In 2003, Alan used a new type of pesticides in order to increase the net yield of his crops. Because of these new pesticides, the net yield of his crops doubled which made him really happy and he decided to continue which these pesticides. For next four years, the yield of his crops increased consistently compared to the previous years. In 2008, the yield of his crops decreased exponentially, which caused severe financial problems for Alan.
Which of the following statements explains the reason for the drop in net yield this year?