Host-specificity of the Mediterranean weevil, Coniatus tamarisci : an unsuitable biological control agent of invasive Tamarix taxa in South Africa

Abstract

Numerous trees within the genus Tamarix L. (Tamaricaceae) have been introduced globally for phytoremediation, soil stabilisation, and ornamental purposes, subsequently naturalising and frequently becoming invasive. In South Africa, two species, Tamarix chinensis and T. ramosissima, along with their hybrid taxon, are considered invasive, and have been targeted for biocontrol since 2014. Biocontrol research to date has been unsuccessful in identifying suitably host-specific agents for release in South Africa, largely owing to the presence of the congeneric indigenous T. usneoides. Given the growing socio-economic and ecological impacts of Tamarix invasions in South Africa, assessment of additional biocontrol candidates is required. This study investigated the suitability of the Mediterranean weevil, Coniatus tamarisci F. (Coleoptera: Curculionidae), as a biocontrol agent of invasive Tamarix trees in South Africa. Laboratory no-choice and multi-choice tests were undertaken using several taxa within the Tamarix genus. Under no-choice conditions, C. tamarisci displayed indiscriminate oviposition, completed larval development, and fed extensively on all Tamarix taxa, including the indigenous (non-target) T. usneoides. Similarly, multi-choice assessments highlighted that C. tamarisci showed no distinct preference for any Tamarix present in South Africa, ovipositing indiscriminately and feeding extensively on all taxa. Overall, C. tamarisci appears to be a generalist Tamarix weevil, and as such presents a strong likelihood for unwanted non-target effects to the indigenous South African T. usneoides. Therefore, it is recommended that investigations into C. tamarisci as a biocontrol agent of invasive Tamarix in South Africa be halted, in favour of other potentially more suitable candidates.