Crop losses: The need to quantify the effects of pests,diseases and weeds on agricultural production

Crop losses caused by pests, including rodents and birds, and by diseases and weeds, are defined, and previous studies reviewed. Information about losses is needed to monitor the effects of pests and diseases on crop production, in individual countries and worldwide, important for food policy and economic reasons; to make decisions and allocate resources on the study, management and control of pests and diseases; to stimulate action against them; to provide guidelines for research on crop improvement to prevent pest and disease attack; and as a basis for judgement on the importance of pests, diseases and weeds in relation to agriculture and the environment.The causes of such losses, why information must be up to date, and methods of assessing pests, diseases, weeds and losses are examined. The part played by losses in the development and use of economic pest and disease control action thresholds is discussed, including pest, disease, crop, climatic, economic and farmer attitude factors. The types of relation between yield and pest and disease attack are reviewed, how to measure the relation in practice, how to obtain different amounts of attack, surveys of losses, and the difficulties met in the study of losses: interaction; compensation; interplot interference; variation; and distribution. Sources of information on world losses, training and the work of the FAO are reviewed. General guidelines on crop loss assessment are given.