The corn in the field

I was glad to see the growing corn in the nearby open field. After taking a selfie, I noticed something that is amiss. There were parts of the leaves that are missing. In a closer inspection, I saw that there were pests. The farmer said that a spray is needed to kill the pests otherwise there will be no harvest.

I cannot imagine if the cornfield is large like 10 hectares. How can you fight the pests that would be jumping from hectare to hectare in eating the young leaves of the corn plants. Maybe they should have cleaned the field first before planting.