Risk assessment of the laboratory host range and a molecular characterisation determining the field host range of Lixus aemulus, for the biological control of Chromolaena odorata in South Africa

Chromolaena odorata (Asteraceae: Eupatorieae) is a sprawling shrub native to the Americas, and a destructive invader of much of the humid tropics and subtropics of the Old World. Opportunistic native-range exploration in 1995 identified a stem-boring weevil, Lixus aemulus, as a promising biological control candidate agent. Host-specificity testing was conducted on L. aemulus in South Africa using laboratory no-choice and paired-choice tests. Three invasive alien plants closely related to C. odorata may be utilized by L. aemulus but no indigenous, ornamental or crop species in South Africa was or is expected to be attacked by the weevil. A native-range field survey was conducted in Brazil to determine the exact identity of the host plant L. aemulus had been collected in 1995, and to identify additional host-plant species. Genetic assessments of the Lixus sp.(p.). adults collected on the three host plants (C. odorata, Chromolaena laevigata and Heterocondylus vitalbae) reveal these individuals are L. aemulus and the weevil can be classed as an oligophage in its native range. Over 5,500 adults were released in South Africa, but overall establishment has been poor. The most likely explanation appears to be a climate mismatch between the region of South Africa invaded by C. odorata and the collection locality in Rio Branco, Acre state, Brazil. Additionally, because the full extent of the native range of L. aemulus is unknown, it is uncertain whether individuals can be sourced from an area whose climate resembles that of South Africa. Furthermore, despite being oligophagous, L. aemulus may perform sub-optimally on the southern African C. odorata biotype.