Remotely piloted vehicles exist to take pilots out of danger. They still require pilots however. There is no inherent danger in a mooted spray operation therefore taking the pilots out of the operation would be pointless because you still need them.
Drones are not remotely piloted, hence the name. There would not be a lot of them due to their limited uses. They exist exist mainly to take the boredom out of long duration general surveillance missions or to get shot at. A large drone- anything larger or faster than a Predator size, pilotless, would be an accident going somewhere to happen. Large aircraft still require human input to safely fly, therefore there is no gain to be had by making them a "drone".
There are no RPV's cureently known that have the payload to effectively spray anything...leaving aside the arguments about the efficiency of high altitude spraying.