Instead of spraying a field with chemicals a plane disperses tens of thousands of tiny wasps over the growing plants What is the most likely reason for such an action?

A very good idea! This is using nature to get rid of pests, rather than fighting nature by using chemicals.

The farmer obviously has a problem with insect pests in the crop. The wasps will kill and eat these pests. They will eventually die out so will not become a pest themselves.