A study of the effect of Chrysomela hyperici Forst. on St. John's Wort in the Mannus Valley, NSW

Abstract

Conditions in those parts of the Mannus Valley, N.S.W., where Chrysomela hyperici Forst. has been liberated appear either unfavourable or just tolerable to the species, which has increased in numbers and spread to a limited extent in the more favourable parts of the area infested with St. John's wort. In the colony site studied intensively, the gradual multiplication of C. hyperici led to great destruction of the weed in places, but this destruction was soon followed by regeneration. Because of this and the way in which the beetles aggregate and move on definite fronts, there has been a history of waves of insects passing back and forth over a small area around the liberation site followed by waves of regeneration of the host plant. Very young and large old crowns of Hypericum were readily destroyed by the insect, but the mortality of mature crowns of moderate size was relatively low. Where few crowns were killed, vegetative reproduction soon resulted in the reoccupation of vacant spaces. Under some conditions, defoliation by the insect appeared to stimulate suckering and thereby produced a stand of higher crown density than the original one. This addition of new crowns must have rejuvenated some stands, and probably increased their resistance to further beetle attacks for a short time. The ability of St. John's wort to maintain its position of predominance in the community in the face of heavy insect damage was due partly to the absence of effective plant competitors. The heavy grazing by rabbits and kangaroos on the plants associated with the weed, and their habit of removing the ground cover by scratching, favoured the regeneration of Hypericum from seed in places where most of the original wort plants had been destroyed.