n African Entomology - Biological control of spear thistle, Cirsium vulgare (Asteraceae), in South Africa : a modest start for Rhinocyllus conicus (Coleoptera : Curculionidae)

Abstract

Rhinocyllus conicus is a flower-feeding weevil that has played a pivotal role in the biological control of several thistle species around the world. More recently it has achieved notoriety because of the damage it causes on several species of native thistles in North America. From 1984 to 1986, R. conicus was released for biological control of spear thistle, Cirsium vulgare, in three different regions of South Africa. Although the weevils have been established in the country since then, no proper surveys have been made to determine the extent of damage they are causing on C. vulgare. To redress the situation, samples of flower-heads from several localities were examined and damage due to R. conicus was recorded. The study showed that R. conicus has not dispersed more than 2.5 and 3.9 km from two of the original release sites where establishment was confirmed. The weevil is seldom particularly abundant (levels of damaged flower-heads exceeded 65 % at only two of 17 weevil-inhabited sites). Flower-heads set earlier in the season suffered higher levels of damage by the weevils, and infected flower-heads had significantly fewer seeds compared to uninfected flower-heads. All indications are that the contribution by R. conicus to biological control of C. vulgare in South Africa has been meagre due to unidentified constraints, at least in the two localized areas that the weevils currently occupy. Manual redistribution is required to determine whether or not other regions might be more suitable for R. conicus.