Total food production and distribution systems and energy. Subsidy to yield relationships. A plant or insect out of place. At times this distinction is clear, at other times it is subjective. There are no biological differences between pest and non pest organisms 2.
Losses are great in economic terms and in human suffering.
Calculating loss as if it is possible to have no weeds or pests results in an inflated loss rate. Methods of control. The conversion of crop land to non farm uses. Water shortages potentially caused by non-agricultural water demands. Regulation, price support, taxation, and market value influencing harvest.