How do farmers use pesticides and fertilizers?

To prevent bugs such as white fly, caterpillars and grasshoppers off of their crop so they don't lose any of it.

Farmers use pesticides to help control or suppress any living thing that is damaging the crop or stealing away anything the crop needs for healthy growth. A pest can be a weed, an insect, certain arthropoda, rodents, birds, bacteria, fungus, etc.