species (Pinaceae) in South Africa

Nine Pin us species (Pinaceae) have become invasive plants in South Africa after being deliberately introduced and cultivated in commercial forests, for timber. A proposal to use biological control to contain the problem raised concerns among foresters who immediately identified a number of difficulties that could arise for the forestry industry if biological control agents were to be introduced. As a compromise, plans were made to target, initially at least, two pine species, Pinus pinaster Aiton and Pinus halepensis Mill., that currently have no commercial value. A cone-feeding weevil from Portugal, Pissodes validirostris Gyllenhal (Coleoptera: Curculionidae), was identified as the most promising agent. Formerly regarded as a single species, extensive preparatory studies revealed that there are several different forms (perhaps a complex of sibling species) of P. validirostris each associated with different pine species in different regions of Europe. Screening tests in arboreta in France showed that the prospective agent was host-specific enough to be used with safety in South Africa. Despite this positive finding, the programme did not proceed much further because subsequent trials in quarantine in South Africa showed that damage caused by the adult weevils feeding on leader shoots of pines allowed ingress of pitch canker, Fusarium circinatum Nirenberg and O'Donnell (Hypocreales: Nectriaceae), an increasingly problematic pathogen in pine forests in South Africa. However, given the escalating negative consequences of pine tree invasions, especially in the Cape Floral Region, the question of whether or not this biological control programme should have been discontinued in 2009 remains open for debate and the opposing views on the subject are presented.