Fundamentals of biological control of weeds

Abstract does not appear. First page follows. Preface The critical phase of biological control work against weeds is the selection of species that will not harm other plants, or at least useful plants. All other considerations are subordinate, and a suitable species for introduction into a country against a weed is one that is safe to introduce, irrespective of its other characteristics.—J. R. Williams (1954)3 Introduction Since the beginning of agriculture man has engaged in a never-ending struggle to rid his lands of weeds. In this age of chemistry, there are yet millions of acres of land on which weeds flourish and where they either do not yield to chemicals used against them, or else this solution has proved impracticable for other reasons. With some of the worst weeds known where chemical, mechanical, or precautionary measures have failed, biological control, or the employment of natural enemies, has proved eminently successful. The objective in this work is not eradication, but the reduction of weed densities to levels largely noninjurious to man’s interests. In this field where host-specific agents must be employed, eradication is inconceivable over any appreciable natural area. As will be shown later, control may be accomplished either by direct action of the introduced agents or through other actions set in motion by such agents. The fauna and flora of infested lands constitute an ecological entity which may represent either a barrier or an open highway to success. Employment of biological methods of weed control has been hesitantly approached for two reasons: 1) fear that the risks involved are too great as compared with the chances of success, and 2) the conflict in general acceptance of a given plant as a weed, coupled with the fact that introduced natural enemies of weeds would be free to move into other lands where the plant may be considered of value. The first and most important reason for hesitancy is losing its force because of accumulating evidence of successes and greater assurances against disproportionate risk.