The development of agriculture over the last 10,000 years or more has had great impact on man and his environment. During the evolution of early agriculture, man developed relatively stable agroecosystems. This was achieved more by necessity and chance than by design. Man obviously harvested those plants which survived pest damage and thereby unconsciously selected for pest resistance. Agriculture, throughout its history, has been beset by severe pest outbreaks. The effects of mildew diseases and insect pests, including recurrent locust plagues, are recorded in the Old Testament. Ireland lost one third of its population either by death, starvation, or emigration because of the potato blight epidemic in 1845. The greatest crop loss of corn in history, approximately $1 billion, caused by southern corn leaf blight, occurred in the U.S. in 1970.