A farmer releases tens of thousands of wasps ino his field. Why might he do this?

In my opinion, the most likely reason that a farmer would
release wasps into his field would be if he meant to use them as a form of biological
pest control.  In other words, the farmer might be using wasps as a form of pesticide so
that he does not have to use chemical pesticides.

This can
be done if the farmer has problems (or wants to prevent problems) with some sort of
pests that the wasps can prey upon.  He will then release the wasps and they will kill
the pests.  That way, he does not have to use chemicals to kill the
pests.

The link I have provided shows one example of using
wasps in this way.