Successful Examples of biological Control of Pest Insects and Plants

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The selection of examples where the biological control method has been successful depends somewhat upon the individual viewpoint. A casual perusal of the literature would lead one to believe that predators and parasites had been unusually successful against many pests. In a recent communication from H. S. Smith it was stated : “ If you go over the old literature on this subject in California, you will find that the black scale problem was solved by the introduction of Rhizobius in 1890. It was solved again in 1900 by the introduction of Scutellista. But we still spend in excess of a million dollars a year in southern California on spraying and fumigation for this pest. The same was true in Hawaii. For many years it was stated that one of the greatest examples of successful biological control was that of the sugar-cane leafhopper by the egg parasites, Paranagrus and Ootetrastichus. Later Cyrtorhinus was introduced with success, and it was admitted that control by the two egg parasites was not satisfactory. No doubt all these introductions were valuable and represent different degrees of control, each succeeding introduction resulting in a greater degree of control than previously existed. The difficulty lies in the absence of a satisfactory criterion by which to measure and record the relative effects of the introductions.” A critical analysis of the data submitted to support the contentions for the early examples mentioned above is far from convincing. However, it should be recalled that the standards for measuring the success of biological control have undergone considerable change during the intervening period.