The biology of Hypericum perforatum L. var. angustifolium DC (St. John's Wort) in the Ovens Valley, Victoria, with particular reference to entomological control

Abstract

A study of the biology of Hypericum perforatum L. var. angustifolium DC in the Ovens Valley, Victoria, has shown that the weed possesses characteristics that make it a difficult species for effective entomological control. They include the ability to prosper under a diversity of environmental conditions after successful establishment from seed; a great capacity for vegetative reproduction by suckering; and the delayed germination of a proportion of the seed for at least 6 years, probably owing to the presence of an inhibitor in the sticky exudate of the seed capsule. The problem of control is made more difficult by the unfavourableness of a large part of the area for the growth of other herbaceous plants capable of competing effectively with either mature Hypericum or its seedlings.

The leaf-eating beetles Chysomela gemellata Rossi and C. hyperici Forst. differ greatly in their ability to destroy well-established stands of the weed. The susceptibility of mature Hypericum to defoliation depends on environmental conditions, e.g. the physical condition of the soil, which determine the life span of the individual crown and the rate and amount of vegetative reproduction. C. hyperici is highly destructive only in stands characterized by large crowns and limited vegetative reproduction; whereas C. gemellata can eliminate any type of stand occurring in treeless areas. The difference between the insects is due to the fact that C. gemellata can sustain the process of defoliation much longer than C. hyperici.

It so happens that environmental conditions most favourable for the initial multiplication of both insects occur in the very sites in which the weed is most readily controlled, i.e, where the established plants are of the type most susceptible to insect attack, and where seedling regeneration is most likely to be suppressed by the associated flora.

The stands of Hypericum that occur in areas less favourable for the initial colonization by the insects are genaally more resistant to defoliation. Some of these stands, e.g. those present in eucalypt forest, cannot be destroyed by hand defoliation sustained for longer periods than the insects can maintain the process. Frequently, plants capable of ousting mature wort damaged by insect attacks or of suppressing seedling regeneration are scarce or absent, e.g. many areas of gold dredgings. Consequently, if the insects do succeed in destroying the original stands in such sites, the weed has a good chance of successful regeneration from seed. In many such areas in which the weed was destroyed in 1946 or 1948 St. John's wort is already re-established as the predominant plant. It has either achieved its former density or is well on the way towards doing so.