BIOCONTROL OF COMMON ST. JOHNSWORT (HYPERICUM PERFORATUM) WITH CHRYSOLINA HYPERICI AND A HOST-SPECIFIC COLLETOTRICHUM-GLOEOSPORIOIDES

Common St. Johnswort is widespread in eastern Canada but it seldom con stitutes a serious weed problem. A demographic study conducted in 1993 and 1993 at four typical undisturbed sites indicated that 36 to 96% o f established St. Johnswort shoots died during the growing season. Mor tality was always associated with infection by a host-specific Colleto trichum gloeosporioides. The leaf-feeding beetle Chrysolina hyperici o ccurred at all sites and caused maximum midsummer defoliation of 27% i n 1993 and 51% in 1994. Healthy plants readily recovered from defoliat ion during pupation of the fourth instar of the insect in June and fol lowing adult estivation in August, Although widespread, C. hyperici po pulations appear transient and alone do not cause sustained feeding pr essure resulting in weed control. When Chrysolina larvae and adults we re collected at six field sites and placed on healthy seedlings under controlled conditions, up to 36% of the plants became infected with C. gloeosporioides. Scanning electron micrographs commonly showed Collet otrichum conidia among the setae on legs, tarsal pads, and antennae of adults and larvae. In a series of three experiments conducted under c ontrolled conditions in which Chrysolina larvae and adults were placed on healthy plants after feeding on diseased ones, the incidence of in fection ranged from 63 to 100%. Hence, under favorable conditions Chry solina adults may selectively transmit the pathogen in the field. This study demonstrated the potential of enhancing biological control of w eeds by insects with the integration of an effective, host-specific pa thogen.