1. The World Bank http://data.worldbank.org/indicator/AG.YLD.CREL.KG
2. Withers JA Jess S Matthews D Kelly T 2011
3. Withers JA Jess S Matthews D Kelly T 2010
4. Lavery MK Jess S Kirbas JM Matthews D Kelly T 2011
5. Thematic Strategy on Sustainable Use of Pesticides http://ec.europa.eu/environment/ppps/home.htm