The present status of St. John's wort ( Hypericum perforatum L.) and its biological control agents in Victoria, Australia

Abstract

A study of the weed St. John's wort, Hypericum perforatum, and the biological control agents Chrysolina quadrigemina and C. hyperici, at open, semi-shaped and shaded sites, showed that the weed could be successfully controlled under open but not under shaded conditions. If beetles moved into shaded areas some defoliation occurred, but migration rarely took place. At shaded sites, plants appeared less aggressive than elsewhere and their growth habits differed from those of plants growing at open or semi-shaded sites. C. hyperici were collected in low numbers at open sites only, whereas C. quadrigemina were found at all sites. C. hyperici numbers were lower than observed 30 years earlier. C. quadrigemina numbers were frequently high at open sites, but considerably lower at both semi-shaded and shaded sites. C. quadrigemina is now the dominant species and the main insect exerting some control on the plant. Both Chrysolina species aestivate during the hot dry summer months, but aestivating beetles were only found under the bark of Eucalyptus trees. Another biological agent, Zeuxidiplosis giardi, was mainly found at shaded sites. Neither it nor any indigenous insect species contributed to the control of the plant.