The ecology of Chrysomela gemellata Rossi and c.Hyperici forst, and their effect on St. John's Wort in the Bright District, Victoria.

Abstract

This paper describes the basic entomological study necessary in an investigation of the effectiveness of Chrysomela gemellata Rossi and C. hyperici Forst. as controls of St. John's wort, Hypericum perforatum L. var. angustifolium DC, in the Bright district, Victoria. The investigation has shown that the conditions prevailing in the Bright district during the study period must have been generally near the limit of tolerance of both C. gemellata and C. hyperici, parts being a little above and parts definitely below this limit. Furthermore, largely because of certain behaviour responses, neither insect has been able to colonize the timbered areas infested by Hypericum. The total area favouring the multiplication of the insects forms only a small part of the area colonized successfully by Hypericum. Within the zone favourable to the beetles, they have failed to control Hypericum effectively except in small, treeless areas adjoining densely timbered country infested by the weed (in such places the timbered area has acted as a damping factor to population fluctuation, enabling the insects to persist in moderate numbers). Although adverse conditions have played an important part in limiting the effectiveness of the insects, their failure has been due primarily to their own characteristics. Both species have poor powers of dispersal and a relatively poor ability to multiply. In the most favourable sites, both species are able to multiply within a few years to a density high enough to cause complete defoliation of the host plant, which usually dies out. By destroying their food and reducing their protective cover, both species suffer very heavy losses and greatly limit their own numbers. Although many individuals succumb to starvation, a large proportion of the mortality is caused by density-independent factors, e.g. frost and predation, which are allowed to operate in an intensified manner by removal of the protective cover provided by the foliage of the host plant. After completing the defoliation of the weed, the surviving beetles migrate elsewhere in search of more food. Their desertion of the site gives the host plant a chance to recover by regeneration from seed. Numerous environmental factors unfavourable to the insects, combined with their poor mobility and slow rate of increase, frequently allow the plant to recover almost completely before beetle numbers become sufficiently high to defoliate it again. Where other wort-controlling agencies are lacking, the insects have caused their host plant to fluctuate violently in density in both space and time without producing a great overall reduction in quantity. By destroying stands of Hypericum in areas in which the soils are capable of growing a dense pasture, the insects have paved the way for successful wort control by other factors, e.g. shade-producing herbage and grazing animals. Such areas virtually cease to provide habitats for the insects after the original stand of Hypericum is destroyed.