THE CONTROL OF BRACKEN

Bracken is a weed of economic importance to hill farming, conservation, and forestry in many parts of the world. It is an aggressive competitor, capable of rapid spread by means of its persistent underground rhizomatous system, and it is highly toxic and carcinogenic to domestic stock. In addition, it harbors large populations of ticks that transmit disease to animals and humans. Control by husbandry or by mechanical frond destruction is labor-intensive and often inadequate. Chemical control continues to be a subject of intensive research, with particular emphasis on the use of adjuvants, including surfactants. There is evidence that formulation of systemic herbicides such as asulam or glyphosate with low concentrations (e.g., 0.05%) of certain nonionic surfactants can effectively increase foliar absorption and translocation to the underground rhizome buds. Biological methods of control using either insects or fungi are under investigation. Certain bracken-feeding South African insects are specific to the variety of bracken (Pteridium aquilinum ssp. aquilinum var. aquilinum) that is also common in the U.K. It is proposed that the moth Conservula consigna would occupy a vacant ecological niche if introduced into the U.K. bracken ecosystem and thereby give effective control. The insect feeds on the pinnae and completes its life cycle on bracken. The fungus Ascochyta pteridis is indigenous to the U.K., where, under certain climatic conditions, it causes the disease of bracken known as curl-tip. The potential of a mycoherbicidal formulation, in which the spores are mixed with adjuvants designed to give the fungus a competitive advantage, is under investigation. Oil and water emulsions, for example, are expected to be of particular importance: they assist adhesion of spores to the host surface while providing water for their germination. This review examines the problems posed by bracken and the progress of research toward the development of economically and environmentally acceptable methods of control of this noxious and aggressive weed.