Biocontrol of Common St. Johnswort (Hypericum perforatum) withChrysolina hypericiand a Host-SpecificColletotrichum gloeosporioides

Common St. Johnswort is widespread in eastern Canada but it seldom constitutes a serious weed problem. A demographic study conducted in 1993 and 1994 at four typical undisturbed sites indicated that 36 to 96% of established St. Johnswort shoots died during the growing season. Mortality was always associated with infection by a host-specificColletotrichum gloeosporioides. The leaf-feeding beetleChrysolina hypericioccurred at all sites and caused maximum midsummer defoliation of 27% in 1993 and 51% in 1994. Healthy plants readily recovered from defoliation during pupation of the fourth instar of the insect in June and following adult estivation in August. Although widespread,C. hypericipopulations appear transient and alone do not cause sustained feeding pressure resulting in weed control. WhenChrysolinalarvae and adults were collected at six field sites and placed on healthy seedlings under controlled conditions, up to 36% of the plants became infected withC. gloeosporioides. Scanning electron micrographs commonly showedColletotrichumconidia among the setae on legs, tarsal pads, and antennae of adults and larvae. In a series of three experiments conducted under controlled conditions in whichChrysolinalarvae and adults were placed on healthy plants after feeding on diseased ones, the incidence of infection ranged from 63 to 100%. Hence, under favorable conditionsChrysolinaadults may selectively transmit the pathogen in the field. This study demonstrated the potential of enhancing biological control of weeds by insects with the integration of an effective, host-specific pathogen.